CHAPTER-2: (ACIDS, BASES& SALTS)
We use a number of materials in our daily life, some of which are sweet, sour, bitter and salty in taste. Now it is very interesting to know that what substances are present in them which make them sour, bitter, sweet or salty? And the answer is
(i) Sour taste of substances is due to acid (latin word acidus which means sour) present in them and are called acidic substances eg. Imli, vinegar etc.
(ii) Bitter taste of substance is due to base present in them and are called basic substances eg. Washing soda, baking soda, etc.
(iii) Sweet taste of substances is due to sugar present in them and are called sugars e.g. cane-sugar, glucose etc.
(iv) Salty taste of substances is due to presence of salts in them and are called salts e.g. sodium chloride, silver nitrate etc.
We can distinguish between acids and bases without tasting them by using certain chemicals called indicators.
Tests to distinguish between acids and bases: The tests used to distinguish between acids and bases are done with some indicators known as acid base indicators.
Acid – Base Indicators: These are the substances which show one characteristic property (colour, odouretc) in the acidic medium and a different property in the basic medium and thus distinguish between acids and bases. There are two types of acid base indicators depending upon their property.
1.1 INDICATORS SHOWING DIFFERENT COLOURS IN ACIDIC AND BASIC MEDIUM
1.1.1 Litmus solution
Litmus solution is a purple coloured dye. extracted from the lichen plant. It is very interesting to note that litmus solution (purple colour) itself is neither acidic nor basic. To use it as an indicator,it is made acidic or alkaline.
The alkaline form of litmus solution is blue in colour and called blue litmus solution.
The acidic form of litmus solution is red in colour and called red litmus solution.
Blue litmus solution(blue in colour): It is obtained by making the purple litmus extract. alkaline. Thus it is basic in nature and acts as an acid-indicator by giving a characteristic change in its colour in acids.
Red litmus solution(red in colour): It is obtained by making the litmus extract acidic. Thus it is acidic in nature and acts as a base-indicator by giving a characteristic change in its colour in bases.
Change in colour is due to acid or base present in sample solution.
Litmus solution itself is neither acidic nor basic, but its alkaline form (i.e. red litmus solution) act as base indicator and its acidic form (i.e blue litmus solution) act as an acid indicator.
Turmeric used in kitchen can also be used to test a basic solution i.e. it act as base indicator by giving brown colour in basic medium. In other words yellow colour of haldi turns into brown in basic substances (due to base present in them) and thus distinguishes between acids and bases.
Example: While eating food, if curry falls on the white clothes, a yellow stain is produced in the clothes. When we apply soap solution (basic in nature) on the cloth, the yellow stain becomes brown due to base present in soap solution.
This example shows that turmeric (haldi) act as base indicator by giving brown colour in basic substances.
1.2 SYNTHETIC INDICATORS
The chemical substances which change their colour in acids and bases and thus distinguish between them are called synthetic indicators. Since they distinguish between acids and bases, so they are also called synthetic acid base indicators. The two most common synthetic indicators are
(a) Phenolphthalein and (b) Methyl orange.
(i) Methyl orange (whose naturalcolour is orange) gives pinkcolour in acidic medium or solution i.e. it turns into pink,if the sample solution to be tested is acidic.
(ii) Methyl orange gives yellow colour in basic medium or solution i.e. it turns into yellow, if the sample solution to be tested is basic.
1.3 INDICATORS GIVING DIFFERENT ODOURS IN ACIDIC AND BASIC MEDIUM (Olfactory Indicators)
These are another type of acid-base indicators which distinguish between acids and base by giving different odour or smell in acidic and basic medium i.e. they give one type of odour or smell in acidic medium and a different odour or smell in basic medium and thus distinguish between acids and bases.
These indicators which give different odours or smell in acidic and basic medium are called olfactory indicators.
A few of these are given below:
(a) onionodoured cloth strip (b) vanilla extract (c) clove oil
Those substances which turn blue litmus solution red are called acids. The term ‘acid’ has been derived from the Latin word ‘acidus’ which means sour. Acids are sour in taste.
2.1 PHYSICAL PROPERTIES OF ACIDS
(i) Sour taste: Almost all acidic substances have a sour taste.
(ii) Action on litmus solution: Acids turn blue litmus solution red.
(iii) Action on methyl orange: Acids turn methyl orange pink.
(iv) Action on phenolphthalein: Phenolphthalein remains colourless in acid.
(v) Conduction of electricity: The aqueous solution of acid conducts electricity.
(vi) Corrosive nature: Most acids are corrosive in nature. They produce a burning sensation on the skin and make holes on surfaces on which they fall.
2.2 CHEMICAL PROPERTIES OF ACIDS
2.2.1 Reaction of acids with metals
When acid reacts with a metal, then a salt and hydrogen gas are formed.
i.e. metal + acid salt + hydrogen gas
Example:Reaction of dil with zinc metal.
Experiment: Take about 5 ml of dil H2SO4 in a test tube and add a few pieces of zinc granules in it. Pass the gas evolved through soap solution. The soap bubbles filled with gas rise.
Test for gas: Bring a burning candle near the gas filled soap bubble.
Observation: The gas present in soap bubble burns with pop sound which shows the gas evolved during reaction is H2 (hydrogen) gas (figure).
Figure-reaction of Zn granules with dil. and test for hydrogen gas by burning
The reaction involved is
In this reaction more active metal zinc displaces less active hydrogen from H2SO4 and this hydrogen is evolved as gas.
Thus it is an example of displacement reaction.
Some more examples of reaction of different metals with a particular acid:
Mg(s) + 2HCl (aq) MgCl2 (aq) + H2 (g)
magnesium hydrochloricacid magnesiumchloride hydrogen
(a metal) (dil) (a salt) gas
Zn (s) + 2HCl(aq) ZnCl2 (aq) + H2(g)
zinc hydrochloricacid zinc chloride hydrogen
(a metal) (dil) (a salt) gas
Fe(s) + 2HCl(aq) FeCl2 (aq) + H2(g)
iron hydrochloricacid iron(II) chloride hydrogen
(a metal) (dil) (a salt) gas
Cu + 2HCl(aq) no reaction
(a metal) (dil)
The order of reactivities of above metals with same acid (dilHCl) is Mg > Zn > Fe > Cu i.e. these metals do not react with same acid with same vigour.
It is observed that at room temperature
(i) Mg reacts most vigorously (ii) Zn reacts less vigorously than Mg
(iii) Fe reacts slowly (iv) Cu does not react at all
Conclusion:From above reactions we lead to a conclusion that all metals do not react with same acid with same vigour.
The reason is the different reactivities or activities of metals towards acid.
In the above reactions we observe that metals like Mg, Fe, Zn being more active than hydrogen, displaces hydrogen from acid HCl and release H2 gas. Thus above reactions are displacement reactions, Cu being less reactive than hydrogen, cannot displace hydrogen from dilHCl. Thus no reaction takes place.
In General : Acid + Metal Metal Salt + Hydrogen gas
2.2.2 Reaction of acid with metal carbonates and metal hydrogen carbonate (or metal bicarbonates)
When an acid reacts with a metal carbonate or metal hydrogen carbonate (metal bicarbonate), then a salt, CO2 gas and H2O are formed.
i.e. Metal carbonate + Acid Salt + CO2 + H2O
Metal hydrogen carbonate (or metal bicarbonate) + Acid Salt + CO2 + H2O
Example: Reaction of sodium carbonate (Na2CO3) or sodium hydrogen carbonate (NaHCO3) with dilHCl.
Experiment: Take about 0.5g of Na2CO3 or NaHCO3 in a test tube and add about 2 ml of dilHCl acid to it. Pass the gas evolved through lime water (taken in another test tube).
The reactions taking place are
Na2CO3(s) + 2HCl(aq) 2NaCl(aq) + H2O(l) + CO2(g)
sodium carbonate hydrochloric acid sodium chloride water carbondioxide
(dil) (a salt)
Ca(OH2) (aq) + CO2(g) CaCO3(s) + H2O(l)
lime water carbondioxide calcium carbonate water
(i) The lime water on passing CO2 gas turns milky due to formation of white ppt. of CaCO3 (insoluble in water) having milky appearance.
(ii) On passing excess of CO2 gas through lime water milkiness disappears due to dissolution of white ppt of CaCO3 and clear solution is formed due to formation of soluble calcium bicarbonate [Ca(HCO3)2]
CaCO3(s) + H2O(l) + CO2(g) Ca(HCO3)2(aq)
calciumcarbonte water carbondioxide calcium bicarbonate
(whiteppt) (soluble in water)
(insoluble in water)
Some more examples of reaction of metal carbonate and metal bicarbonates with dil acids
MCO3 + acid salt + H2O + CO2(g)
M stands for metal MHCO3 + acid salt + H2O + CO2(g)
MgCO3 (s) + 2HCl (aq) MgCl2 (aq) + H2O (l) + CO2
Mg(HCO3)2 (aq) + 2HCl (aq) MgCl2 (aq) + 2H2O (l) + 2CO2
CaCO3 (s) + H2SO4(aq) CaSO4(aq) + H2O (l) + CO2(g)
Ca(HCO3)2(aq) + H2SO4(aq) CaSO4(aq) + 2H2O (l) + 2CO2(g)
ZnCO3 (s) + 2HCl (aq) ZnCl2(aq) + H2O (l) + CO2(g)
Zn(HCO3)2(aq) + 2HCl(aq) ZnCl2(aq) + 2H2O (l) + 2CO2(g)
2.2.3 Reaction of acids with bases
When an acid reacts with a base then a salt and water are formed, i.e
Acid + Base Salt + water
This reaction is called neutralization reaction, because when acid and base react with each other, they neutralize each other’s effect (i.e base destroys the acidic property of acid and acid destroys the basic property of base).
Example: Reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH).
Experiment: Take about 10 ml of dilNaOH solution in a conical flask and add 2-3 drops of phenolphthalein indicator to it. The solution will turn pink (showing that it is basic in nature). Now add dilHCl solution from burette into flask slowly till the pink colour in the solution disappears.
Observation:This point (at which pink colour disappear) is called end point.
At end point:
(i) The dilNaOH solution in flask has been completely neutralised by dilHCl solution added from burette, dilNaOH has completely reacted with dilHCl.
(ii) [H+] = [OH–] (in case of NaOH&HCl)
(from acid) (from base)
In case of NaOH and HCl, the reaction mixture has become neutral.
The chemical reaction can be written as
NaOH (aq) + HCl (aq) NaCl(aq) + H2O(l)
sodium hydroxide hydrochloric acid sodium chloride water
(base) (acid) (salt)
This reaction of acid and base to form salt and water is called neutralization reaction or neutralization of base by an acid
Figure-neutralisation of NaOH solution by HCl solution using phenolphthalein indicator
In the solution, NaOH, HCl and NaCl ionize completely into ions, so the above reaction can be written as:
Canceling out the common ions on both sides, we get:
hydroxide ion hydrogen ion water
(from base) (from acid)
Hence, neutralization may also be defined as the reaction between H+ ions given by acid with the ions given by base to form water
2.2.4 Reaction of acids with metallic oxides
Acid react with metal oxide to form salt and water.
i.e. Metal oxide + Acid Salt + Water
This reaction is similar to the neutralization reaction between acid and a base to form salt and water. Thus, the reaction between acids and metal oxides is a kind of neutralization reaction and shows that metallic oxides are basic oxides.
Example: Reaction of copper (II) oxide with dilute hydrochloric acid:
Experiment: Take about 1- 2g of copper (II) oxide (black in colour) in a beaker. Add dilHCl slowly with constant stirring.
Observation: Black CuOdissolves in dilHCl and a bluish green solution is formed due to formation of copper (II) chloride (CuCl2) as salt.
The reaction taking place is:
copper (II)oxide hydrochloric acid copper (II)chloride water
2.3 WHAT DO ALL ACIDS HAVE IN COMMON ORCHEMICAL NATURE OF ACIDS
To see what is common in all acids, let us perform the following experiment with different acids:
Experiment to illustrate chemical nature of acids or what do all acids have in common:
Take four test tubes and label them as A, B, C and D. Place them in a test tube stand. Take about 2 ml of each dilHCl, dil , dil and dil CH3COOH in test tubes A, B, C and D respectively. Now add few pieces of zinc granules in each test tube.
Figure-reaction of Zn metal with different acids
Observation: There is evolution of hydrogen gas (H2)in each test tube which burns with a pop sound on bringing a burning a candle near the mouth of tubes.
Conclusion: Hydrogen is common in all acids i.e. all acids contain hydrogen which they liberate when they react with active metals.
Thus we can say that acids are the substance which contain hydrogen, which they liberate when they react with active metals.
All acids contain hydrogen but all hydrogen containing compounds are not acids, for example, glucose (C6H12O6) and alcohol (C2H5OH) contain hydrogen but they are not acids.
It can be explained more clearly by following experiment.
Experimentto show that all compounds containing hydrogen are not acids: The experiment is based on the fact that acids conduct electricity through their aqueous solutions.
(i) Take aqueous solutions of hydrogen containing compounds like hydrochloric acid (HCl), sulphuricacid , glucose and alcohol in 4 beakers respectively.
(ii) Fix two iron nails on the rubber cork and place the cork in each beaker
(iii) Connect the nails to the two terminals of a 6 volt battery through a switch and a bulb (fig) in each beaker
(iv) Switch on the current in each case
(i) Bulb starts glowing in arrangements a and c containing aqueous solutions of HCl and acids respectively.
It shows aqueous solutions of hydrochloric acid (HCl) and sulphuric acid conduct electricity.
(ii) Aqueous solutions of glucose and alcohol do not conduct electricity (i.e. they do not allow electricity to pass through them) as bulb does not glow in arrangements b and d containing aqueous solutions of glucose and alcohol.
Explanation: Conduction of electricity through the aqueous solutions of acids (HCl and H2SO4) is due to the ions present in them. For example, aqueous solution of contains and ions. These ions can carry electric current and thus are responsible for conduction of electricity through HCl and solutions. On the other hand aqueous solutions of glucose and alcohol (hydrogen containing compounds) do not contain ions or any other ions. Due to absence of ions, aqueous solutions of glucose and alcohol do not conduct electricity.
Conclusion: From the above experiment, we lead to a conclusion that only those hydrogen containing compounds are acidic which when dissolved in water give ionsin the solution. Thus the definition of acid is modified as:
Acids are the substances which contain hydrogen and which when dissolved in water give ions in the solution. This is called Arrhenius definition of acids given by Arrhenius in 1884.
2.4 ROLE OF WATER IN SOLUTION OF AN ACID OR WHAT HAPPENS TO AN ACID IN WATER SOLUTION?
It is observed that acidic behaviour of acids is due to the presence of H+ ions in them, which they give only in presence of water. So in the absence of water, a substance will not form ions and hence will not show its acidic behavior. It can be explained more clearly by following experiments:
Experiment: Take about 1–2 g of NaCl in a dry test tube. Add some concentrated into the test tube. Following reaction takes place producing hydrogen chloride (HCl) gas
sodium chloride sulphuric acid sodiumhydrogensulphatehydrogen chloride gas (conc.)
Now bring a dry blue litmus paper and a wet (or moist) blue litmus paper near the mouth of test tube (which contains HCl gas)
(i) The dry litmus paper does not turn red. It shows that HCl gas does not behave as an acid in absence of water (since there is no water in dry litmus paper).
(ii) The wet (or moist) litmus paper turns red. It shows that HCl gas act as an acid only in presence of water (which is present in moist or wet litmus paper).
(i) When HCl gas come in contact with dry litmus paper, then HCl does not dissociate into ions (i.e and ions) due to absence of water in dry litmus paper.
Since ions are responsible for acidic behaviour of acids, HCl gas does not show acidic behaviour with dry litmus paper and thus it does not turn the blue litmus red (due to absence of H+ ion in dry HCl gas).
Dissociation does not occur.
(ii) When HCl gas comes in contact with wet litmus paper, then HCl dissociates into and ions due to dissociation of HCl in water present in wet litmus paper.
Since ions are responsible for acidic behaviour of acids. HCl gas shows acidic behaviour with wet litmus paper and thus it turns it into red (due to presence of ions in wet HCl gas) i.e.
(acts as acid) (dissociation occurs)
Such dissociation of a covalent molecule like HCl into ions in the presence of water is called ionization. The ionization of water is shown more clearly as follows:
Dissociation of HCl into H+and Cl ions in presence of water.
It is clear from the figure that, after dissociation of HCl, a number of water molecules remain attached to . Hence they are represented as H+(aq) and Cl–(aq) (aq indicating water molecules)
Alternatively, ions combine with water molecule to form an ion called hydronium ion.
hydrogen ion water molecule hydronium ion
Thus does not exist freely in water, but exist in combination with water molecules. Hence, we represent it as .
Conclusion: The properties of an acid is due to ions or hydronium ions which it gives in the aqueous solution.
Acidic properties of acids are due to presence of ions or ions which they produce only in presence of water.
In absence of water, a substance will not form ions or ions and hence will not show its acidic behaviour.
Greater the amount of H+(aq) ions in the solution, stronger is the acid .
2.5 CLASSIFICATION OF ACIDS ON THE BASIS OF DEGREE OF IONIZATION OR STRENGTH OF ACIDS ON BASIS OF DEGREE OF IONIZATION
The acids are classified into two categories on the basis of the degree of ionization as follows:
1. Strong acids
2. Weak acids
2.5.1 Strong acid
An acid which is completely ionized in water and thus produces a large amount of ions is called a strong acid e.g. acids like hydrochloric acid (HCl), nitric acid and sulphuric acid are completely ionized in water and thus produce large amounts of ions in the solution. So these are called strong acids. The ionization of these acids are represented as follows:
hydrochloric acid hydrogen ion chloride ion
nitric acid hydrogen ion nitrate ion
sulphuric acid hydrogen ion sulphate ion
Single arrow pointing towards ions indicates complete ionization of acids.
Characteristics of strong acids
Due to large amounts of (aq) ions in the solutions of strong acids,
(i) They react rapidly with other substances (such as metals, metal carbonates and metal hydrogen carbonates or metal bicarbonates).
(ii) They have a high electrical conductivity.
(iii) They are strong electrolytes.
2.5.2 Weak acids
An acid which is partially ionized in water and thus produces small amount of (aq) ions is called a weak acid. e.g. Acids like acetic acid formic acid , carbonic acid and phosphoric acid etc, are partially ionised in water and thus produce small amounts of (aq) ions in the solution, so these are called weak acids. The ionization of these acids are represented as follows:
acetic acid acetate ion hydrogen ion
carbonic acid hydrogen ion carbonate ion
phosphoric acid hydrogen ion phosphate ion
formic acid formate ion hydrogen ion
The double arrow pointing towards ions indicates partial ionization of acids.
Characteristics of weak acids:
Due to small amounts of ions in the solutions of weak acids,
(i) They react quite slowly with other substances (such as metals, metal carbonates and metal bicarbonates etc).
(ii) They have low electrical conductivity.
(iii) They are weak electrolytes.
Greater the degree of ionization, greater is the amount of ions produced in solution and stronger is the acid (or greater is strength of acid).
Smaller the degree of ionization, smaller is the amount of ions produced in the solution and weaker is the acid (or smaller is the strength of acid).
Thus,STRENGTH OF AN ACID DEGREE OF IONIZATION
2.6 DILUTION OF CONCENTRATED ACIDS–AN EXOTHERMIC REACTION
2.6.1 Concentrated acid
Pure acid is generally known as the concentrated acid.
2.6.2 Dilute acid
A concentrated acid mixed with water is called a dilute acid and this process of mixing of water to a concentrated acid is called dilution.
The dilution of an acid with water is an exothermic reaction i.e. on mixing water to an acid, heat is produced.
Experiment to verify that dilution of a concentrated acid is exothermic
Take a small amount of water in a beaker. Note its temperature. Now put a few drops of conc. or conc. acid into it and note the temperature of beaker again.
There is rise in temperature in each case.
Thus dilution of conc. acid is an exothermic reaction and is accompanied by ionization of acid as follows:
sulphuric acid(conc)water hydronium ion sulphate ion released
nitric acid (conc.) water hydronium ion nitrate ion released
From the above experiment we lead to a conclusion that dilution of concentrated acid in water is an exothermic (or heat releasing) reaction.
2.6.6 Usefulness of certain acids
(i) Hydrochloric acid (HCl) produced in the stomach kill the harmful bacteria that may enter into the stomach along with the food we eat.
(ii) Vinegar (acetic acid) is used in the pickling of food as a method of preservation of food.
Those substances which change red litmus solution blue are called bases. They are bitter in taste.
3.1 PHYSICAL PROPERTIES OF BASES
(i) Bitter taste: Almost all basic substances have a bitter taste.
(ii) Action on litmus solution: Bases turn red litmus solution into blue.
(iii) Action on methyl orange: Bases turn methyl orange into yellow.
(iv) Action on phenolphthalein:Bases turn phenolphthalein into pink.
(v) Conduction of electricity: Like acid, the aqueous solution of a base also conducts the electricity.
3.2 CHEMICAL PROPERTIES OF BASES
3.2.1 Reaction of bases with metals
When a base reacts with a metal then a metal salt and (g) are formed
i.e. Metal + Base Salt + H2gas
Reaction of Sodium hydroxide (NaOH) with zinc metal (Zn)
Experiment: Take 2-3 pieces of zinc granules in a test tube and add about 2-3 ml of conc. NaOH solution in to it and warm the contents.
Observation:There is evolution of gas which burns with a pop sound (on bringing a burning candle near the mouth of tube).
The reaction involved is:
zinc sodium hydroxide sodium zincate hydrogen
(a metal) (conc.) (a salt) gas
Figure-study of the reaction of sodium hydroxide with Zn metal
All metals do not react with bases to form salts and hydrogen gas.
3.2.2 Reaction of Bases with Acids
When a base reacts with an acid then salt and water are formed
This reaction is called neutralization reaction, because when base and acid react with each other, they neutralize each others effect (i.e. acid destroys the basic property of a base and a base destroys the acidic property of an acid)
sodium hydroxide hydrochloric acid sodium chloride water
(base) (acid) (salt)
sodium hydroxide sulphuric acid sodium sulphate water
(base) (acid) (salt)
These reactions of bases and acids to form salt and water are called neutralization reactions or neutralization of acids and bases. (As discussed in detail in neutralization reaction of acids and bases in the topic on Acids)
Conclusion: Reaction of a base with an acid is a neutralization of an acid by base i.e. in the reaction of base with acid, base neutralize or destroy the acidic properties of an acid.
3.2.3 Reaction of base with non-metal oxide
Bases react with non-metal oxide to form salt and water
i.e. Non metal oxide + Base Salt + water
This reaction is similar to the neutralization reaction between acid and base to form salt and water. Thus, the reaction between bases and metal oxides is a kind of neutralization reaction and shows that non-metal oxides are acidic oxides.
Reaction of calcium hydroxide (lime water) with carbon dioxide.
Calcium hydroxide (lime water) is a base and carbon dioxide is a non-metal oxide, so when they react with each other, salt and water are produced according to the reaction:
calcium hydroxide carbondioxide calcium water
(lime water) (non-metal oxide) carbonate
Thus, it is an example of neutralization reaction (reaction of acid and base to form salt and water). It shows that non-metal oxides are acidic in nature.
Reaction of bases with non-metal oxides are neutralization reactions which show the acidic nature of non-metals.
3.3 CHEMICAL NATURE OF BASES
Like acids, bases also possess some general characteristic properties which show that chemically bases must have something in common.
It is found that when bases are dissolved in water, they produce ions (hydroxide ions) in the solution according to the reactions:
sodium hydroxide sodium ions hydroxide ion
potassiumhydroxide potassium ion hydroxide ion
ammonium hydroxide ammoniumion hydroxide ion
Thus, bases when dissolved in water produce ions in the solution. Such dissociation of bases into ions in presence of water is called ionization. These ions are responsible for the basic properties of the bases which they give only when dissolved in water.
Hence a base can be defined as:
A base is a substance which when dissolved in water gives hydroxide ions in the solution. This is called Arrhenius definition of bases which was put by Arrhenius in 1884.
The properties of base are due to ions which it gives in the aqueous solution
(or with water).
Basic properties of bases are due to presence of ions which they produce only in presence of water.
In absence of water, a substance will not form ions and hence will not show its basic behaviour.
Necessary and sufficient condition for a substance or compound to be a base is that it should give ions in presence of water (or when dissolved in water).
(i) In solution, like ions are represented by the ions are also represented by .
(ii) All bases contain group, but all group containing compound are not bases. For example (ethyl alcohol) contains group, but since, it does not ionize in water to give ions it is not a base.
(iii) Greater the amount of ions in the solution, stronger is the base.
3.4 CLASSIFICATION OF BASES ON THE BASIS OF DEGREE OF IONIZATION
The bases are classified into two categories on the basis of degree of ionization as follows:
(i) Strong bases
(ii) Weak bases
3.4.1 Strong Bases
A base which is completely ionized in water and thus produces a large amount of (aq) ions in the solution is called a strong base e.g. bases like sodium hydroxide (NaOH), potassium hydroxide are completely ionized in water and thus produce large amounts of ions in the solution. So these are called strong bases.
The ionization of these bases is represented as follows:
sodium hydroxide sodium ion hydroxide ion
potassium hydroxide potassium ion hydroxide ion
Single arrows pointing towards ions indicate complete ionization of bases.
3.4.2 Weak bases
A base which is partially ionized in water and thus produces a small amount of ions in the solution is called a weak base, e.g. bases like ammonium hydroxide , calcium hydroxide , magnesium hydroxide are weak bases as they ionize partially in water to produce small amounts of ions in the solution.
The ionization of these bases is represented as follows:
Ammonium hydroxide Ammonium ion hydroxide ion
calcium hydroxide calcium ion hydroxide ion
magnesium hydroxide magnesium ion hydroxide ion
The double arrow pointing towards ions indicate partial ionization of bases.
Greater the degree of ionization, greater is the amount of ions produced in the solution and thus stronger is the base (or greater is the strength of base).
Smaller the degree of ionization, smaller is the amount of ions produced in the solution and thus weaker is the base (or smaller is the strength of base). Thus,
STRENGTH OF BASE DEGREE OF IONIZATION.
3.5 DILUTION OF BASE: AN EXOTHERMIC REACTION
Like acids, dilution of bases with water or mixing of bases with water is an exothermic process e.g. if we dissolve bases like NaOH, KOH in water, the solution is found to be hotter. This shows that dissolution of bases in water is an exothermic process.
3.6 EFFECT OF DILUTION ON STRENGTH OF A BASE
Like acids, on dilution of base with water, in the solution decrease and thus, solution becomes less basic (or strength of base decrease)
Strength of an acid or base can be measured on pH scale.
pH scale: A scale of numbers from 0 to 14 on which the strength of an acid or base is measured is known as pH scale.
pH is defined as negative logarithm of
e.g. if then
It is clear from the above expression that pH of a solution is the magnitude of the negative power to which 10 must be raised to express the of the solution in mol L-1.
In other words, pH stands for power of hydrogen ions (p stands for power and H stands for hydrogen) e.g.
If then pH = 1
If then pH = 2
If then pH = 6 and so on
and If then and pH = 8
[ at 298K ]
similarly if then
and pH = 0
if [so that ] and pH = 14 and so on
pH values for acidic or basic or neutral solution in terms of can be expressed as follows:
4.1 FOR A NEUTRAL SOLUTION (OR WATER)
its pH = 7 (magnitude of negative power to which 10 must be raised to express )
4.2 FOR AN ACIDIC SOLUTION
its pH < 7 (i.e. 6, 5, 4, ….0)
4.3 FOR A BASIC SOLUTION
or (i.e. etc.)
or mol L1 (i.e. etc so that )
its pH > 7 (i.e. 8, 9, 10, ….14)
Hence, acidic or basic strength or neutral nature of solution may be expressed on the pH scale from 0 to 14 as follows:
Conclusion: From the above figure, we lead to a conclusion that
(i) for a neutral solution, pH = 7
(ii) for a basic solution, pH > 7
(iii) for an acidic solution, pH < 7
What will be the concentration of H+(aq) ions in the solution when pH = 0?
(i) The solution with pH = 0-3 are strongly acidic, with pH = 3-5 are moderately acidic while with pH = 5–7 are weakly acidic.
(ii) The solution with pH = 7-9 are weakly basic, with pH = 9-12 are moderately basic, while with pH = 12-14 are strongly basic.
The pH paper gives only the approximate value of pH of the solution.
4.5 ROLE OF pH IN EVERYDAY LIFE
The pH plays an important role in many activities of our daily life. Let us discuss in detail
4.5.1 pH in our digestive system
Our stomach produces hydrochloric acid (of pH 1.2) which helps in digesting food without harming the stomach.
However when we take too much of spicy food, the amount of hydrochloric acid produced increases beyond the required limit and thus causes sharp pain and irritation in the stomach due to indigestion of food. The problem is called acidity.
In order to cure indigestion and get rid of pain, we take bases called ‘antacids’ (antacid means anti-acid). Antacids being basic in nature react with excess acid in the stomach and neutralize it and thus gives relief to the person concerned.
The two common antacids used for curing indigestion due to acidity are magnesium hydroxide (milk of magnesia) and sodium hydrogen carbonate or sodium bicarbonate (baking soda).
4.5.2 pH change as the cause of tooth decay
When we eat food containing sugar, then the bacteria present in our mouth break down the sugar to form acids (such as lactic acid). This acid lowers the pH in the mouth (making it more acidic).
Tooth decay starts when pH of acid formed in the month falls below 5.56.
This is because when the acid becomes strong enough to attack the enamel of our teeth and corrode it. This sets in tooth decay. Though tooth enamel is made of calcium phosphate (which is hardest material in our body), but it starts getting corroded when the pH in mouth is lower than 5.5.
Prevention of tooth decay
(i) The best way to prevent tooth decay is to clean the mouth thoroughly after eating food (by rinsing it with lots of clean water).
(ii) Many toothpastes contain bases to neutralize the mouth acid (The pH of toothpaste being about 8.0). So using toothpastes (which are generally basic) for cleaning the teeth can neutralize the excess acid in mouth and prevent tooth decay.
4.5.3 Self defense by animals and plants through chemical warfare
We feel pain and irritation, when we are stung by an honeybee or yellow ant due to acid (formic acid i.e. methanoic acid) injected into our skin by bee or the ant.
To get relief, we apply the solution of mild base like baking soda, to neutralize the ant’s sting.
Similarly we feel pain and irritation, when we are stung by the nettle plants (having leaves with sting) due to formic acid injected by these leaves into our skin.
To get relief we rub the injected area with the leaf of the dock plant (which provides the base) neutralize the acidic sting of nettle plant.
Salts are the ionic compounds consisting of two parts, one carrying a positive charge (called positive ion or cation) and the other part carrying a negative charge (called a negative ion or anion).
For example is a salt having as cation and as anion.
5.1 FORMATION OF SALTS
The most common method of formation of salts is by the neutralization of acids and bases (i.e. reaction between an acid and a base). e.g.
sod. hydroxide hydrochloric acid sodiumchloride water
(base) (acid) (a salt)
magnesiumoxide sulphuricacid magnesium sulphatewater
(base) (acid) (salt)
(iii) + H2O
ammonium hydroxide hydrochloric acid ammonium. chloride water
(base) (acid) (salt)
It is clear from the above reactions that positive part of the salt comes from the base (called basic radical) and the negative part of the salt comes from the acid (called acidic radical). e.g.
salt, (comes from NaOH) is called the basic radical and (comes from HCl) is called the acidic radical.
5.3 HYDROLYSIS OF SALTS AND pH OF THEIR SALT SOLUTIONS
Salts formed by the reactions between acid & bases undergo ionization in water. The cations/anions formed by ionization of salts interact with water to form corresponding acids/ bases depending upon the nature of salt.
The process of interaction between water and cations/anions or both of salts is called salts hydrolysis.
5.4 COMMON SALT (SODIUM CHLORIDE, NaCl)
Chemically, common salt is sodium chloride with formula NaCl. It is also called “table salt” as it is used as an important food material. The table salt used as food material is not pure NaCl, but it contains small amounts of KI and thus table salt sold in market is labelled as “iodized salt”. The presence of iodide is essential to protect us from thyroid disorders (disorder of the throat).
5.4.1 Sodium hydroxide NaOH (caustic soda)
Sodium hydroxide is commonly known as caustic soda having chemical formula NaOH. It is a strong base.
(i) Manufacture of sodium hydroxide (NaOH)
Sodium hydroxide is manufactured by the electrolysis of a concentrated aqueous solution of sodium chloride (called brine) i.e., when the electricity is passed through a concentrated aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide, chlorine and hydrogen.
sodium chloride water sodium hydroxide chlorine hydrogen
(brine) (caustic soda)
This process is called chlor-alkali because of products formed: chlor for chlorine and alkali for sodium hydroxide.
During electrolysis, gas is produced at the anode (positive electrode), gas is produced at the cathode (negative electrode) and NaOH solution is produced near the cathode. These products i.e. obtained by chlor-alkali process have a large number of uses described below one by one.
(ii) Uses of sodium hydroxide (NaOH)
Sodium hydroxide (NaOH) is used
(a) For making soaps and detergents.
(b) For making artificial textile fibres.
(c) For making paper.
(d) In de-greasing metals.
(e) As reagent in laboratory.
(f) In absorbing poisonous gases.
(g) In petroleum industry.
(iii) Uses of chlorine (Cl2)
Chlorine is used
(a) As disinfectant and germicide for sterilization of drinking water and water in swimming pools.
(b) In manufacture of chlorofluorocarbons used as refrigerants.
(c) In manufacture of PVC (polyvinyl chloride) used for making shoe soles.
(d) In bleaching of wood pulp and cotton fibres.
(e) In manufacture of pesticides.
5.4.2 Bleaching powder (calcium oxychloride CaOCl2)
Bleaching powder is chemically calcium oxychloride with the formula .
(i) Manufacture of bleaching powder
The bleaching powder is manufactured by the action of chlorine gas (produced as a by product during manufacture of caustic soda) on dry slaked lime . The reactions involved are
Slaked lime chlorine bleaching powder water
(ii) Uses of bleaching powder
Bleaching powder is used
(a) In textile industry for bleaching cotton and linen.
(b) In paper industry for bleaching wood pulp.
(c) In laundry for bleaching washed clothes.
(d) Making wood unshrinkable.
(e) For disinfecting drinking water to make it free from germs.
(f) As an oxidizing agent in many chemical industries because in the presence of insufficient acid, it gives nascent oxygen.
bleaching powder sulphuricacid calciumsulphate calciumchloride hypochlorousacid
(g) In manufacture of chloroform .
Which by product of chlor-alkali process is used to manufacture bleaching powder?
5.4.3 Baking soda, (Sodium hydrogen carbonate NaHCO3)
Baking soda, is chemically sodium hydrogen carbonate with chemical formula .
(i) Manufacture of baking soda
(a) On large scale: Baking soda is produced on a large scale by reacting a cold and concentrated solution of sodium chloride (called brine) with ammonia and carbon dioxide.
sodium chloride water ammonia carbondioxide Baking sodaammonium chloride
This process is called Solvay’s process.
(b) On small scale: On a small scale baking soda can be prepared in the laboratory by passing gas through aqueous sodium carbonate solution.
sodium carbonate water carbon dioxide baking soda
(ii) Properties of Baking soda
(a) Colourand state
It is a white crystalline solid.
(b) Alkaline nature
It is mild, non-corrosive base. The alkaline nature of baking soda is due to salt hydrolysis.
baking soda water strong base weak acid
(sodium hydrogen carbonate)
Thus, salt solution is basic due to hydrolysis of ions
(c) Action of heat
When solid baking soda (or its solution) is heated it decomposes to give sodium carbonate with the evolution of gas.
baking soda soda carbonate water carbon dioxide gas
The above reaction takes places when baking soda is heated during the cooking of food. Since baking soda gives on heating, it is used as a constituent of baking powder.
(iii) Uses of baking soda (NaHCO3)
(a) As an antacid in medicine
Baking soda is used as an antacid in medicine to remove acidity of the stomach because of its alkaline or basic nature.
Being alkaline baking soda neutralises the excess acid present in the stomach and relieves indigestion.
(b) In making baking powder
Baking soda is used in making baking powder (used in making cakes, bread etc) as follows:
Baking powder is a mixture of baking soda and an acid like tartaric acid or citric acid. When baking powder mixes with water (present in dough made for baking cake or bread) the baking soda reacts with tartaric acid to evolve gas.
baking soda (from tartaric acid) (sodium ion)
The gas thus produced gets trapped in the wet dough and bubbles out slowly making the cake (or bread) to rise and becomes soft and strong. If however baking powder is not added in the preparation of cake (or bread), then the cake (or bread) obtained will be hard and quite small in size.
The function of adding tartaric acid or citric acid to baking soda in preparation of baking powder is to neutrilise the sodium carbonate (Na2CO3) formed during heating of baking soda(during baking), as otherwise the cake or bread will taste bitter (sodium carbonate is basic in nature and have bitter taste)
What are antacids?
(c) In fire extinguisher
The baking soda is used in soda acid fire extinguishers as follows:
Soda acid fire extinguisher contain a solution of baking soda and in separate container inside them. When the knob of fire extinguisher is pressed then acid mixes with baking soda solution to produces gas. The pressure of gas forces a stream of liquid to fall on burning substance. The gas (coming out along with liquid) surrounds the fire. As a result supply of air is cutoff, (because CO2 gas is heavier than air) and the fire extinguished.
Preparation of soda acid fire extinguisher
Take about 20 ml of a saturated solution of baking soda in a wash bottle. Take about 5 ml dilute acid in a small test tube and tie a thread on its neck and then suspend it in wash bottle. Now tilt the wash bottle so that acid in tube mixes into the baking soda solution. Immediately the reaction starts and gas starts coming out of the nozzle of the wash bottle with brisk effervescence.
Direct the nozzle towards burning candle. It is immediately extinguished because gas being heavier than air surrounds the fire and cuts off supply of air.
Making a soda-acid fire extinguisher
Which chemicals are used in soda-acid fire extinguisher?
5.4.4 Washing soda (sodium carbonate, Na2CO310H2O)
Washing soda is chemically sodium carbonate decahydrate with formula i.e. one mole of contains 10 moles of water of crystallization.
Anhydrous sodium carbonate Na2CO3 is called soda ash.
(i) Manufacture of washing soda
Washing soda is manufactured from sodium chloride (or common salt) in the following three steps:
Manufacture of sodium hydrogen carbonate (baking soda) by solvay process: A cold and concentrated solution of sodium chloride (brine) is reacted with ammonia and to obtain sodium hydrogen carbonate
sodium chloride water ammonia carbondioxide sodium bicarbonate ammonium chloride
Thermal decomposition of sodium hydrogen carbonate (or baking soda): on heating sodium hydrogen carbonate decomposes to form sodium carbonate.
sod. hydrogen sod. carbonate carbon dioxide water
Re-crystallization of sodium carbonate (or soda ash)
Anhydrous sodium carbonate (or soda ash) obtained in step 2 is dissolved in water and subjected to re-crystallization. As a result, crystal of washing soda (sodium carbonate decahydrate) is obtained.
soda ash water washing soda
(ii) Properties of Washing Soda
(a) Colour and state: It is a transparent crystalline solid (when freshly prepared) c containing 10 molecules of water of crystallisation.
(b) Action of air: On exposure to air, washing soda crystals lose 9 molecules of water of crystallisation to form a monohydrate which is a white powder
This process is called efflorescence
(c) Action of heat: On heating, washing soda loses all the molecule of water and becomes anhydrous.
washing soda Anhydrous sodium water
carbonate (soda ash)
(iii) Uses of washing soda (sodium carbonate)
Washing soda is used
(a) In laundry for cleaning of clothes.
(b) For removing permanent hardness of water.
(c) In manufacture of glass, soap, paper, borax caustic soda etc.
(d) In textile and petroleum refining.
(e) As cleaning agent for domestic purposes.
(f) As laboratory reagent.
5.5 ARE THE CRYSTALLINE SALTS REALLY DRY
The crystalline salts or crystals of salts appear to be dry but actually they are not. They contain water of crystallization. It can be explained as follows:
Water of crystallization: The fixed number of water molecules present in one formula unit of salt is called water of crystallization.
(i) Sodium carbonate crystals (washing soda crystals) contains 10 molecules of water of crystallization in one formula unit and hence written as .
(ii) Calcium sulphate crystals (gypsum crystals) contain 2 molecules of water of crystallization in one formula unit and hence written as .
(iii) Copper sulphate crystals contain 5 molecules of water of crystallization in one formula unit and hence written as .
It is clear from the above examples that water of crystallization is not free water, so it does not wet the salts. Thus the crystalline salts which seem to be dry contain water of crystallization. It can be explained more clearly by following experiment.
Experiment to test the presence of water of crystallization in a crystalline salt.
Take a few crystals of copper sulphate in a dry test tube. Copper sulphate crystals are blue in colour. Heat the test tube.
(i) Blue copper sulphate crystals turn white.
(ii) Water vapours appear on the upper parts inside the test tube.
Figure-experiment to test the presence of water of crystallisation in copper sulphate crystals
Explanation: The blue copper sulphate crystals on heating give out water vapour which condensed on the upper parts of the test tube and the salt left behind was anhydrous copper sulphate which was white in colour.
Copper sulphate copper sulphate
crystals (white, anhydrous)
Crystalline salt contain water of crystallization which are lost on heating.
5.5.1 Types of salts on the basis of water of crystallization
Salts are classified into two types on the basis of water of crystallization
(i)Anhydrous saltswhich contain no water ofcrystallization e.g. .
The salts which contain a fixed number of water molecules of crystalisation. A few examples of these salts are
(a) Copper sulphate .
(b) Washing soda .
(c) Gypsum .
(d) Plaster of paris(CaSO4½H2O).
Out of these plaster of paris is very useful salt which is discussed below:
5.5.2 Plaster of Paris (CaSO4½H2O) or P.O.P.
Chemically, plaster of Paris is calcium sulphate hemihydrate with formula It is obtained from gypsum which is mainly found in Paris.
(i) Manufacture: Plaster of Paris is obtained by heating gypsum at
373 K in a kiln.
gypsum plaster of paris water
(ii) Properties of plaster of Paris (POP)
(a) Colourand state: It is a white powder.
(b) Reaction with water: (setting of plaster of Paris (or POP).
When POP is mixed with water and left for half an hour to one hour, it sets to a hard mass due to rehydration of POP to gypsum.
P.O.P. water gypsum
(c) Effect of heat: When POP is heated at 473K, it forms anhydrous calcium sulphate which is known as dead burnt plaster. It has no setting property as it takes up water very slowly.
(P.O.P.) dead burnt plaster
(iii) Use of plaster of Paris (POP)
(a) For setting fractured bones in right position and in making casts in dentistry.
(b) In making blackboard chalks.
(c) For making smooth surface and ornate designs on walls and ceilings.
(d) In laboratories for sealing air gaps in apparatus so as to make it airtight.
An acid may be defined as a substance which releases one or more H+ ions in aqueous solution.
Antacid is called as ‘Milk of Magnesia’
Metal carbonate and metal hydrogen carbonates are also considered to be bases because they neutrilize the acids.
A base which is soluble in water is called an alkali. E.g. Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide [Ca(OH)2].
Basicity of an acid does not depend upon the number of the H–atoms present but upon number of replaceable H–atoms. For example CH3COOH has four H–atoms.
Acidity of a base may be defined as: “The number of replaceable hydroxyl groups present in a base which it can release as ions when dissolved in water or in aqueous solution.”
Salts are formed when acids react with bases.
Neutral salt solutions: Salt solutions of strong acids and strong bases
Acidic salts: An acidic salt still contains some replaceable hydrogen atoms.
Basic salts: A basic salt still contains some replaceable hydroxyl groups.
A neutral salt solution has pH = 7
An acidic salt solution has pH < 7
A basic salt solution has pH > 7
Na2CO3 is also called soda ash. It is locally also known as Reh and Sajji.
Efflorescence: Certain hydrate crystalline salts when exposed to the atmosphere at ordinary temperature, lose their water of crystalline molecule and become anhydrous. These are known as efflorescent substances/salts and this property is known as efflorescence.
The plaster of paris made of gypsum which is mainly found in paris. So it is called plaster of paris. It’s chemical formula is CaSO4 H2O. It is calcium sulphate hemihydrate.
It is very soft mineral composed of calcium sulphatedihydrate with the chemical formula CaSO4•2H2O
The Solvay process, also referred to as the ammonia–soda process, is the major industrial process for the production of soda ash (sodium carbonate).