Class 10 Science Acid Bases and Salts Exam Notes

Class 10 Science Acid Bases and Salts Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Intorduction

These Class 10 Science exam notes on Acids, Bases and Salts cover the meaning and properties of acids and bases, the classification of acids, the preparation and uses of acids and bases, neutralisation, indicators, the nature and types of salts, their preparation and properties, the pH scale, and the role of pH in everyday life.

INTRODUCTION

A chemical substance is a kind of matter that cannot be separated into any other kind of matter by a physical method. Pure substances have uniform composition and always have some properties like taste, smell, texture, etc.

We use several materials in our daily life for various purposes. Some of them are sweet in taste, some are salty, some are sour and some are bitter.

The sweet taste of a substance is due to the sugar present in it. Certain food materials are sour in taste, like curd, vinegar, tamarind, lemon juice, green mango, tomatoes, orange and unripened grapes. Others are sweet, whereas some are salty and some are bitter. What substances are present in them that make them sour, bitter or salty?

ACIDS

The term 'acid' has its origin in the Latin word acidus, meaning sour. In fact, anything that tastes sour contains an acid. For example, lemon juice, tomato, vinegar, etc., all taste sour, so each of these substances must contain an acid.

Some naturally occurring substances and the acids present in them:

Aqueous solutions of acids are generally sour in taste. Acids turn blue litmus red, conduct electricity and react with bases to form salts and water. (Bases and salts are discussed later.)

An acid may be defined in various ways. Here, we study the definition given by Liebig in 1838. According to Liebig, an acid is a compound which contains hydrogen that can be replaced partially or wholly by a metal, or by a group of elements acting like a metal, to produce a salt.

For example, sulphuric acid (H₂SO₄) is an acid for the following reasons:
(i) It contains hydrogen atoms in its molecule.
(ii) The two hydrogen atoms present in its molecule can be replaced partially or wholly by a metal like sodium (Na) to produce sodium hydrogensulphate or sodium sulphate.
H₂SO₄ + Na → NaHSO₄ (partial replacement, sodium hydrogensulphate)
H₂SO₄ + 2Na → Na₂SO₄ (complete replacement, sodium sulphate)

The hydrogen atoms in H₂SO₄ can also be partially or wholly replaced by a group of elements, like the ammonium ion (NH₄⁺), to form ammonium hydrogensulphate (NH₄HSO₄) or ammonium sulphate ((NH₄)₂SO₄) respectively.

The substances NaHSO₄, Na₂SO₄, NH₄HSO₄ and (NH₄)₂SO₄ are all salts.
(iii) The acid dissolves in water to make a solution that turns blue litmus red.
(iv) It is sour in taste.
(v) It reacts vigorously with a base to produce a salt.

The hydrogen atoms present in an acid that can be replaced by a metal or a group of elements are called replaceable hydrogen or acidic hydrogen.

Classification of acids

Depending upon the elements present, acids may be classified as follows:
(i) Oxyacids: Acids that contain both hydrogen and oxygen are called oxyacids. For example, nitric acid (HNO₃), sulphuric acid (H₂SO₄) and phosphoric acid (H₃PO₄) are oxyacids.
(ii) Hydracids: Acids that contain hydrogen and other non-metallic element(s), except oxygen, are called hydracids. For example, hydrochloric acid (HCl) and hydrocyanic acid (HCN) are hydracids.

Acids may also be classified as follows:
1. Organic and inorganic acids: All the sour things that we use in our daily food contain acids. These are organic acids. Some common acids used in the laboratory are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃). These are inorganic acids, also called mineral acids. Hydrochloric acid is also present in the gastric juice in our stomach.
2. Concentrated and dilute acids: An acid solution may be concentrated or dilute depending upon the amount of acid present in the solution. When the solution contains a larger amount of the acid, it is said to be concentrated, whereas a dilute solution contains a smaller amount of the acid. Thus, concentrated and dilute solutions of an acid differ only in the proportions of the acid and water in them.
3. Strong and weak acids: The strength of an acid is determined by the amount of hydrogen ions (H⁺) that the acid provides when dissolved in water. Acids which get almost completely dissociated in water to provide hydrogen ions are called strong acids, for example hydrochloric acid (HCl), nitric acid (HNO₃) and sulphuric acid (H₂SO₄). Acids which get only incompletely dissociated in water are called weak acids, for example carbonic acid (H₂CO₃) and acetic acid (CH₃COOH).

Basicity of an acid: The basicity of an acid is the number of replaceable hydrogen atoms present in a molecule of the acid. An acid with one replaceable hydrogen atom is called a monobasic acid (basicity 1). Acids with two or three replaceable hydrogen atoms are called dibasic or tribasic acids (basicity 2 or 3).

Preparation of acids

1. Synthetic method: In this method, acids are prepared by the direct combination of elements. For example, hydrogen and chlorine react under the action of an electric spark to produce hydrogen chloride gas, which is absorbed in water to give hydrochloric acid.
H₂ + Cl₂ → 2HCl (electric spark)

Similarly, sulphuric acid may be obtained from its elements as follows:
S + O₂ → SO₂ (sulphur dioxide)
2SO₂ + O₂ → 2SO₃ (sulphur trioxide)
SO₃ + H₂O → H₂SO₄ (sulphuric acid)

2. By dissolving acidic oxides in water: Some oxides dissolve in water to give acids. These oxides are called acidic oxides. For example, sulphur trioxide (SO₃) dissolves in water to give H₂SO₄.
SO₃ + H₂O → H₂SO₄
Similarly, carbon dioxide (CO₂) dissolves in water to produce carbonic acid (H₂CO₃).
CO₂ + H₂O → H₂CO₃

3. By the action of an acid on the salt of another acid: An acid having a higher boiling point can react with the salt of an acid of lower boiling point to produce an acid. For example, NaCl is a salt of HCl. The boiling point of HCl is lower than that of H₂SO₄. When NaCl reacts with H₂SO₄, HCl is formed.
H₂SO₄ + NaCl → NaHSO₄ + HCl

General properties of acids

1. They are sour in taste.
2. They turn blue litmus paper red.
3. Acids show acidic properties only in the presence of water. Dry hydrogen chloride gas does not produce H⁺ ions in the absence of moisture or water. It produces H⁺ ions only in the presence of moisture or water.
HCl + H₂O → H₃O⁺ + Cl^-
4. Their aqueous solutions conduct electricity.
5. They react with certain metals with the evolution of hydrogen gas.

Examples: Metals like potassium, sodium, calcium, magnesium, aluminium, zinc and iron react with the aqueous solution of an acid to evolve hydrogen gas.
2Na + 2HCl → 2NaCl + H₂ (sodium chloride)
Fe + 2HCl → FeCl₂ + H₂ (ferrous chloride)
Mg + H₂SO₄ → MgSO₄ + H₂ (magnesium sulphate)
2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂ (aluminium sulphate)

These reactions show that certain metals can displace hydrogen from acids to form salts.

Nitric acid reacts only with magnesium and manganese. In both reactions hydrogen gas is evolved.
Mg + 2HNO₃ → Mg(NO₃)₂ + H₂ (magnesium nitrate)
Mn + 2HNO₃ → Mn(NO₃)₂ + H₂ (manganese(II) nitrate)
Nitric acid does not behave like this with any other metal.

6. They react with bases to produce salts and water.

7. They react with carbonates and hydrogencarbonates to form carbon dioxide and a salt.
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ (sodium carbonate, sodium chloride, water, carbon dioxide)
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ (calcium carbonate, calcium chloride)
NaHCO₃ + HCl → NaCl + H₂O + CO₂ (sodium hydrogencarbonate)
2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂

This reaction may be used as a test to determine whether a given substance is an acid or not.

Fire extinguisher: The reaction between sulphuric acid and sodium carbonate or sodium hydrogencarbonate is used in making a fire extinguisher. A sealed glass bottle filled with dilute sulphuric acid is kept inside a container filled with an aqueous solution of sodium carbonate. In case of fire, the plunger is struck against a hard surface to break the bottle. As a result, sulphuric acid comes in contact with the sodium carbonate. The carbon dioxide gas which comes out is directed towards the fire.

[Figure: Fire extinguisher with a plunger at the top, a nozzle at the side, a glass bottle of H₂SO₄ inside, and sodium carbonate solution in the container.]

7. Acids react with the oxides of metals to form salts and water.
Na₂O + 2HCl → 2NaCl + H₂O (sodium oxide)
CuO + H₂SO₄ → CuSO₄ + H₂O (cupric oxide, cupric sulphate)
CaO + 2HCl → CaCl₂ + H₂O (calcium oxide, calcium chloride)

All acids contain hydrogen

All acids have similar chemical properties, which indicates that all acids must have something in common. All acids react with metals to produce hydrogen gas, so hydrogen is the common substance present in all acids. But all compounds containing hydrogen are not acids. For example, hydrochloric acid and sulphuric acid contain hydrogen and are acids, whereas alcohol and glucose also contain hydrogen but they are not acids.

The acidic properties of acids are due to the fact that they produce H⁺ ions in aqueous solution.

Uses of acids

BASES

Bases are substances that feel soapy to touch and are bitter in taste. A base is a substance, usually the oxide or the hydroxide of a metal, which can react with an acid to produce a salt and water.

For example, sodium oxide (Na₂O), calcium oxide (CaO), cupric oxide (CuO), iron oxides (FeO, Fe₂O₃, etc.), sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)₂) are all bases.

Certain substances are also called bases, though they do not fit the above definition. For example, ammonia (NH₃) forms a salt with an acid without giving water, so it should not be treated as a base. But ammonium hydroxide (NH₄OH), the aqueous solution of NH₃, is a base because it reacts with an acid to give a salt and water.
NH₄OH + HCl → NH₄Cl + H₂O

Alkalis

Bases that are soluble in water are called alkalis. For example, sodium hydroxide, potassium hydroxide and calcium hydroxide are soluble in water, so they are alkalis. But bases like copper hydroxide (Cu(OH)₂), ferric hydroxide (Fe(OH)₃) and aluminium hydroxide (Al(OH)₃) do not dissolve in water, so they are not alkalis.

Hence, all alkalis are bases, but all bases are not alkalis.

Lime water, baking soda and washing soda are also considered bases.

Preparation of bases

Bases can be prepared by the following methods.
1. By the direct union of a metal with oxygen: Some metals, when heated in air or oxygen, form the oxides of the metals.
4Na + O₂ → 2Na₂O (sodium oxide)
4K + O₂ → 2K₂O (potassium oxide)
2Ca + O₂ → 2CaO (calcium oxide)
These oxides, when dissolved in water, make the hydroxides of the metals.
Na₂O + H₂O → 2NaOH
K₂O + H₂O → 2KOH
CaO + H₂O → Ca(OH)₂

2. By the action of water or steam on some active metals: Some active metals like sodium and potassium react with cold water to form hydroxides with the evolution of hydrogen gas.
2Na + 2H₂O → 2NaOH + H₂
Magnesium reacts with steam to form magnesium oxide with the evolution of hydrogen gas.
Mg + H₂O (steam) → MgO + H₂
On passing superheated steam over red-hot iron, ferrosoferric oxide is formed and hydrogen gas is evolved.
3Fe + 4H₂O (superheated steam, red-hot iron) → Fe₃O₄ + 4H₂

3. By heating carbonates of some metals: When calcium carbonate is heated, calcium oxide and carbon dioxide are formed.
CaCO₃ → CaO + CO₂
Similarly, when zinc carbonate is heated, zinc oxide and carbon dioxide are formed.
ZnCO₃ → ZnO + CO₂

4. By the action of an alkali on a salt solution: For example, when an aqueous solution of sodium hydroxide is added to an aqueous solution of magnesium sulphate, magnesium hydroxide is precipitated and sodium sulphate remains in the solution.
MgSO₄ + 2NaOH → Mg(OH)₂ (precipitate) + Na₂SO₄

General properties of bases

1. The solutions of bases in water feel soapy to touch. When dissolved in water, they produce hydroxide ions (OH^-) in solution.
NaOH (in H₂O) → Na⁺ + OH^-
Ca(OH)₂ (in H₂O) → Ca²⁺ + 2OH^-
Mg(OH)₂ (in H₂O) → Mg²⁺ + 2OH^-

2. They turn red litmus paper blue. Dip the tip of a red litmus paper into a soap solution; the red litmus paper turns blue, which indicates that the soap solution contains a base.

3. They react with acids to produce salt and water.
NaOH + HCl → NaCl + H₂O
2KOH + H₂SO₄ → K₂SO₄ + 2H₂O
Cu(OH)₂ + H₂SO₄ → CuSO₄ + 2H₂O

In these reactions the acid and the base neutralise each other, so they are called neutralisation reactions. A neutralisation reaction is a reaction between an acid and a base, producing salt and water.

This can be explained as follows. All acids provide H⁺ ions and all bases provide OH^- ions in aqueous solution. When HCl and NaOH react together:
HCl + NaOH → NaCl + H₂O
H⁺ + Cl^- + Na⁺ + OH^- → Na⁺ + Cl^- + H₂O
H⁺ + OH^- → H₂O

Thus, during neutralisation of an acid with a base, or vice versa, H⁺ ions (from the acid) and OH^- ions (from the base) combine to produce H₂O molecules.

4. The oxides which produce acids in aqueous solutions are called acidic oxides. They are usually the oxides of non-metals. Acidic oxides react with bases to give salts and water.
2NaOH + CO₂ → Na₂CO₃ + H₂O (sodium carbonate)
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (calcium carbonate)

5. When a base is heated with an ammonium salt, ammonia gas, another salt and water are produced. For example, when sodium hydroxide is heated with ammonium chloride, the products are sodium chloride, water and ammonia gas.
NaOH + NH₄Cl → NaCl + H₂O + NH₃
Ammonia gas is recognised by its pungent smell.

6. Bases react with certain salts to produce another salt and another base. For example, when NH₄OH is added to a solution of Al₂(SO₄)₃, (NH₄)₂SO₄ and Al(OH)₃ are produced.
6NH₄OH + Al₂(SO₄)₃ → 2Al(OH)₃ + 3(NH₄)₂SO₄

Strong bases and weak bases

The strength of a base is determined by the amount of hydroxide ions (OH^-) that the base provides when dissolved in water. Bases which get almost completely dissociated in water are called strong bases, for example sodium hydroxide and potassium hydroxide. (Bases soluble in water are also called alkalis.) Bases which get only partially dissociated in water are called weak bases, for example magnesium hydroxide and ammonium hydroxide.

Acidity of a base: The acidity of a base is the number of hydroxyl (OH) groups present in a molecule of the base. NaOH, KOH and NH₄OH each have one hydroxyl group, so their acidity is 1. Ca(OH)₂ and Ba(OH)₂ have two hydroxyl groups, so their acidity is 2. Fe(OH)₃ and Al(OH)₃ have three hydroxyl groups, so their acidity is 3.

A base with one hydroxyl group is called a monoacidic base, one with two is a diacidic base and one with three is a triacidic base. Thus, NaOH, Ca(OH)₂ and Fe(OH)₃ are monoacidic, diacidic and triacidic bases respectively.

Common bases and their uses

INDICATORS

An acid-base indicator is a substance that shows different colours in acidic, basic and neutral solutions. Litmus, methyl orange and phenolphthalein are some of the most commonly used acid-base indicators.

Litmus: It is a natural dye made from small plants called lichens. Blue and red litmus solutions are prepared from two different varieties of lichens.

Litmus paper: Blue or red litmus paper is prepared by dipping a strip of filter paper in blue or red litmus solution, then removing and drying it. Blue litmus paper turns red in an acidic solution, and red litmus paper turns blue in a basic solution.

Phenolphthalein: It is a colourless compound. An alcoholic solution of phenolphthalein is used as an indicator. It is colourless in an acidic solution, but becomes pink (red) in a basic solution.

Methyl orange: A very small amount of solid methyl orange is dissolved in hot water and filtered. The filtrate is used as an indicator. It turns red in acid solutions and yellow in basic solutions.

Household indicators

(i) Turmeric juice is yellow in colour. It remains yellow in acidic or neutral solutions but turns deep brown in a basic solution.
(ii) Red-cabbage juice is purple in colour. It turns red in an acidic solution, but green in a basic solution.

Household indicators can be used to test whether some substances of daily use are acidic or basic.

Olfactory indicators: Substances like onion juice, vanilla essence and clove oil change their smell to indicate whether a sample solution is acidic or basic. These are called olfactory indicators.

SALTS

In neutralisation reactions, an acid and a base react to produce salt and water. For example, the neutralisation between NaOH and HCl gives the salt NaCl and water.
NaOH + HCl → NaCl + H₂O

A salt is a compound formed by the reaction of an acid with a base, in which the hydrogen of the acid is replaced by a metal.

In polybasic acids, more than one hydrogen atom is present in a molecule. These hydrogen atoms can be replaced partially or completely, so two kinds of salts are possible.
(i) H₂SO₄ + NaOH → NaHSO₄ + H₂O (partial replacement, sodium hydrogensulphate)
(ii) H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O (complete replacement, sodium sulphate)

NaHSO₄ and Na₂SO₄ represent two kinds of salts.

Types of salts

The different types of salts are: normal salt, acid salt, basic salt and double salt.

1. Normal salt: A salt that does not contain any replaceable hydrogen atoms or hydroxyl groups is called a normal salt. Na₂SO₄, obtained from H₂SO₄ and NaOH, is a normal salt because both hydrogen atoms of H₂SO₄ are completely replaced. Similarly, calcium sulphate (CaSO₄), sodium phosphate (Na₃PO₄) and potassium phosphate (K₃PO₄) are normal salts.

2. Acid salt: When a polybasic acid is not completely neutralised by a base, the salt produced contains replaceable hydrogen atoms. Such a salt can react further with a base as an acid, and is called an acid salt. For example, NaHSO₄ is an acid salt because it reacts further with NaOH to produce the normal salt Na₂SO₄.
H₂SO₄ + NaOH → NaHSO₄ + H₂O
NaHSO₄ + NaOH → Na₂SO₄ + H₂O
A salt that contains replaceable hydrogen atoms is called an acid salt. Examples: NaHSO₄, NaH₂PO₄ and Na₂HPO₄.

3. Basic salt: When a polyacidic base reacts with a lesser amount of acid than is necessary for complete neutralisation, the salt produced contains hydroxyl group(s) as well. Such a salt is called a basic salt.

Example: One mole of Pb(OH)₂ requires 2 moles of HCl for complete neutralisation. But when 1 mole of Pb(OH)₂ reacts with 1 mole of HCl, some Pb(OH)₂ is left unreacted, and the salt produced is not PbCl₂ but Pb(OH)Cl (lead oxychloride).
Pb(OH)₂ + HCl → Pb(OH)Cl + H₂O
Similarly, when one mole of Bi(OH)₃ reacts with one mole of HNO₃, the salt Bi(OH)₂NO₃ is formed.
Bi(OH)₃ + HNO₃ → Bi(OH)₂NO₃ + H₂O

Salts like Pb(OH)Cl and Bi(OH)₂NO₃ contain the OH group and are called basic salts, because they can further react with acids to form water and the corresponding normal salts.
Pb(OH)Cl + HCl → PbCl₂ + H₂O
Bi(OH)₂NO₃ + HNO₃ → Bi(OH)(NO₃)₂ + H₂O
Bi(OH)(NO₃)₂ + HNO₃ → Bi(NO₃)₃ + H₂O

4. Double salt: In a double salt there are two different negative ions and/or two different positive ions. For example, the mineral dolomite, CaCO₃·MgCO₃, contains both Ca²⁺ and Mg²⁺ ions, so it is a double salt. Potash alum, K₂SO₄·Al₂(SO₄)₃·24H₂O, is also a double salt.

Double salts exist only in the solid state. When dissolved in water, they break up into a mixture of two separate salts. For example, when potash alum is dissolved in water, it breaks up as follows:
K₂SO₄ → 2K⁺ + SO₄^2-
Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄^2-

Preparation of salts

1. By the reaction between a metal and an acid: Certain metals (for example, Zn and Mg) react with HCl or H₂SO₄ to form a salt and hydrogen.
Zn + 2HCl → ZnCl₂ + H₂
Zn + H₂SO₄ → ZnSO₄ + H₂

2. By the reaction between an acid and a base: All acid-base (neutralisation) reactions produce salts.
NaOH + HCl → NaCl + H₂O
CuO + 2HCl → CuCl₂ + H₂O

3. By direct union of a metal and a non-metal: Sodium and chlorine combine directly to form sodium chloride.
2Na + Cl₂ → 2NaCl
Similarly, when sulphur is heated with iron filings, ferrous sulphide (FeS) is formed.

4. By the union of an acidic oxide and a basic oxide:
CO₂ + CaO → CaCO₃ (calcium carbonate)
SO₃ + Na₂O → Na₂SO₄ (sodium sulphate)

5. By the reaction between a metal and a base: When zinc is heated with an aqueous solution of NaOH, sodium zincate (a salt) is formed with the evolution of hydrogen gas.
Zn + 2NaOH → Na₂ZnO₂ + H₂ (sodium zincate)

General properties of salts

1. Reaction with an acid: When a salt reacts with an acid, another salt and another acid are formed. For example, when sodium chloride is heated with sulphuric acid, sodium hydrogensulphate is formed at low temperature and sodium sulphate at high temperature, and hydrogen chloride gas is evolved.
NaCl + H₂SO₄ → NaHSO₄ + HCl (at low temperature)
2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl (at high temperature)

2. Reaction with a base: A salt reacts with a base to produce another salt and another base.
(NH₄)₂SO₄ + 2NaOH → Na₂SO₄ + 2NH₄OH

3. Reaction with a metal: A salt solution may react with a metal. For example, when an iron nail is dipped into an aqueous solution of copper sulphate, copper gets deposited on the surface of the nail and ferrous sulphate remains in the solution.
CuSO₄ + Fe → FeSO₄ + Cu
This reaction shows that iron is more reactive than copper. Thus, a more reactive metal can displace a less reactive metal from a solution of its salt.

4. Behaviour of salts towards water: When a salt is dissolved in water, the solution may be neutral, acidic or alkaline, depending upon the nature of the salt.
(i) A normal salt derived from a strong acid and a strong base gives a neutral solution. For example, aqueous solutions of NaCl and K₂SO₄ are neutral to litmus.
(ii) A normal salt derived from a weak acid and a strong base gives an alkaline solution. For example, aqueous solutions of sodium carbonate (Na₂CO₃) and sodium acetate (CH₃COONa) are alkaline.
Na₂CO₃ + H₂O → NaHCO₃ + NaOH
CH₃COONa + H₂O → CH₃COOH + NaOH
(iii) A salt derived from a strong acid and a weak base gives an acidic solution. For example, aluminium chloride (AlCl₃) and ammonium chloride (NH₄Cl) give acidic aqueous solutions.
AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl
NH₄Cl + H₂O → NH₄OH + HCl
(iv) Solutions of acid salts are acidic to litmus, that is, they turn blue litmus paper red. For example, a solution of sodium hydrogensulphate (NaHSO₄) turns blue litmus paper red. Sodium hydrogencarbonate (NaHCO₃) solution, however, is slightly alkaline.

Uses of salts

THE pH SCALE

A litmus solution or litmus paper can be used to determine whether a given solution is acidic or basic. But suppose you have two acidic solutions containing different amounts of acids. How can you say which solution is more acidic? The same problem arises with basic solutions. This problem is solved by using a scale known as the pH scale, introduced by S. P. Sorensen in 1909. The pH of a solution indicates which solution is more acidic or more basic than the other.

The acidity or basicity (alkalinity) of a solution is usually expressed in terms of a function of the H⁺ ion concentration. This function is called the pH of the solution.

The pH of an aqueous solution is the negative logarithm of its H⁺ ion concentration:
pH = -log [H⁺]
pOH = -log [OH^-]
Here [H⁺] and [OH^-] denote the concentrations of H⁺ and OH^- ions respectively.

The concentrations of H⁺ and OH^- ions in aqueous solutions are usually very small numbers and are difficult to work with. Since pH is the negative logarithm of [H⁺], we get positive numbers and the inconvenience of dealing with small numbers is eliminated.

pH is only a number, because we take the logarithm of a number and not of a unit. Therefore, the pH of a solution is a dimensionless quantity.

In a neutral solution, [H⁺] = 1.0 × 10^-7 M.
pH = -log (1.0 × 10^-7) = 7.
So the pH of a neutral solution is 7.

In an acidic solution, [H⁺] > 1.0 × 10^-7 M. Let us assume [H⁺] = 1.0 × 10^-5 M.
pH = -log (1.0 × 10^-5) = 5.
So the pH of an acidic solution is less than 7.

In an alkaline solution, [H⁺] < 1.0 × 10^-7 M. Let us assume [H⁺] = 1.0 × 10^-9 M.
pH = -log (1.0 × 10^-9) = 9.
So the pH of an alkaline solution is more than 7.

The pH values of different solutions at 298 K can be shown on the pH scale:

[Figure: pH scale running from 1 to 14. Values 1 to 6 are marked as acidic solution, 7 as neutral solution, and 8 to 14 as alkaline solution.]

Rules for the pH scale (at 298 K)

1. Acidic solutions have pH less than 7.
2. The lower the pH, the more acidic the solution.
3. Neutral solutions, or pure water, have pH equal to 7.
4. Basic solutions have pH greater than 7.
5. The higher the pH, the more basic the solution.

pH values of some common solutions

How is pH measured? The pH of a solution is generally determined with the help of pH paper or universal indicator. The pH paper gives a particular colour with a solution of a particular pH. The colour is compared with a chart which has different colours at different pH values.

Role of pH in everyday life

1. In our digestive system: Hydrochloric acid produced in our stomach helps the digestion of food without harming the stomach. But when the amount of acid goes beyond a certain limit due to indigestion, pain and irritation are created in the stomach. To neutralise the effect of the excess acid, a mild base called an antacid is taken. Magnesium hydroxide (milk of magnesia) is a mild base that is usually used as an antacid.

2. Acids cause tooth decay: When we eat sugary food, it is degraded by bacteria present in the mouth and an acid is formed. When the pH falls below 5.5, tooth enamel gets corroded. Saliva, which is slightly alkaline, neutralises some of the acid, but the excess acid remains. The excess acid can be removed only by using toothpaste, which is alkaline. Neem sticks contain alkaline juice, so cleaning teeth with a neem stick also helps to reduce tooth decay.

3. Acid is produced in fatigued muscles: As a result of physical exercise, stiffness and pain in the muscles start due to the formation of lactic acid. The supply of oxygen in the muscle is reduced. This causes difficulty in the release of energy, leading to an increase in the rate of anaerobic metabolism. As a result, lactic acid gets accumulated in the muscles.

4. Some animals and plants contain acids: A honey-bee injects an acid through its sting, which causes pain and irritation. Hence, a mild base like baking soda is applied to treat the wound. Similarly, nettle leaves, which have stinging hairs, inject formic acid into our body when touched, causing a burning pain. Nettle is a stinging plant. The affected area is rubbed with the dock plant, which is alkaline and neutralises the effect of the acid.

5. The brilliance of a tarnished copper vessel can be restored by using an acid: Lemon juice contains an acid. To clean a copper vessel, we rub it with a piece of lemon. The tarnish on the vessel is caused by a layer of basic copper oxide. Since lemon juice contains citric acid, it reacts with the copper oxide to form copper citrate, which is washed away. The vessel then regains its shining appearance.

[Photograph of a lemon slice, a glass of liquid and pH paper strips (PDF page 21)]

6. pH of soil: Soils are generally acidic. Plants require a definite pH range for proper growth and do not grow well in highly acidic or highly alkaline soil. So, highly acidic soil is treated by spreading quicklime, slaked lime or calcium carbonate to lower its acidity.

pH of salts

The aqueous solutions of all salts do not have the same pH value.
(i) Salts of strong acids and strong bases: Sodium chloride (NaCl), potassium nitrate (KNO₃), sodium sulphate (Na₂SO₄), etc. Their aqueous solutions are neutral, with pH equal to 7.
(ii) Salts of strong acids and weak bases: Aluminium chloride (AlCl₃), copper sulphate (CuSO₄), zinc sulphate (ZnSO₄), etc. Their aqueous solutions are acidic, with pH less than 7.
(iii) Salts of weak acids and strong bases: Sodium acetate (CH₃COONa), sodium carbonate (Na₂CO₃) and sodium hydrogencarbonate (NaHCO₃). Their aqueous solutions are basic, with pH more than 7.

POINTS TO REMEMBER

● Everything that tastes sour contains an acid.
● Acetic acid, citric acid and tartaric acid are a few organic acids. Sulphuric acid, nitric acid and hydrochloric acid are examples of inorganic acids.
● Acids turn blue litmus red, whereas bases turn red litmus blue.
● When a solution of an acid contains a larger amount of the acid, it is said to be concentrated; a solution containing a smaller amount of the acid is said to be dilute.
● Metals like sodium, potassium and calcium react with an acid to liberate hydrogen gas.
● Acids react with bases to produce salts and water.
● Acids react with carbonates and hydrogencarbonates to give carbon dioxide gas.
● The hydrogen atoms of an acid which can be partially or completely replaced by an atom or a group of atoms are called replaceable hydrogen atoms, also called acidic hydrogen.
● The number of replaceable hydrogen atoms present in a molecule of the acid is known as the basicity of the acid.
● A compound that reacts with an acid to form a salt and water is called a base.
● Bases that are soluble in water are called alkalis. All alkalis are bases, but all bases are not alkalis.
● The reaction between an acid and a base is called a neutralisation reaction. In such a reaction, the acid and the base destroy each other's properties.
● The number of hydroxyl groups (OH) present in a molecule of the base is called the acidity of the base.
● A salt is a compound formed by the reaction of an acid with a base.
● Na₂SO₄, CaSO₄ and Na₃PO₄ are normal salts, whereas NaHSO₄, NaHCO₃ and Na₂HPO₄ are acid salts.
● A strong acid is one which gets almost completely dissociated when dissolved in water to give hydrogen ions, whereas a weak acid gets only partially dissociated.
● A strong base gets almost completely dissociated when dissolved in water to give hydroxide ions (OH^-), whereas a weak base gets only partially dissociated.

Examples of a few acids with their basicities are given in the table below.

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