CBSE Class 10 Science Acids Bases And Salts

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Class 10 Science: Acids, Bases and Salts

Chapter 2: Acids, Bases and Salts

1. Acids: substances which turn blue litmus solution red are called acids. Acids are sour in taste.
2. Bases: substances which change red litmus solution blue are called bases. They are bitter in taste.
3. Mineral acids: acids which are obtained from minerals like sulphates, nitrates, chlorides, etc. are called mineral acids, e.g. H\(_2\)SO\(_4\) (sulphuric acid), HNO\(_3\) (nitric acid) and HCl (hydrochloric acid).
4. Organic acids: acids which are obtained from plants and animals are called organic acids, e.g. citric acid, ascorbic acid, tartaric acid, lactic acid, acetic acid.
5. Hydronium ions: they are formed by the reaction of H\(^+\) (from the acid) and H\(_2\)O, because H\(^+\) is unstable.
6. Universal indicator: a universal indicator is a mixture of indicators which shows a gradual but well-marked series of colour changes over a very wide range of change in the concentration of H\(^+\) ions.
7. Strong acids: acids which dissociate into ions completely are called strong acids, e.g. H\(_2\)SO\(_4\), HCl.
8. Weak acids: acids which do not dissociate into ions completely are called weak acids, e.g. citric acid, acetic acid.

Chemical properties of acids

1. Acids react with active metals to give salt and hydrogen gas.
2. Acids react with metal carbonates and metal hydrogen carbonates to give salt, water and carbon dioxide.
3. Acids react with bases to give salt and water. This reaction is called a neutralization reaction.
4. Acids react with metal oxides to give salt and water.

Chemical properties of bases

1. Reaction with metals: certain metals such as zinc, aluminium and tin react with alkali solutions on heating and hydrogen gas is evolved.
2. Reaction with acids: bases react with acids to form salt and water.

Indicators: indicators are substances which indicate the acidic or basic nature of a solution by their colour change.pH scale: a scale for measuring the hydrogen ion concentration in a solution. The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration in moles per litre.

\[ \text{pH}=-\log[\text{H}^+]=-\log[\text{H}_3\text{O}^+] \]

where [H\(^+\)] or [H\(_3\)O\(^+\)] represents the concentration of hydrogen ions in the solution.

1. The pH of a neutral solution is 7.
2. The pH of an acidic solution is < 7.
3. The pH of a basic solution is > 7.

Some important compounds and their uses

Common name	Chemical name	Chemical formula	Uses
Washing soda	Sodium carbonate decahydrate	Na\(_2\)CO\(_3\)·10H\(_2\)O	Manufacture of borax and caustic soda; softening of hard water
Baking soda	Sodium hydrogen carbonate	NaHCO\(_3\)	Used as an antacid; ingredient of baking powder
Bleaching powder	Calcium oxychloride	CaOCl\(_2\)	Bleaching clothes; used as an oxidizing agent; disinfecting water; manufacture of chloroform
Plaster of Paris	Calcium sulphate hemihydrate	CaSO\(_4\)·\(\frac12\)H\(_2\)O	Plastering fractured bones; making toys, decorative materials and statues

Equations of acids, bases and salts

Acid + Metal \(\rightarrow\) Salt + Hydrogen gas

\[ \text{H}_2\text{SO}_4+\text{Zn}\rightarrow \text{ZnSO}_4+\text{H}_2 \]

Base + Metal \(\rightarrow\) Salt + Hydrogen gas

\[ 2\text{NaOH}+\text{Zn}\rightarrow \text{Na}_2\text{ZnO}_2+\text{H}_2 \quad(\text{sodium zincate}) \]

Base + Acid \(\rightarrow\) Salt + Water

\[ \text{NaOH(aq)}+\text{HCl(aq)}\rightarrow \text{NaCl(aq)}+\text{H}_2\text{O(l)} \]

Acids give hydronium ions in water:

\[ \text{HCl}+\text{H}_2\text{O}\rightarrow \text{H}_3\text{O}^++\text{Cl}^- \]

Bases generate OH\(^-\) ions in water:

\[ \text{NaOH(aq)}\xrightarrow{\text{H}_2\text{O}}\text{Na}^+\text{(aq)}+\text{OH}^-\text{(aq)} \]

Reactions of important chemical compounds

Preparation of bleaching powder (by the action of chlorine on dry slaked lime):

\[ \text{Ca(OH)}_2+\text{Cl}_2\rightarrow \text{CaOCl}_2+\text{H}_2\text{O} \]

On heating, baking soda liberates CO\(_2\):

\[ 2\text{NaHCO}_3\xrightarrow{\text{heat}}\text{Na}_2\text{CO}_3+\text{H}_2\text{O}+\text{CO}_2 \]

Washing soda (sodium carbonate decahydrate):

\[ \text{Na}_2\text{CO}_3+10\text{H}_2\text{O}\rightarrow \text{Na}_2\text{CO}_3\cdot 10\text{H}_2\text{O} \]

Preparation of plaster of Paris:

\[ \text{CaSO}_4\cdot 2\text{H}_2\text{O}\xrightarrow{373\ \text{K}}\text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O}+1\tfrac12\text{H}_2\text{O} \]

On mixing plaster of Paris with water, gypsum is obtained:

\[ \text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O}+1\tfrac12\text{H}_2\text{O}\rightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O} \]

Mind map

Question bank

Very short answer type questions (1 mark)

Write the name of the products formed by heating gypsum at 373 K. Write one use of it.

Answer: Plaster of Paris and water. Plaster of Paris is used for plastering fractured bones.

Write the chemical name and formula of the compound which is used as an antacid.

Answer: Sodium hydrogen carbonate (sodium bicarbonate), NaHCO\(_3\).

Short answer type questions (2 marks)

Given below are the pH values of different liquids: 7.0, 14.0, 4.0 and 2.0. Which of these could be that of (a) lemon juice, (b) distilled water, (c) sodium hydroxide solution, (d) tomato juice?

(a) Lemon juice: 2.0 (very acidic)
(b) Distilled water: 7.0 (neutral)
(c) Sodium hydroxide solution: 14.0 (strongly basic)
(d) Tomato juice: 4.0 (weakly acidic)

What is baking powder? How does it make the cake soft and spongy?

Answer: Baking powder is a mixture of sodium hydrogen carbonate and tartaric acid (a mild edible acid). On heating (or when moistened) it liberates CO\(_2\), which makes the cake soft and spongy.

Short answer type questions (3 marks)

Write the chemical name of plaster of Paris. Write a chemical equation to show the reaction between plaster of Paris and water. Name the compound produced in this reaction.

Plaster of Paris is calcium sulphate hemihydrate.

\[ \text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O}+1\tfrac12\text{H}_2\text{O}\rightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O} \]

The compound produced is gypsum.

A gas X reacts with lime water and forms a compound Y which is used as a bleaching agent in the chemical industry. Identify X and Y. Give the chemical equation of the reaction involved.

X is chlorine and Y is CaOCl\(_2\) (calcium oxychloride, bleaching powder), used as a bleaching agent.

\[ \text{Ca(OH)}_2+\text{Cl}_2\rightarrow \text{CaOCl}_2+\text{H}_2\text{O} \]

Long answer type questions (5 marks)

(a) A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Mention the pH range within which our body works. (c) Explain how antacids give relief from acidity. (d) Mention the nature of toothpastes. How do they prevent tooth decay?

(a) It is done to prevent the formation of lactic acid, which spoils the milk (alkaline milk takes a longer time to set as curd).

(b) The pH range is 7.0–7.8.

(c) Antacids neutralize the excess acid in our stomach and give relief.

(d) Toothpastes are basic. They neutralize the acid formed in the mouth by the bacteria, and so prevent tooth decay.

(a) Crystals of a substance changed their colour on heating in a closed test tube but regained it after some time when they were allowed to cool down. Name the substance and write its formula. Explain the phenomenon. (b) How is sodium carbonate prepared? Give two uses of the compound.

(a) The substance is copper sulphate pentahydrate, CuSO\(_4\)·5H\(_2\)O. It is blue. It becomes white on heating due to the loss of its water of crystallization:

\[ \text{CuSO}_4\cdot 5\text{H}_2\text{O}\xrightarrow{\text{heat}}\text{CuSO}_4+5\text{H}_2\text{O} \]

It regains its blue colour on cooling by taking up water (the water vapour condensed in the tube, or moisture from the atmosphere):

\[ \text{CuSO}_4+5\text{H}_2\text{O}\rightarrow \text{CuSO}_4\cdot 5\text{H}_2\text{O} \]

(b) Sodium carbonate is prepared by passing CO\(_2\) through ammoniated brine (a saturated solution of NaCl in ammonia), which gives sodium hydrogen carbonate; this on heating gives sodium carbonate (Na\(_2\)CO\(_3\)), which on recrystallization gives washing soda. Uses: in the production of washing powder and in the manufacture of glass.

Base + Acid → Salt + Water

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)