Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

(i) An indicator is a chemical substance which is used to indicate acidic or basic or neutral nature of an aqueous solution of a substance.

(ii) Indicators may be natural (litmus, turmeric and red cabbage), or synthetic (phenolphthalein and methyl orange). Some indicators (onion paste, vanilla essence and clove oil) distinguish acids and bases with their odour. Such indicators are called olfactory indicators.

2. Acids:

(i) Acids are substances that dissociate in water to give hydrogen ions (${\mathrm{H}}^{+}$) or hydronium ions (${\mathrm{H}}_3{\mathrm{O}}^{+}$).

(ii) Acids may be classified as:

(a) Organic and inorganic acids (On the basis of the source from which they are obtained).

(b) Strong and weak acids (On the basis of their extent of dissociation in aqueous solution).

(c) Concentrated and dilute acids (On the basis of their concentration in water).

(iii) The dilution of acid is highly exothermic, so it must be done with utmost care. Always, concentrated acid should be added to water (not water to acid) with constant stirring. This is done so that the heat evolved may be absorbed by the large amount of water present.

(iv) Acids taste sour and turn blue litmus to red. They are corrosive in nature. Their aqueous solutions conduct electricity.

(v) Acids react with metals to form salt and hydrogen gas.

(vi) Acids react with metal carbonates and metal hydrogen carbonates to form a salt, carbon dioxide and water.

(vii) Acids react with bases to form salt and water. This reaction is called a neutralisation reaction.

(viii) Acids react with basic metallic oxides to form salt and water.

3. Bases:

(i) Bases are substances that dissociate in water to give hydroxide $\left({\mathrm{OH}}^{–}\right)$ ions. A base which is soluble in water is known as alkali.

(ii) Bases are classified as strong and weak bases on the basis of whether they dissociate completely in water or not.

(iii) Bases are bitter to taste and turn red litmus to blue. They are soapy in touch and corrosive in nature. They conduct electricity in an aqueous medium.

(iv) Bases react with some active metals like zinc on heating to form salt and hydrogen gas.

(v) Non-metallic oxides being acidic in nature react with bases to form salt and water.

4. pH scale:

(i) $\mathrm{pH}$ is a numeric scale which is used to specify the strength of acids and bases in aqueous solutions.

(ii) At $298 \mathrm{K}$, the pH value of any solution ranges from $0$ to $14$. $\mathrm{pH}$ of middle value $7$ indicates neutral solution. $\mathrm{pH}$ values below $7$ indicate that the solution is acidic in nature while the values above $7$ indicate that the solution is basic in nature.

(iii) A universal indicator comprising of a solution of several compounds (dyes) that exhibits different colours for different $\mathrm{pH}$ values is used as a $\mathrm{pH}$ indicator.

5. Salts:

(i) A salt is a neutral ionic compound that is formed by the neutralisation reaction of an acid and a base.

(ii) Salts when dissolved in water can form neutral, acidic or basic solutions.

(iii) An aqueous solution of a salt of strong acid and strong base is neutral.

(iv) An aqueous solution of a salt of strong acid and a weak base is acidic.

(v) An aqueous solution of a salt of a weak acid and strong base is basic.

(vi) The number of water molecules bonded to a molecule of salt is called water of crystallisation.

(vii) Sodium chloride is also known as table salt or common salt. It is used in the manufacturing of baking soda, washing soda and sodium hydroxide.

(viii) Electrolysis of an aqueous solution of sodium chloride produces hydrogen gas at the cathode, chlorine gas at the anode and sodium hydroxide in the solution.

(ix) Washing soda is sodium carbonate decahydrate $\left({\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}\right)$. It is prepared by heating sodium bicarbonate.

(x) Baking soda is sodium hydrogen carbonate $\left({\mathrm{NaHCO}}_3\right)$ or sodium bicarbonate. It is manufactured by passing ${\mathrm{CO}}_2$ gas through an ammoniacal solution of sodium chloride.

(xi) Bleaching powder is calcium oxychloride $\left({\mathrm{CaOCl}}_2\right)$. It is manufactured by passing chlorine gas through dry slaked lime.

(xii) Plaster of Paris is calcium sulphate hemihydrate $\left({\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\right)$. It is prepared by heating gypsum under controlled conditions.