Key Points

Acids are sour in taste and change the colour of blue litmus paper to red. Bases are bitter in taste, feel soapy to touch, and change the colour of red litmus paper to blue.

Indicators are substances that show a change in colour when brought in contact with acids and bases. For example, phenolphthalein is colourless in acidic solutions and pink in basic solutions, while methyl orange is red in acids and yellow in bases.

Acids react with most metals to form a salt and liberate hydrogen gas. The general equation for this reaction is: $Acid + Metal \rightarrow Salt + Hydrogen \text{ gas}$. For example, $H_2SO_4 + Zn \rightarrow ZnSO_4 + H_2$.

Acids react with metal carbonates and metal hydrogencarbonates to produce a salt, carbon dioxide gas, and water. For example, $2HCl + Na_2CO_3 \rightarrow 2NaCl + H_2O + CO_2$.

The reaction between an acid and a base to form a salt and water is known as a neutralisation reaction. The general form is: $Acid + Base \rightarrow Salt + Water$. For instance, $HCl + NaOH \rightarrow NaCl + H_2O$.

Metallic oxides are generally basic in nature because they react with acids to form salt and water. Non-metallic oxides are acidic in nature because they react with bases to form salt and water.

Acids produce hydrogen ions ($H^+$ or $H_3O^+$) only in the presence of water. Similarly, bases produce hydroxide ions ($OH^-$) in water. This is why aqueous solutions of acids and bases conduct electricity.

The pH scale is used to measure the strength of acids and bases, ranging from 0 to 14. A pH less than 7 indicates an acidic solution, a pH of 7 is neutral, and a pH greater than 7 indicates a basic solution.

The strength of an acid depends on the concentration of $H^+$ ions it produces in solution. Strong acids like $HCl$ and $H_2SO_4$ dissociate completely, while weak acids like acetic acid ($CH_3COOH$) dissociate partially.

Living organisms survive in a narrow pH range (human body: 7.0-7.8). Tooth decay begins when the pH of the mouth falls below 5.5. Antacids are mild bases used to neutralize excess stomach acid during indigestion.

Salts of a strong acid and strong base are neutral (pH=7). Salts of a strong acid and weak base are acidic (pH<7). Salts of a strong base and weak acid are basic (pH>7).

When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas. The reaction is $2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$.

Bleaching powder ($CaOCl_2$) is produced by the action of chlorine gas on dry slaked lime ($Ca(OH)_2$). It is used as a bleaching agent in textile and paper industries and as a disinfectant for water.

Baking soda is sodium hydrogencarbonate ($NaHCO_3$). It is a mild, non-corrosive base used in baking powder, antacids, and soda-acid fire extinguishers. On heating, it releases $CO_2$: $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$.

Washing soda is hydrated sodium carbonate ($Na_2CO_3 \cdot 10H_2O$). It is used in glass, soap, and paper industries, as a cleaning agent, and for removing the permanent hardness of water.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. For example, hydrated copper sulphate ($CuSO_4 \cdot 5H_2O$) is blue, while anhydrous copper sulphate ($CuSO_4$) is white.

Plaster of Paris is calcium sulphate hemihydrate ($CaSO_4 \cdot \frac{1}{2}H_2O$). On mixing with water, it changes to gypsum ($CaSO_4 \cdot 2H_2O$), a hard solid mass used for setting fractured bones.