Acids, Bases and Salts — NCERT Solutions, AI Tutor & Practice

Understand: Curd and sour foods contain acids (curd contains lactic acid).

Analyse: Acids react with metals to form salts and release hydrogen gas: $\text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{H}_2$ Brass and copper react with these acids, forming toxic metal compounds.

Conclude: Storing sour foods in brass or copper vessels causes the acids to corrode the metal and produce harmful substances that spoil the food and can cause food poisoning. Hence such foods should be stored in glass, stainless steel or earthen vessels.

Understand: Electrical conduction in a solution requires the presence of free-moving ions.

Analyse: When an acid such as HCl dissolves in water, it dissociates to give ions: $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$ The $\text{H}^+$ ions (present as hydronium $\text{H}_3\text{O}^+$) and the anions carry the current.

Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) dissolves in water but does not ionise; no free ions are produced.

Conclude: An acid solution has ions and therefore conducts electricity, whereas a glucose solution has only neutral molecules and cannot conduct electricity.

Understand: An acid reacting with a base is a neutralisation reaction.

Analyse: The $\text{H}^+$ ions of the acid combine with the $\text{OH}^-$ ions of the base to form water, while the remaining ions form a salt.

Balanced equation: $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

Conclude: Sodium chloride (a salt) and water are produced. This is a neutralisation reaction and it is exothermic.

Understand: The acidic character is due to hydrogen ions, $\text{H}^+$.

Analyse: Dry HCl gas exists as molecules and cannot release $\text{H}^+$ ions in the absence of water. Only in the presence of water does it ionise: $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$

Conclude: Dry litmus has no water, so dry HCl gives no $\text{H}^+$ ions and litmus does not turn red. Moist litmus provides water, HCl ionises to give $\text{H}^+$ ions, and the litmus turns red. This proves acidic behaviour requires water.

Understand: The pH scale runs from 0 to 14. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Lower pH means higher $\text{H}^+$ concentration.

Analyse:

• Solution with pH $2$: strongly acidic.
• Solution with pH $11$: strongly basic (alkaline).

Since pH decreases as $\text{H}^+$ increases, the solution with pH $2$ has a far greater hydrogen-ion concentration. Each pH unit is a factor of 10, so pH 2 has about $10^{9}$ times more $\text{H}^+$ than pH 11.

Conclude: pH 2 is acidic and has the higher $\text{H}^+$ concentration; pH 11 is basic.

Understand: Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a salt.

Analyse: Copper sulphate crystals are blue and have the formula: $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ There are 5 molecules of water of crystallisation per formula unit.

On heating, the water is driven off and the salt turns white (anhydrous): $\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \xrightarrow{\text{heat}} \text{CuSO}_4 + 5\text{H}_2\text{O}$ Adding water back restores the blue colour.

Conclude: Water of crystallisation is responsible for the shape and colour of the crystals; its removal gives the anhydrous form.

Understand: Baking soda is sodium hydrogencarbonate, $\text{NaHCO}_3$; washing soda is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$.

Analyse — preparation:

1. Heating baking soda gives sodium carbonate: $2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$
2. Recrystallising sodium carbonate with water gives washing soda: $\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$

Uses:

• Baking soda: used as an antacid and in baking powder to make cakes fluffy.
• Washing soda: used for softening hard water and in cleaning.

Conclude: Washing soda is made by heating baking soda to sodium carbonate, then recrystallising it with water.

Understand: Bleaching powder is calcium oxychloride, $\text{CaOCl}_2$.

Analyse — manufacture: It is made by passing chlorine gas over dry slaked lime (calcium hydroxide): $\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$

Uses:

1. For bleaching cotton, linen and wood pulp in the paper industry.
2. For disinfecting drinking water (as an oxidising and germ-killing agent).

Conclude: Bleaching powder is prepared by the action of chlorine on slaked lime and is widely used for bleaching and disinfection.

Understand: Tooth enamel is made of calcium phosphate, the hardest substance in the body; it does not dissolve in water but corrodes in acid.

Analyse: Bacteria in the mouth break down sugar to produce acids. When the pH of the mouth falls below 5.5, the enamel starts to dissolve, leading to tooth decay.

Prevention: Using toothpaste, which is basic, neutralises the excess acid and raises the pH above 5.5, protecting the enamel. Cleaning the mouth after eating also removes food and bacteria.

Conclude: Tooth decay begins when the mouth becomes too acidic (pH < 5.5); brushing with a basic toothpaste neutralises the acid and prevents decay.

Understand: HCl is a strong acid and NaOH is a strong base; when equal amounts react they neutralise completely.

Analyse: The reaction is: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ Equal volumes of equal concentration provide equal moles of $\text{H}^+$ and $\text{OH}^-$, which combine fully to form water. NaCl formed is the salt of a strong acid and strong base, so it does not affect pH.

Conclude: The solution is neutral, with pH = 7.

Understand: Plaster of Paris (POP) is calcium sulphate hemihydrate, $\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O}$.

Analyse: On contact with water it reacts and sets into a hard solid, gypsum: $\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ If POP absorbs moisture from the air, it will set and become useless before it can be used.

Conclude: POP must be kept in a moisture-proof (dry, sealed) container so that it does not set prematurely by reacting with atmospheric water vapour.

Understand: Acids are classified by how completely they ionise in water.

Analyse:

• Strong acids ionise almost completely, releasing a large amount of $\text{H}^+$ ions. Example ionisation: $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$
• Weak acids ionise only partially, giving fewer $\text{H}^+$ ions: $\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+$

Classification:

• Strong acids: hydrochloric acid (HCl), sulphuric acid ($\text{H}_2\text{SO}_4$).
• Weak acids: acetic acid ($\text{CH}_3\text{COOH}$), carbonic acid ($\text{H}_2\text{CO}_3$).

Conclude: Whether an acid is strong or weak depends on the degree of ionisation in water, not on its concentration.