2. Acids, Bases and Salts

Curd and sour substances contain acids. These acids react with brass and copper. Poisonous compounds are formed, which spoil the food. So they should not be kept in such vessels.

Hydrogen gas is usually liberated.

Test: Bring a burning candle near the gas. The gas burns with a pop sound. This shows that the gas is hydrogen.

The gas that puts off a burning candle is carbon dioxide. Since calcium chloride is formed, compound A is calcium carbonate (CaCO₃).

HCl and HNO₃ produce H⁺ ions in water. These H⁺ ions are the reason for acidic character.

Alcohol and glucose also contain hydrogen, but they do not give H⁺ ions in water. So they do not show acidic character.

An acid in water produces ions such as H⁺(aq) and Cl^-(aq). These ions carry electric current. So the solution conducts electricity.

Dry HCl gas does not form H⁺ ions without water. H⁺ ions are needed to show acidic behaviour. So dry HCl gas does not change the colour of dry litmus paper.

Mixing an acid with water is a highly exothermic process. If water is added to acid, a lot of heat is produced at once. The mixture may splash out and cause burns. The glass container may also break.

When acid is added slowly to water with stirring, the heat spreads out in the large amount of water. So it is safe.

When an acid is diluted, the concentration of hydronium ions (H₃O⁺) per unit volume decreases.

The concentration of hydroxide ions (OH^-) increases.

Lower pH means higher hydrogen ion concentration. So solution A (pH 6) has more hydrogen ion concentration.

Solution A (pH 6) is acidic. Solution B (pH 8) is basic.

Higher the concentration of H⁺(aq) ions, more acidic is the solution and lower is its pH. Lower the concentration of H⁺(aq) ions, less acidic (or more basic) is the solution.

Yes, basic solutions also have H⁺(aq) ions. But in these solutions, the concentration of OH^-(aq) ions is more than that of H⁺(aq) ions. So they are basic.

A farmer would treat the soil with these substances when the soil is too acidic. These are bases. They neutralise the excess acid in the soil.

Answer: Bleaching powder

Answer: Dry slaked lime [Ca(OH)₂]

Answer: Washing soda (Na₂CO₃.10H₂O)

On heating, sodium hydrogencarbonate breaks down. Sodium carbonate, water and carbon dioxide gas are formed.

Plaster of Paris combines with water and changes back into gypsum. It sets into a hard solid mass.

Answer: (d) 10

A solution that turns red litmus blue is basic. A basic solution has pH more than 7.

Answer: (b) HCl

Egg-shells contain calcium carbonate. HCl reacts with it to give carbon dioxide, which turns lime water milky.

Answer: (d) 16 mL

Double the volume of NaOH needs double the volume of HCl. So 2 × 8 = 16 mL.

Answer: (c) Antacid

An antacid is a mild base. It neutralises the excess acid in the stomach.

Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

Hydrochloric acid + Iron → Iron(II) chloride + Hydrogen

• Fix two nails on a cork. Place the cork in a 100 mL beaker.
• Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch.
• Pour some dilute HCl in the beaker and switch on the current. The bulb glows.
• Repeat the activity with glucose solution and alcohol solution. The bulb does not glow.

Conclusion: Dilute HCl gives H⁺ ions in water, which conduct electricity. Glucose and alcohol do not give H⁺ ions. So they are not acids, though they contain hydrogen.

Distilled water does not contain ions. So it does not conduct electricity.

Rain water dissolves gases like carbon dioxide from the air. These form acids, which give ions in water. These ions conduct electricity. So rain water conducts electricity.

Acids show acidic behaviour only when they produce H⁺ ions. Dry HCl gas cannot form H⁺ ions in the absence of water. So it does not show acidic behaviour.

(a) Neutral: D (pH 7)

(b) Strongly alkaline: C (pH 11)

(c) Strongly acidic: B (pH 1)

(d) Weakly acidic: A (pH 4)

(e) Weakly alkaline: E (pH 9)

Increasing order of hydrogen-ion concentration:

Fizzing will be more vigorous in test tube A.

HCl is a strong acid. It gives more H⁺ ions than acetic acid, which is a weak acid. So hydrogen gas is produced faster in test tube A.

The pH will decrease (become less than 6).

When milk turns into curd, lactic acid is formed. This acid makes the curd more acidic. So its pH falls.

Milk becomes sour when acid forms in it. Baking soda is a base. It makes the milk slightly alkaline. So the milk does not turn sour quickly and can be kept for a longer time.

The acid formed during curd setting is first used up in neutralising the baking soda. Only after that the milk becomes acidic enough to set. So it takes a longer time to set as curd.

Plaster of Paris absorbs water from moisture. It changes into gypsum and becomes a hard solid mass.

Then it cannot be used. So it is stored in a moisture-proof container.

The reaction between an acid and a base to give a salt and water is called a neutralisation reaction.

Washing soda:

• It is used in glass, soap and paper industries.
• It is used for removing permanent hardness of water.

Baking soda:

• It is used in baking powder for making cakes and bread soft and spongy.
• It is used as an antacid to give relief from acidity. It is also used in soda-acid fire extinguishers.