Curd and sour foods contain acids. Acids react with copper and brass to form soluble metal salts, which can make the food poisonous and unfit to eat. Therefore such substances should not be kept in copper or brass vessels.
Hydrogen gas is usually liberated. For example, zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂. The gas is tested by bringing a burning candle near it; hydrogen burns with a pop sound.
The gas is carbon dioxide, so compound A is calcium carbonate. The balanced equation is: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O. Carbon dioxide extinguishes a burning candle.
HCl and HNO₃ ionise in water to produce H⁺/H₃O⁺ ions, which are responsible for acidic character. Alcohol and glucose contain hydrogen, but they do not ionise in water to produce H⁺ ions, so their aqueous solutions are not acidic.
An aqueous acid produces ions in water, mainly H⁺ or H₃O⁺ ions along with negative ions. These ions carry charge through the solution, so the solution conducts electricity.
Dry HCl gas does not produce H⁺/H₃O⁺ ions in the absence of water. Since acidic behaviour is shown only when these ions are present in aqueous solution, dry HCl does not change dry litmus paper.
Dilution of concentrated acid is highly exothermic. If water is added to acid, the heat produced may cause the mixture to splash. Adding acid slowly to a larger volume of water lets the heat dissipate safely.
When an acid is diluted, the number of H₃O⁺ ions per unit volume decreases. Therefore the concentration of hydronium ions decreases and the pH increases.
Adding excess base increases the concentration of OH⁻ ions in the solution. The solution becomes more basic and its pH increases.
Solution A has more hydrogen ion concentration because lower pH means higher H⁺ concentration. Solution A (pH 6) is acidic, while solution B (pH 8) is basic.
The concentration of H⁺(aq) ions determines the acidic nature of a solution. Higher H⁺ concentration makes the solution more acidic and lowers its pH.
Yes. Basic aqueous solutions also contain some H⁺ ions because water ionises slightly. They are basic because the concentration of OH⁻ ions is greater than the concentration of H⁺ ions.
A farmer treats the soil with quick lime, slaked lime or chalk when the soil is too acidic. These basic substances neutralise the excess acid and make the soil suitable for plant growth.
The common name of Ca(ClO)₂ is bleaching powder.
Dry slaked lime, Ca(OH)₂, gives bleaching powder when treated with chlorine.
Washing soda, sodium carbonate decahydrate (Na₂CO₃·10H₂O), is used for softening hard water.
On heating, sodium hydrogencarbonate decomposes to give sodium carbonate, carbon dioxide and water: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O.
Plaster of Paris combines with water to form gypsum and sets hard: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
- a. 1
- b. 4
- c. 5
- d. 10
A basic solution turns red litmus blue. Among the options, pH 10 is basic.
(d) 10
- a. NaCl
- b. HCl
- c. LiCl
- d. KCl
Egg-shells contain calcium carbonate. Acids react with carbonates to release CO₂, which turns lime-water milky.
(b) HCl
- a. 4 mL
- b. 8 mL
- c. 12 mL
- d. 16 mL
The volume of acid required is directly proportional to the volume of alkali of the same concentration. Doubling NaOH from 10 mL to 20 mL doubles HCl from 8 mL to 16 mL.
(d) 16 mL
- a. Antibiotic
- b. Analgesic
- c. Antacid
- d. Antiseptic
Indigestion caused by excess stomach acid is relieved by antacids, which neutralise the acid.
(c) Antacid
(a) Zinc + sulphuric acid → zinc sulphate + hydrogen; Zn + H₂SO₄ → ZnSO₄ + H₂
(b) Magnesium + hydrochloric acid → magnesium chloride + hydrogen; Mg + 2HCl → MgCl₂ + H₂
(c) Aluminium + sulphuric acid → aluminium sulphate + hydrogen; 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
(d) Iron + hydrochloric acid → iron chloride + hydrogen; Fe + 2HCl → FeCl₂ + H₂
Use the electrical conductivity activity. Place two nails in a cork, connect them to a battery, bulb and switch, and dip them in solutions separately. Dilute HCl or H₂SO₄ makes the bulb glow because it produces ions. Glucose and alcohol solutions do not make the bulb glow because they do not produce H⁺ ions in water. This shows that hydrogen-containing compounds are acidic only if they ionise to give H⁺/H₃O⁺ ions.
Distilled water has almost no dissolved salts or ions, so it does not conduct electricity. Rain water dissolves carbon dioxide and other substances from air and contains ions; these ions conduct electricity.
Dry HCl does not ionise to produce H⁺/H₃O⁺ ions without water. Since acidic behaviour depends on these ions, dry HCl gas does not show acidic behaviour.
A = pH 4, B = pH 1, C = pH 11, D = pH 7, E = pH 9.
(a) Neutral: D.
(b) Strongly alkaline: C.
(c) Strongly acidic: B.
(d) Weakly acidic: A.
(e) Weakly alkaline: E.
Increasing order of hydrogen-ion concentration is: pH 11 < pH 9 < pH 7 < pH 4 < pH 1, so C < E < D < A < B.
Fizzing will be more vigorous in test tube A. HCl is a strong acid and produces a higher concentration of H⁺ ions than acetic acid, which is a weak acid. Therefore magnesium reacts faster with HCl, releasing hydrogen gas more rapidly.
The pH will decrease below 6. As milk turns into curd, bacteria produce lactic acid, increasing acidity and lowering the pH.
(a) Baking soda is basic, so it makes the milk slightly alkaline and prevents it from souring quickly.
(b) Curd forms when lactic acid makes milk acidic. The added baking soda neutralises the acid formed, so it takes longer for enough acid to accumulate and set the milk as curd.
Plaster of Paris reacts with water to form gypsum and sets into a hard solid: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. If exposed to moisture during storage, it will harden and become useless for moulding.
A neutralisation reaction is a reaction in which an acid reacts with a base to form salt and water. Examples: HCl + NaOH → NaCl + H₂O; H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.
Uses of washing soda: it is used for softening hard water and in the manufacture of glass, soap and paper. Uses of baking soda: it is used in baking powder to make cakes and bread soft and spongy, and as an ingredient of antacids or soda-acid fire extinguishers.