CBSE Class 10 Science Acids Bases and Salts MCQs Set 10

Answer: (b) Yellow

Explanation:
1. Methyl orange is orange in neutral, red in acidic and yellow in basic solution.
2. A base therefore turns it yellow.

Teacher's Note:
1. Methyl orange: red in acid, yellow in base.
2. It is a synthetic indicator.

Answer: (b) Blue litmus paper becomes red.

Explanation:
1. Sulphur burns in air to form sulphur dioxide: S + O₂ → SO₂.
2. SO₂ is the oxide of a non-metal, so it is acidic and forms sulphurous acid with moisture.
3. Hence moist blue litmus turns red and red litmus is unchanged.

Teacher's Note:
1. Non-metal oxides are acidic in nature.
2. Moist litmus is needed because the oxide must dissolve to form an acid.

Answer: (d) All

Explanation:
1. Na₂CO₃·10H₂O, NaHCO₃ and Na₂CO₃ are all salts of a strong base (NaOH) and a weak acid (H₂CO₃).
2. Each gives a basic solution that turns red litmus blue.

Teacher's Note:
1. Salt of strong base + weak acid: pH above 7.
2. The water of crystallisation does not change this nature.

Answer: (b) Clove oil

Explanation:
1. A visually impaired student cannot judge a colour change, so an olfactory indicator is needed.
2. Clove oil (like onion and vanilla) changes its smell in acid and base.
3. Litmus and red cabbage extract depend on colour.

Teacher's Note:
1. Olfactory indicators change odour, not colour.
2. They can be used by visually impaired students.

Answer: (d) The distilled water does not have ions present in it.

Explanation:
1. Distilled water is almost free from ions.
2. Electric current is carried through a solution by ions, so the bulb does not glow.
3. A bulb works on AC also and graphite is a good conductor.

Teacher's Note:
1. Rain water conducts because it has dissolved ions.
2. Pure water is a poor conductor.

Answer: (c) Dissociation

Explanation:
1. When an ionic compound dissolves in water, its positive and negative ions separate.
2. This process is called dissociation (ionisation).

Teacher's Note:
1. Acids and bases dissociate to give H⁺ and OH^–.
2. Dissociation explains why their solutions conduct electricity.

Answer: (b) 1 and 2 are correct

Explanation:
1. pH 11, 5, 7 and 2 mean A is basic, B is weakly acidic, C is neutral and D is strongly acidic.
2. Statement 1 is true: C (pH 7) is neutral.
3. Statement 2 is true: A (base) reacts with D (acid) to give a salt and water.
4. Statements 3 and 4 are false: C is neutral and B and D are both acids.

Teacher's Note:
1. Neutralisation always forms water.
2. Two acids cannot react to give hydrogen.

Answer: (d) calcium oxide

Explanation:
1. CaO + 2Na → Ca + Na₂O.
2. In this reaction CaO loses oxygen, so it is reduced.
3. The substance that gets reduced is the oxidising agent, so CaO is the oxidising agent.

Teacher's Note:
1. Oxidising agent: gets reduced.
2. Reducing agent: gets oxidised (here Na).

Answer: (d) Al₂O₃

Explanation:
1. Oxides that react with both acids and bases are amphoteric.
2. Al₂O₃ is amphoteric, while K₂O, Na₂O and CuO are basic.

Teacher's Note:
1. Amphoteric oxides: Al₂O₃ and ZnO.
2. They give salt and water with both acids and bases.

Answer: (a) Lime juice

Explanation:
1. Lime juice contains citric acid, so it is acidic.
2. Human blood is slightly basic, lime water is basic and an antacid is basic.

Teacher's Note:
1. Citrus fruits are sources of citric acid.
2. Test with blue litmus: it turns red in acids.

Answer: (c) combining with water

Explanation:
1. Plaster of Paris is CaSO₄·½H₂O.
2. On mixing with water it forms gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
3. This combination with water makes a hard mass.

Teacher's Note:
1. Setting of plaster is hydration, not release of CO₂.
2. That is why the powder must be kept moisture-proof.

Answer: (b) HCl

Explanation:
1. Egg shells are mainly calcium carbonate.
2. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
3. The CO₂ turns lime water milky, so the solution is HCl.

Teacher's Note:
1. Metal carbonate + acid gives salt, water and CO₂.
2. NaCl, LiCl and KCl are neutral salts.

Answer: (b) high concentration of hydroxide ion (OH^–) and low concentration of hydronium ion (H₃O⁺)

Explanation:
1. Detergent-rich water is basic, so its pH is above 7.
2. A basic solution has a high concentration of OH^– ions and a low concentration of H₃O⁺ ions.

Teacher's Note:
1. Basic: [OH^–] more than [H₃O⁺].
2. Neutral: both are equal.

Answer: (b) pH = 1.8

Explanation:
1. The lower the pH, the higher the H⁺ ion concentration.
2. Among 2.5, 1.8, 7 and 10 the lowest pH is 1.8.

Teacher's Note:
1. pH 1.8 is a strong acid.
2. A difference of one pH unit means ten times the concentration.

Answer: (b) Na – positive radical; Cl – negative radical

Explanation:
1. NaCl is made of Na⁺ and Cl^– ions.
2. Sodium is the metal, so it forms the positive radical; chlorine forms the negative radical.

Teacher's Note:
1. Metals form positive ions (cations).
2. Non-metal radicals form negative ions (anions).

Answer: (a) X

Explanation:
1. In the salt XY, X comes from the base XOH and Y from the acid HY.
2. X carries the positive charge, so it is the cation.

Teacher's Note:
1. The cation of a salt comes from the base.
2. The anion comes from the acid.

Answer: (b) Baking soda

Explanation:
1. Blue vitriol (CuSO₄·5H₂O), washing soda (Na₂CO₃·10H₂O) and gypsum (CaSO₄·2H₂O) contain water of crystallisation.
2. Baking soda, NaHCO₃, is anhydrous.

Teacher's Note:
1. Water of crystallisation is the fixed number of water molecules per formula unit.
2. Baking soda has none.

Answer: (b) hydrogen chloride

Explanation:
1. NaCl + H₂SO₄ (conc.) → NaHSO₄ + HCl.
2. The gas produced is hydrogen chloride.

Teacher's Note:
1. HCl gas fumes in moist air.
2. It is dried using CaCl₂ in a guard tube.

Answer: (c) Dilute NaOH: bulb glows

Explanation:
1. Glucose and alcohol do not give ions, so the bulb does not glow in their solutions.
2. Dilute HCl and dilute NaOH give ions, so the bulb glows.
3. Only the statement about NaOH is correct.

Teacher's Note:
1. Ions carry current in solutions.
2. Bulb glowing shows the presence of ions.

Answer: (a) Both Statement-I and Statement-II are correct.

Explanation:
1. Bleaching powder (CaOCl₂) acts as a bleaching agent by liberating chlorine, so Statement-I is correct.
2. Sodium carbonate removes the hardness of water, so Statement-II is also correct.

Teacher's Note:
1. Bleaching powder gives chlorine with CO₂ of air.
2. Washing soda softens hard water.

Answer: (a) H⁺ in water

Explanation:
1. According to Arrhenius, an acid is a substance that gives H⁺ ions in water.
2. A base gives OH^– ions in water.

Teacher's Note:
1. Acid: H⁺ in water. Base: OH^– in water.
2. The ions appear only in aqueous solution.

Answer: (b) The powder would react to moisture and turn into solid mass.

Explanation:
1. The white powder (Plaster of Paris) reacts with moisture from the air.
2. It would set into a hard mass of gypsum, so it is kept in a moisture-proof, airtight container.

Teacher's Note:
1. Setting is a reaction with water.
2. Even a little moisture spoils the powder.

Answer: (b) Plaster of Paris

Explanation:
1. The doctor mixed a white powder in water to make a paste that hardens around the fracture.
2. The white powder used for supporting fractured bones is Plaster of Paris.

Teacher's Note:
1. Plaster of Paris is CaSO₄·½H₂O.
2. On setting it changes to gypsum.

Answer: (d) Crystallisation

Explanation:
1. Plaster of Paris takes up water and changes to gypsum, CaSO₄·2H₂O.
2. Gypsum crystals interlock and form a hard solid mass; this is crystallisation (hydration).

Teacher's Note:
1. No burning, oxidation or mineral formation is involved.
2. Setting is crystallisation of gypsum.

Answer: (c) Reactant is calcium sulphate hemihydrate, product is gypsum.

Explanation:
1. In the reaction CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O the reactant is the hemihydrate.
2. The product CaSO₄·2H₂O is gypsum.

Teacher's Note:
1. Plaster of Paris is calcium sulphate hemihydrate.
2. Gypsum is calcium sulphate dihydrate.

Answer: (a) Hydrochloric acid

Explanation:
1. The stomach produces hydrochloric acid for digestion.
2. Too much HCl during indigestion causes pain and irritation.

Teacher's Note:
1. HCl in the stomach is a mineral acid.
2. Antacids relieve the problem.

Answer: (c) below 5.6

Explanation:
1. Normal rain water has pH about 5.6 because of dissolved CO₂.
2. Rain with pH below 5.6 is called acid rain.

Teacher's Note:
1. Acid rain harms plants, aquatic life and marble buildings.
2. Remember the figure 5.6.

Answer: (c) baking soda

Explanation:
1. Bee sting contains an acid and needs a mild base for neutralisation.
2. Baking soda is mildly basic and non-corrosive, so it gives relief.

Teacher's Note:
1. Never use a strong base like caustic soda on skin.
2. Baking soda: mild base.

Answer: (d) Antacid

Explanation:
1. Hyperacidity is due to excess HCl in the stomach.
2. An antacid is a mild base that neutralises it.

Teacher's Note:
1. Antacids: Mg(OH)₂ and NaHCO₃.
2. Antiseptics, antibiotics and analgesics have other uses.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Tap water has dissolved salts that give ions, so it conducts electricity.
2. Distilled water has almost no ions, so it does not conduct.
3. The reason correctly explains the assertion.

Teacher's Note:
1. Ions carry current in solutions.
2. Pure water is a poor conductor.

Answer: (d) A is false, but R is true.

Explanation:
1. A basic salt solution is alkaline and turns red litmus blue, not blue litmus red, so the assertion is false.
2. The reason is true: a salt of a strong base and a weak acid gives a basic solution.

Teacher's Note:
1. Basic solution: red litmus turns blue.
2. Acidic solution: blue litmus turns red.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Oxides of non-metals are acidic: CO₂ + H₂O → H₂CO₃.
2. They react with bases to give a salt and water, just as an acid does, which explains why they are acidic.

Teacher's Note:
1. Metal oxides are basic; non-metal oxides are acidic.
2. CO₂ + Ca(OH)₂ → CaCO₃ + H₂O.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Acid rain has a low pH, so the pH of river water falls; the assertion is true.
2. Aquatic life becomes difficult in acidic water, which is also true.
3. The reason is a result of the fall in pH, not an explanation of why the pH falls.

Teacher's Note:
1. The survival of aquatic life requires a pH close to neutral.
2. Both true does not always mean R explains A.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Bases that dissolve in water are called alkalis, so all alkalis are bases but not all bases are alkalis.
2. The reason correctly explains the assertion.

Teacher's Note:
1. Example: NaOH is an alkali. Cu(OH)₂ is an insoluble base.
2. Solubility decides whether a base is an alkali.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Methyl orange and phenolphthalein are synthetic indicators used to test acids and bases.
2. Indicators are chemicals that change colour in the presence of an acid or a base, which explains the use.

Teacher's Note:
1. Natural indicators: litmus, turmeric.
2. Synthetic indicators are made in the laboratory.