CBSE Class 10 Science Acids Bases and Salts MCQs Set 09

Answer: (b) Solution becomes red

Explanation:
1. Methyl orange is orange in neutral solution, red in acidic solution and yellow in basic solution.
2. HCl is a strong acid, so the methyl orange solution turns red.

Teacher's Note:
1. Learn the colour chart: methyl orange is red in acid and yellow in base.
2. Phenolphthalein is colourless in acid and pink in base.

Answer: (d) they change the colour of red litmus to blue

Explanation:
1. Acids taste sour, can change the colour of indicators and turn blue litmus red.
2. They never turn red litmus blue; that is the action of a base.
3. So statement (d) is the incorrect one.

Teacher's Note:
1. Acids: blue litmus to red. Bases: red litmus to blue.
2. In a negative question first find the three true statements and eliminate them.

Answer: (a) Dry blue litmus: no change; wet blue litmus: turns red

Explanation:
1. Concentrated H₂SO₄ is added to solid NaCl and the HCl gas formed is passed through CaCl₂ to dry it.
2. Dry HCl has no H⁺(aq) ions, so dry blue litmus does not change colour.
3. In wet litmus the gas dissolves in water, forms H₃O⁺ and turns blue litmus red.

Teacher's Note:
1. Acidic behaviour needs water.
2. This experiment proves that an acid releases H⁺ ions only in the presence of water.

Answer: (d) Aluminium

Explanation:
1. An element that reacts with both an acid and a base to give hydrogen is amphoteric.
2. Aluminium reacts with HCl and also with NaOH (forming sodium aluminate).
3. Sulphur and copper do not give hydrogen with either, and magnesium reacts only with acids.

Teacher's Note:
1. Zn and Al are the two metals to remember.
2. Their oxides, ZnO and Al₂O₃, are amphoteric oxides.

Answer: (b) All metal oxides react with water to give salt and acid

Explanation:
1. Metal carbonates react with acids to give a salt, water and CO₂, and some metals react with acids to give H₂.
2. Non-metal oxides such as CO₂ form acids with water.
3. Metal oxides are basic and do not form a salt and an acid with water, so statement (b) is wrong.

Teacher's Note:
1. Metal oxide + water gives a hydroxide (base), not a salt plus an acid.
2. Use this to correct statements about oxides.

Answer: (b) II and III

Explanation:
1. Most metal oxides react only with acids because they are basic.
2. Zinc oxide and aluminium oxide are amphoteric, so they react with bases as well as with acids.
3. MgO and CaO are purely basic.

Teacher's Note:
1. Amphoteric oxides: ZnO and Al₂O₃.
2. They give salt and water with both an acid and a base.

Answer: (a) Water soluble base

Explanation:
1. A base that dissolves in water is called an alkali.
2. NaOH and KOH are alkalis, but Cu(OH)₂ and Fe(OH)₃ are insoluble bases.

Teacher's Note:
1. All alkalis are bases, but all bases are not alkalis.
2. Solubility in water is the key word.

Answer: (c) Al

Explanation:
1. Metals such as Zn, Al and Sn react with a hot strong alkali like NaOH to give hydrogen.
2. Aluminium does so: 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂.
3. Mg, Cu and Ag do not react with NaOH.

Teacher's Note:
1. Hydrogen gas can be tested by the pop sound with a burning matchstick.
2. Only amphoteric metals react with alkalis.

Answer: (c) Carbon dioxide

Explanation:
1. Vinegar is acetic acid and baking soda is sodium hydrogencarbonate.
2. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂.
3. So carbon dioxide is released with brisk effervescence.

Teacher's Note:
1. Acid + metal hydrogencarbonate gives salt, water and CO₂.
2. Confirm the gas by passing it through lime water.

Answer: (b) Rock salt

Explanation:
1. Common salt obtained from mines is found as large, brownish crystals because of impurities.
2. This impure sodium chloride is called rock salt.

Teacher's Note:
1. Rock salt is the mineral form of NaCl.
2. Its brown colour is due to impurities.

Answer: (c) Mixing acid or base with water

Explanation:
1. Dilution means decreasing the concentration of an acid or base by adding water.
2. It lowers the number of H₃O⁺ or OH^– ions per unit volume.

Teacher's Note:
1. Always add acid to water slowly with stirring.
2. Never add water to a concentrated acid.

Answer: (c) S < Q < R < P

Explanation:
1. The lower the pH, the higher the hydronium ion concentration.
2. The pH values are P = 2, R = 5, Q = 9 and S = 11.
3. So the order of increasing [H₃O⁺] is S < Q < R < P.

Teacher's Note:
1. pH and [H₃O⁺] are inversely related.
2. A change of one pH unit means a ten-fold change in concentration.

Answer: (b) Sodium hydroxide (NaOH)

Explanation:
1. A strong base ionises almost completely in water.
2. NaOH gives Na⁺ and OH^– completely, whereas NH₄OH ionises only partly.
3. Water is neutral and HNO₃ is an acid.

Teacher's Note:
1. Strong bases: NaOH, KOH.
2. Weak bases: NH₄OH, Mg(OH)₂.

Answer: (a) Heat is evolved

Explanation:
1. Dissolving an acid in water releases a large amount of heat; the process is highly exothermic.
2. Hence heat is evolved and the concentration of the acid decreases.

Teacher's Note:
1. This is why acid is added to water slowly with stirring.
2. Adding water to acid can cause splashing and burns.

Answer: (d) Aqua regia

Explanation:
1. Gold does not dissolve in HCl, H₂SO₄ or HNO₃ alone.
2. Aqua regia is a 3 : 1 mixture of concentrated HCl and concentrated HNO₃.
3. It can dissolve gold and platinum.

Teacher's Note:
1. Aqua regia means royal water.
2. It dissolves the noble metals that single acids cannot.

Answer: (a) P–3, Q–4, R–1, S–2

Explanation:
1. Antacid is made from NaHCO₃, so P matches 3.
2. Vinegar (acetic acid) is used for preserving food, so Q matches 4.
3. Soap is made with NaOH (R–1) and H₂SO₄ is used in car batteries (S–2).

Teacher's Note:
1. Link each chemical to one everyday use.
2. Correct pairing: P–3, Q–4, R–1, S–2.

Answer: (c) absorb moisture from the gas.

Explanation:
1. HCl gas is made by heating NaCl with concentrated H₂SO₄.
2. On a humid day the gas carries moisture.
3. CaCl₂ in the guard tube absorbs this moisture, so dry HCl gas is obtained.

Teacher's Note:
1. Calcium chloride is a drying agent.
2. It does not absorb the HCl gas itself.

Answer: (c) copper(II) chloride

Explanation:
1. CuO is a black basic oxide and reacts with dilute HCl.
2. CuO + 2HCl → CuCl₂ + H₂O.
3. The copper(II) chloride formed makes the solution blue-green.

Teacher's Note:
1. Metal oxide + acid gives salt and water.
2. The colour change is a good test of this reaction.

Answer: (d) C₂H₅OH

Explanation:
1. NaOH, KOH and NH₄OH all give OH^– ions in water, so they are bases.
2. C₂H₅OH (ethanol) has an OH group but does not ionise to give OH^–, so it is not a base.

Teacher's Note:
1. An OH group alone does not make a compound a base.
2. Alcohols and glucose are neither acids nor bases.

Answer: (c) Antacid

Explanation:
1. Indigestion is caused by excess acid in the stomach.
2. Antacids are mild bases that neutralise this acid and give relief.

Teacher's Note:
1. Examples of antacids: milk of magnesia and baking soda.
2. Antibiotics kill bacteria, analgesics relieve pain.

Answer: (d) H₂SO₄

Explanation:
1. Basicity is the number of replaceable H⁺ ions in one molecule of an acid.
2. H₂SO₄ gives two H⁺ ions, so it is dibasic.
3. HCl and HNO₃ are monobasic and H₃PO₄ is tribasic.

Teacher's Note:
1. Dibasic acids: H₂SO₄, H₂CO₃.
2. Count only ionisable hydrogens.

Answer: (b) Carbonic acid

Explanation:
1. Soft drinks are made by dissolving CO₂ in water under pressure.
2. CO₂ + H₂O → H₂CO₃ gives carbonic acid.

Teacher's Note:
1. Carbonic acid is a weak acid.
2. Boric, oxalic and acetic acids are not used for aerated drinks.

Answer: (a) NaHCO₃

Explanation:
1. Compound C forms a white powder, is a constituent of baking powder and is used in antacids.
2. These properties belong to sodium hydrogencarbonate, NaHCO₃ (baking soda).

Teacher's Note:
1. Baking soda is mildly basic and non-corrosive.
2. Do not confuse it with washing soda, Na₂CO₃·10H₂O.

Answer: (b) Na₂CO₃

Explanation:
1. On heating, NaHCO₃ decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
2. The anhydrous product D is sodium carbonate, Na₂CO₃.

Teacher's Note:
1. Sodium carbonate formed this way is called soda ash.
2. Recrystallising it gives washing soda.

Answer: (b) Alkaline

Explanation:
1. Na₂CO₃ is the salt of a strong base (NaOH) and a weak acid (H₂CO₃).
2. In water it gives more OH^– than H⁺, so the solution is alkaline.

Teacher's Note:
1. Salt of strong base + weak acid is basic (pH above 7).
2. Use this rule for Na₂CO₃, CH₃COONa and similar salts.

Answer: (b) H₂CO₃

Explanation:
1. Na₂CO₃ dissolves in water to give a strong base (NaOH) and a weak acid.
2. The weak acid formed is carbonic acid, H₂CO₃.

Teacher's Note:
1. Carbonic acid is a weak acid.
2. NaOH ionises completely, so the solution is alkaline.

Answer: (a) CaCO₃

Explanation:
1. A solid that gives CO₂ and a solid residue on heating is a carbonate.
2. CaCO₃ → CaO + CO₂, so A is calcium carbonate (limestone).

Teacher's Note:
1. Limestone, chalk and marble are forms of CaCO₃.
2. Heating CaCO₃ is thermal decomposition.

Answer: (c) Ca(OH)₂

Explanation:
1. The residue CaO (quicklime) reacts with water: CaO + H₂O → Ca(OH)₂.
2. So B is calcium hydroxide (slaked lime).

Teacher's Note:
1. This reaction is highly exothermic.
2. A solution of Ca(OH)₂ is lime water.

Answer: (a) Ca(HCO₃)₂

Explanation:
1. Passing excess CO₂ through lime water first gives milky CaCO₃.
2. With more CO₂ the precipitate dissolves: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂.
3. So C is calcium hydrogencarbonate.

Teacher's Note:
1. Short passage of CO₂: milky. Excess CO₂: clear again.
2. Calcium hydrogencarbonate is soluble in water.

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

Explanation:
1. Assertion: salts are formed when an acid reacts with a base, so it is true.
2. Reason: a salt may be acidic, basic or neutral depending on the acid and base used, which is also true.
3. But the reason does not explain why salts are products of the acid-base reaction.

Teacher's Note:
1. Both statements true does not always mean R explains A.
2. Ask whether the reason answers 'why' for the assertion.

Answer: (d) Both Assertion and Reason are false.

Explanation:
1. In a neutralisation reaction an acid reacts with a base to give a salt and water, not a metal with a non-metal, so the assertion is false.
2. The reason is also wrong in fact: an acid contains H⁺ ions and a base contains OH^– ions.
3. So both A and R are false.

Teacher's Note:
1. Neutralisation: H⁺ + OH^– → H₂O.
2. Metals and non-metals are not the reactants.

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

Explanation:
1. Hydrogen gas is not evolved when most metals react with nitric acid, so the assertion is true.
2. Nitric acid is a strong oxidising agent, not a weak one, so the reason is false.
3. HNO₃ oxidises the hydrogen formed to water.

Teacher's Note:
1. Only Mg and Mn give H₂ with very dilute HNO₃.
2. Nitric acid gives oxides of nitrogen instead.

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

Explanation:
1. Ammonia is a weak base, so the assertion that it is acidic is false.
2. NH₃ + H₂O ⇌ NH₄⁺ + OH^–, so it forms OH^– in water and the reason is true.

Teacher's Note:
1. Ammonia solution turns red litmus blue.
2. Correct the assertion, not the reason.

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

Explanation:
1. Plaster of Paris absorbs moisture and sets into hard gypsum, so it must be stored in a dry place.
2. Both statements are true, and the reason explains the assertion.

Teacher's Note:
1. CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
2. Use moisture-proof containers for storage.

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

Explanation:
1. Glucose does not ionise in water, so its solution does not conduct electricity; the assertion is false.
2. Electric current in a solution is carried by ions, so the reason is true.

Teacher's Note:
1. Acids, bases and salts conduct in solution because they give ions.
2. Glucose and alcohol are non-electrolytes.