CBSE Class 7 Science Advanced Chemistry Acids, Bases and Salts MCQs Set 04

Answer: (b) BCAD

Explanation:
1. In a basic solution methyl orange is yellow, turmeric is red, litmus is blue and phenolphthalein is pink.
2. The indicators in the order yellow, red, blue, pink are B, C, A, D: option (b).

Teacher's Note:
1. Make a table of acid and base colours for each indicator.
2. Methyl orange is yellow in base and phenolphthalein pink.

Answer: (a) (i) → (C); (ii) → (D); (iii) → (A); (iv) → (B)

Explanation:
1. Baking soda is sodium bicarbonate: (i) → (C).
2. Chile saltpetre is sodium nitrate: (ii) → (D).
3. Washing soda is sodium carbonate: (iii) → (A).
4. Nitre is potassium nitrate: (iv) → (B).
5. This is option (a).

Teacher's Note:
1. Chile saltpetre comes from Chile and is sodium nitrate; nitre is potassium nitrate.
2. Na₂S₂O₃ (hypo) and CaOCl₂ (bleaching powder) are extra entries.

Answer: (d) KOH

Explanation:
1. Soap is made by boiling fats or oils with a strong alkali such as NaOH or KOH.
2. Among the options only KOH is a strong alkali used for this: KOH.

Teacher's Note:
1. Hard soaps use NaOH; soft soaps use KOH.
2. Cu(OH)₂ and Mg(OH)₂ are insoluble bases.

Answer: (a) tea

Explanation:
1. Tea leaves contain tannic acid.
2. Curd has lactic acid and rancid butter has butyric acid.
3. The answer is tea.

Teacher's Note:
1. Tannic acid makes tea taste slightly bitter.
2. Palm oil contains fatty acids such as palmitic acid.

Answer: (d) Mohr's salt

Explanation:
1. Calcium chloride is a normal salt, and bleaching powder is a mixed salt.
2. Potassium ferrocyanide is a complex salt.
3. Mohr's salt, \(\ce{FeSO4.(NH4)2SO4.6H2O}\), is made of two simple salts crystallising together, so it is a double salt.

Teacher's Note:
1. A double salt breaks into all its simple ions in water.
2. Alum is another double salt.

Answer: (c) 3

Explanation:
1. Phosphoric acid is tribasic.
2. The reaction \(\ce{H3PO4 + 3NaOH -> Na3PO4 + 3H2O}\) replaces all three hydrogens.
3. So 3 molecules of NaOH are needed.

Teacher's Note:
1. Number of NaOH needed = basicity of the acid.
2. Normal salt here is sodium phosphate.

Answer: (b) X → Blue, Y → White

Explanation:
1. Hydrated copper sulphate, \(\ce{CuSO4.5H2O}\), is blue because of its water of crystallisation.
2. On heating, it loses the water and becomes anhydrous copper sulphate, which is white.
3. So X is blue and Y is white: (b).

Teacher's Note:
1. The change is reversible: adding water to white CuSO₄ makes it blue again.
2. This is used as a test for water.

Answer: (b) Zinc chloride

Explanation:
1. A substance that absorbs moisture and dissolves in it is deliquescent.
2. Zinc chloride is deliquescent. Cobalt chloride and calcium oxide absorb moisture without dissolving, and pure sodium chloride does not do so.
3. The answer is zinc chloride.

Teacher's Note:
1. Deliquescent examples: CaCl₂, MgCl₂, ZnCl₂, NaOH.
2. Hygroscopic examples: CoCl₂, CaO.

Answer: (b) Plaster of Paris

Explanation:
1. Plaster of Paris is mixed with water to form a paste that sets into a hard solid (gypsum).
2. It can be moulded into statues: Plaster of Paris.

Teacher's Note:
1. Plaster of Paris is also used for broken-bone casts.
2. Potash alum purifies water.

Answer: (c) Sodium sulphide

Explanation:
1. H₂S gives the sulphide radical, so the normal salt is sodium sulphide.
2. The reaction is \(\ce{H2S + 2NaOH -> Na2S + 2H2O}\), so X is sodium sulphide.

Teacher's Note:
1. Hydrosulphuric acid gives sulphide salts.
2. Sulphate comes from H₂SO₄ and sulphite from H₂SO₃.

Answer: (d) yellow

Explanation:
1. Turmeric is yellow itself and stays yellow in acidic solution.
2. It turns red only in a basic solution.
3. The answer is yellow.

Teacher's Note:
1. Turmeric is yellow in acid and neutral, red in base.
2. Litmus is red in acid, and phenolphthalein is colourless in acid.

Answer: (d) 2

Explanation:
1. Configuration 2, 4 is carbon.
2. Its acid is carbonic acid \(\ce{H2CO3}\), which has two replaceable hydrogens.
3. The basicity is 2.

Teacher's Note:
1. Basicity is the number of replaceable hydrogens.
2. Carbonic acid is dibasic.

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

Explanation:
1. A is true: an alkali is a base that dissolves in water.
2. R is true: bases such as copper hydroxide do not dissolve in water.
3. Because some bases are insoluble, they are bases but not alkalis, so R explains A: (a).

Teacher's Note:
1. Alkali = water-soluble base.
2. KOH and NaOH are alkalis; Cu(OH)₂ is only a base.

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

Explanation:
1. A is true: impurities dissolve in nitric acid while gold remains undissolved.
2. R is true: nitric acid is a strong oxidising agent.
3. But gold is purified because it does not dissolve in nitric acid, not because of oxidising power. So R is not the explanation: (b).

Teacher's Note:
1. Gold and platinum do not dissolve in nitric acid.
2. Find the property that really explains A before choosing (a).

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

Explanation:
1. A is true: KOH dissolves in water, so it is an alkali, while Pb(OH)₂ is insoluble.
2. R is true: KOH has one OH group and Pb(OH)₂ has two.
3. The acidity of a base has nothing to do with its solubility, so R does not explain A: (b).

Teacher's Note:
1. Solubility decides whether a base is an alkali.
2. Acidity is the number of OH groups.

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

Explanation:
1. A is true: nitrates such as ammonium nitrate are fertilisers.
2. R is false: nitrates are highly soluble in water, which is why plants can take them up.
3. So A is true and R is false: (c).

Teacher's Note:
1. All nitrates are soluble in water.
2. Plants absorb dissolved nutrients.

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

Explanation:
1. A is true: lead-acid car batteries use sulphuric acid.
2. R is true: sulphuric acid ionises well in water, so it conducts electricity.
3. Because it is a good electrolyte, it is used in the battery: (a).

Teacher's Note:
1. A battery needs an electrolyte to allow current to flow.
2. Strong acids are good electrolytes.

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

Explanation:
1. A is true: the product is sodium chloride, a normal salt.
2. R is true: HCl has one replaceable H and NaOH has one OH.
3. Since both have only one replaceable ion, no acidic or basic salt can form, so R explains A: (a).

Teacher's Note:
1. Only polybasic acids or polyacidic bases give other types of salts.
2. Write the equation: HCl + NaOH → NaCl + H₂O.

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

Explanation:
1. A is true: impurities dissolve in nitric acid and platinum is left behind.
2. R is true: platinum does not dissolve in nitric acid.
3. This insolubility is the reason it can be purified this way, so R explains A: (a).

Teacher's Note:
1. Platinum and gold are noble metals.
2. Impurities such as copper and silver dissolve in nitric acid.

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

Explanation:
1. A is true: \(\ce{CaOCl2}\) dissociates into Ca²⁺, Cl⁻ and OCl⁻, and each ion gives its own tests.
2. R is false: bleaching powder is a mixed salt, not a complex salt.
3. So A is true and R is false: (c).

Teacher's Note:
1. Mixed salt: one cation with two different anions (Cl⁻ and OCl⁻).
2. A complex salt keeps a complex ion together.