CBSE Class 7 Science Advanced Chemistry Acids, Bases and Salts MCQs Set 03

Answer: (c) Carbonic acid

Explanation:
1. Oxalic acid is dibasic, so it forms two salts: a normal salt and an acidic salt.
2. Carbonic acid is also dibasic and forms two salts. Nitric acid and hydrochloric acid are monobasic and form one, and phosphoric acid is tribasic and forms three.
3. The answer is carbonic acid.

Teacher's Note:
1. Number of salts from an acid = its basicity (one normal salt plus the acidic salts).
2. Match oxalic acid with another dibasic acid.

Answer: (b) Decomposition on heating

Explanation:
1. Both are monoacidic bases, both dissolve in water and both react with acids to give salts.
2. Ammonium hydroxide decomposes on heating to give ammonia and water, whereas caustic potash only melts.
3. So the difference is decomposition on heating.

Teacher's Note:
1. Test each option for both bases; the one that differs is the answer.
2. Ammonia gas escapes on heating ammonium hydroxide.

Answer: (b) Preparation of bleaching powder

Explanation:
1. Slaked lime neutralises acids, such as in betel leaves (pan) and acidic soil, and it reacts with carbon dioxide (an acidic oxide) when mortar sets.
2. Preparing bleaching powder is a reaction of slaked lime with chlorine, not neutralisation.
3. The answer is (b).

Teacher's Note:
1. Neutralisation means an acid and a base react to form a salt and water.
2. Making bleaching powder uses chlorine gas, not an acid.

Answer: (d) CDAB

Explanation:
1. Sodium acetate \(\ce{CH3COONa}\) has no replaceable hydrogen (0).
2. Potassium hydrogen sulphate \(\ce{KHSO4}\) has 1.
3. Sodium dihydrogen phosphate \(\ce{NaH2PO4}\) has 2.
4. Phosphoric acid \(\ce{H3PO4}\) has 3.
5. The increasing order is C, D, A, B: option (d).

Teacher's Note:
1. Replaceable hydrogens in acetic acid: only the one in the –COOH group, and it is already replaced in sodium acetate.
2. Count H that can still be replaced by a metal.

Answer: (a) CADB

Explanation:
1. (C) \(\ce{Na[Ag(CN)2] -> Na+ + [Ag(CN)2]-}\) gives 2 ions.
2. (A) \(\ce{CaOCl2 -> Ca^2+ + Cl- + OCl-}\) gives 3 ions.
3. (D) \(\ce{Fe2(SO4)3 -> 2Fe^3+ + 3SO4^2-}\) gives 5 ions.
4. (B) \(\ce{K2SO4.Al2(SO4)3 -> 2K+ + 2Al^3+ + 4SO4^2-}\) gives 8 ions.
5. The increasing order is C, A, D, B: option (a).

Teacher's Note:
1. Complex salts keep the complex ion together as one ion.
2. Add up all ions given by the dissociation.

Answer: (d) (i) → (C); (ii) → (E); (iii) → (D); (iv) → (A); (v) → (B)

Explanation:
1. Potash alum purifies water: (i) → (C).
2. Plaster of Paris is used for statues: (ii) → (E).
3. Sodium carbonate is used in glass making: (iii) → (D).
4. Sodium bicarbonate (baking soda) makes cakes rise: (iv) → (A).
5. Epsom salt is a laxative: (v) → (B).
6. This is option (d).

Teacher's Note:
1. Link each salt with its best-known use.
2. Epsom salt is magnesium sulphate.

Answer: (b) \(\ce{HPO3^2-}\)

Explanation:
1. Phosphorous acid \(\ce{H3PO3}\) is dibasic: only two of its hydrogens can be replaced.
2. In \(\ce{HPO3^2-}\) the remaining H is not replaceable, so this radical forms a normal salt such as \(\ce{Na2HPO3}\).
3. The other radicals still have a replaceable hydrogen and give acidic salts. The answer is (b).

Teacher's Note:
1. A normal salt has no replaceable hydrogen left.
2. In H₃PO₃ one H is bonded directly to P and cannot be replaced.

Answer: (d) \(\ce{NO}\)

Explanation:
1. Carbon dioxide, sulphur dioxide and sulphur trioxide are acidic oxides because they dissolve in water to form acids.
2. Nitric oxide (NO) is a neutral oxide.
3. The odd one is NO.

Teacher's Note:
1. Neutral oxides: CO, NO, N₂O and H₂O.
2. Non-metal oxides are mostly acidic.

Answer: (b) \(\ce{NaOH + H2SO4 -> NaHSO4 + H2O}\)

Explanation:
1. In partial neutralisation the product is still an acidic (or basic) salt that has a replaceable hydrogen.
2. NaHSO₄ still has one replaceable H⁺, so the reaction in (b) is partial neutralisation.
3. (a), (c) and (d) give normal salts, which means complete neutralisation. The answer is (b).

Teacher's Note:
1. Look for H in the salt formula, as in NaHSO₄.
2. Normal salt means complete neutralisation.

Answer: (a) \(\ce{Ca(XO3)2}\)

Explanation:
1. Configuration 2, 8, 7 is chlorine, and its '-ic' oxyacid is chloric acid \(\ce{HClO3}\).
2. The radical is \(\ce{ClO3-}\) with charge –1, and slaked lime gives \(\ce{Ca^2+}\).
3. The normal salt is \(\ce{Ca(ClO3)2}\), which is Ca(XO₃)₂.

Teacher's Note:
1. Cross the valencies: Ca²⁺ and XO₃⁻.
2. Chlorine gives chlorate (ClO₃⁻) as the '-ate' radical of the '-ic' acid.

Answer: (b) The solution changes from pink to colourless.

Explanation:
1. Phenolphthalein is pink in a base such as KOH.
2. When the base is neutralised by the acid, the solution is no longer basic and phenolphthalein turns colourless.
3. The answer is (b).

Teacher's Note:
1. Phenolphthalein: pink in base, colourless in acid.
2. Methyl orange is the indicator that changes between orange red and yellow.

Answer: (d) \(\ce{MgCl2}\), \(\ce{CoCl2}\)

Explanation:
1. X becomes sticky, so it is deliquescent: magnesium chloride is such a substance.
2. Y keeps its physical state after absorbing moisture, so it is hygroscopic: cobalt chloride absorbs water and turns pink while remaining solid.
3. The pair is MgCl₂, CoCl₂.

Teacher's Note:
1. Deliquescent substances become sticky or dissolve; hygroscopic ones keep their state.
2. Cobalt chloride is written CoCl₂ (Co for cobalt).

Answer: (c) Hygroscopic substance absorbs moisture without dissolving in it.

Explanation:
1. Both types of substance absorb moisture from air.
2. A deliquescent substance dissolves in the absorbed moisture; a hygroscopic substance does not.
3. The answer is (c).

Teacher's Note:
1. Loss of water of crystallisation is efflorescence, not hygroscopy.
2. Quicklime is hygroscopic and calcium chloride deliquescent.

Answer: (b) Calcium hydroxide and barium hydroxide

Explanation:
1. M(OH)(NO₃) is a basic salt: M²⁺ is joined to one OH⁻ and one NO₃⁻.
2. So M must come from a diacidic base: calcium hydroxide and barium hydroxide are both diacidic.
3. The answer is (b).

Teacher's Note:
1. Basic salts come from bases with two or more OH groups.
2. Ammonium and potassium hydroxides are monoacidic.

Answer: (d) Aluminium hydroxide and potassium hydroxide

Explanation:
1. Aluminium hydroxide decomposes on heating to give aluminium oxide and water.
2. Potassium hydroxide only melts on strong heating.
3. So X is aluminium hydroxide and Y is potassium hydroxide: (d).

Teacher's Note:
1. Insoluble hydroxides generally decompose on heating, while the strong alkalis NaOH and KOH do not.
2. Ammonium hydroxide decomposes to give ammonia, not an oxide.

Answer: (b) The product of the reaction is highly soluble in water.

Explanation:
1. X is calcium oxide, which with copper sulphate gives Bordeaux mixture.
2. Calcium oxide reacts with water to form slaked lime and releases heat. Slaked lime is used for white washing and as a disinfectant.
3. But slaked lime is only slightly soluble in water, so statement (b) is wrong: (b).

Teacher's Note:
1. Quicklime + water gives slaked lime with a lot of heat.
2. Limewater is a clear solution of the sparingly soluble Ca(OH)₂.

Answer: (c) DCBA

Explanation:
1. (D) \(\ce{K3PO4}\): 3 : 1 = 3.
2. (C) \(\ce{Na2C2O4}\): 2 : 1 = 2.
3. (B) \(\ce{Ca3(PO4)2}\): 3 : 2 = 1.5.
4. (A) \(\ce{BaC2O4}\): 1 : 1 = 1.
5. Descending order: D, C, B, A: option (c).

Teacher's Note:
1. Cross the valencies to get each formula first.
2. Convert each ratio into a number to compare.

Answer: (b) DACEB

Explanation:
1. Mixed salt: sodium potassium carbonate (D).
2. Acidic salt: disodium hydrogen phosphate (A), which has replaceable H.
3. Double salt: Mohr's salt (C).
4. Normal salt: ammonium oxalate (E).
5. Complex salt: potassium ferrocyanide (B).
6. The order is D, A, C, E, B: option (b).

Teacher's Note:
1. Match each salt with its type, then list them in the order of the types given.
2. A mixed salt has two different cations or two different anions.

Answer: (c) (i) → (C); (ii) → (A); (iii) → (B); (iv) → (E); (v) → (D)

Explanation:
1. Soap is made with sodium hydroxide: (i) → (C).
2. Magnesium hydroxide is an antacid (milk of magnesia): (ii) → (A).
3. Nitric acid is used for explosives: (iii) → (B).
4. Sulphuric acid is the battery acid: (iv) → (E).
5. Acetic acid (vinegar) preserves food: (v) → (D).
6. This is option (c).

Teacher's Note:
1. Think of the household use of each chemical.
2. Bases are used for soap and antacids; acids for batteries, explosives and pickles.

Answer: (b) Phosphorous acid

Explanation:
1. Acetic acid is monobasic: only a normal salt.
2. Phosphorous acid \(\ce{H3PO3}\) is dibasic: one normal salt and one acidic salt.
3. Phosphoric acid is tribasic: one normal and two acidic salts.
4. So the answer is phosphorous acid.

Teacher's Note:
1. Dibasic acid = 1 normal + 1 acidic salt.
2. Tribasic acid = 1 normal + 2 acidic salts.

Answer: (d) Both (a) and (c)

Explanation:
1. 2, 4 is carbon, whose oxide CO is neutral.
2. 2, 5 is nitrogen, whose oxide NO is neutral.
3. 2, 8, 6 is sulphur, whose oxides are acidic.
4. So both (a) and (c) are correct: option (d).

Teacher's Note:
1. Neutral oxides: CO, NO, N₂O.
2. Work out the element from its configuration first.

Answer: (b) Nitric acid and potassium hydroxide

Explanation:
1. Partial neutralisation needs either a polybasic acid (giving an acidic salt) or a polyacidic base (giving a basic salt).
2. Nitric acid is monobasic and potassium hydroxide is monoacidic, so only a normal salt can form.
3. The answer is (b).

Teacher's Note:
1. Both partners must have only one replaceable ion for no partial salt.
2. Ca(OH)₂ is diacidic and H₂SO₄ is dibasic.

Answer: (b) \(\ce{M3(XO4)2}\)

Explanation:
1. M has 2 valence electrons, so it forms M²⁺.
2. The acid \(\ce{H3XO4}\) has three replaceable H, so the radical is \(\ce{XO4^3-}\).
3. Cross the valencies: M²⁺ with XO₄³⁻ gives \(\ce{M3(XO4)2}\): option (b).

Teacher's Note:
1. Cross the charges: 2 goes to the radical and 3 goes to the metal.
2. Put brackets around the radical when its subscript is more than 1.

Answer: (d) Pink to yellow

Explanation:
1. Methyl orange is pink (orange red) in an acid such as HCl.
2. On neutralisation by NaOH the solution is no longer acidic and the indicator changes to yellow.
3. The change is pink to yellow.

Teacher's Note:
1. Methyl orange: red or pink in acid, yellow in base.
2. Phenolphthalein is the one that goes colourless and pink.

Answer: (c) \(\ce{Na2CO3.10H2O}\)

Explanation:
1. The substance first loses 9 water molecules on exposure to air (efflorescence) and then loses 1 more on heating.
2. Washing soda \(\ce{Na2CO3.10H2O}\) loses 9 H₂O in air to become \(\ce{Na2CO3.H2O}\), which loses the last H₂O on heating.
3. The answer is (c).

Teacher's Note:
1. Count the total water molecules lost: 9 + 1 = 10.
2. Washing soda is efflorescent; copper sulphate is not.

Answer: (d) Copper sulphate loses water of crystallization on heating.

Explanation:
1. The blue colour of copper sulphate crystals is due to the water of crystallisation in \(\ce{CuSO4.5H2O}\).
2. On heating it loses this water and becomes anhydrous copper sulphate, which has no blue colour (it is white).
3. The answer is (d).

Teacher's Note:
1. Blue vitriol turns white on heating and blue again when water is added.
2. Copper sulphate is not efflorescent: it does not lose water in open air.

Answer: (a) ACB

Explanation:
1. Phosphoric acid \(\ce{H3PO4}\) has basicity 3.
2. Sulphurous acid \(\ce{H2SO3}\) has basicity 2.
3. Hydrochloric acid HCl has basicity 1.
4. The decreasing order is A, C, B: option (a).

Teacher's Note:
1. Basicity is the number of replaceable hydrogens.
2. Take care that sulphurous acid is H₂SO₃, not H₂SO₄.

Answer: (d) BCDA

Explanation:
1. In an acidic solution litmus is red, methyl orange is pink, phenolphthalein is colourless and turmeric is yellow.
2. So the indicators in the order red, pink, colourless, yellow are B, C, D, A: option (d).

Teacher's Note:
1. Learn each indicator's acid and base colours as a table.
2. Turmeric is yellow in acid and red in base.

Answer: (b) (i) → (C); (ii) → (E); (iii) → (A); (iv) → (B)

Explanation:
1. Sodium chloride is a food additive: (i) → (C).
2. Sodium bicarbonate is used in fire extinguishers: (ii) → (E).
3. Washing soda is used in detergents: (iii) → (A).
4. Potassium nitrate is a fertiliser: (iv) → (B).
5. This is option (b).

Teacher's Note:
1. Sodium bicarbonate releases carbon dioxide, which puts out fire.
2. Column B has two extra entries that do not match anything.

Answer: (b) KOH

Explanation:
1. Caustic soda is NaOH.
2. Caustic potash is KOH.

Teacher's Note:
1. Soda goes with sodium and potash goes with potassium.
2. Ca(OH)₂ is slaked lime.

Answer: (b) tartaric acid

Explanation:
1. Grapes and tamarind contain tartaric acid.
2. Lactic acid is in curd, and oxalic acid is in spinach and tomato.
3. The answer is tartaric acid.

Teacher's Note:
1. Remember natural acids: citric in lemon, malic in apple, tartaric in grapes.
2. Stearic acid is a fatty acid in fats.

Answer: (b) normal salt

Explanation:
1. AlCl₃ is formed from aluminium hydroxide and hydrochloric acid with all OH⁻ replaced by Cl⁻.
2. It has no replaceable H⁺ or OH⁻ left, so it is a normal salt.

Teacher's Note:
1. Normal salt = no replaceable H or OH.
2. Acidic salts contain H, basic salts contain OH.

Answer: (a) \(\ce{NaH2PO4}\)

Explanation:
1. One NaOH replaces only one H of H₃PO₄.
2. The reaction is \(\ce{H3PO4 + NaOH -> NaH2PO4 + H2O}\), giving NaH₂PO₄.

Teacher's Note:
1. Each NaOH replaces one hydrogen.
2. Two NaOH would give Na₂HPO₄ and three would give Na₃PO₄.

Answer: (c) Colour changes in A but not in B.

Explanation:
1. Sodium sulphate decahydrate is efflorescent: it loses water molecules, which are taken up by anhydrous blue cobalt chloride to form pink hydrated cobalt chloride.
2. Copper sulphate pentahydrate is not efflorescent and does not give out water, so cobalt chloride stays blue.
3. The colour changes in A but not in B: option (c).

Teacher's Note:
1. Cobalt chloride is a water indicator: blue when dry, pink when wet.
2. Only an efflorescent salt gives out its water.

Answer: (b) calcium chloride, magnesium chloride

Explanation:
1. Pure sodium chloride is not deliquescent.
2. The impurities calcium chloride and magnesium chloride in common salt are deliquescent, so impure salt becomes damp in humid weather.
3. The answer is (b).

Teacher's Note:
1. This is why common salt gets sticky in the rainy season.
2. Table salt has anti-caking agents added.

Answer: (a) calcium sulphate hemihydrate

Explanation:
1. Plaster of Paris is \(\mathrm{CaSO_4\cdot \tfrac{1}{2}H_2O}\).
2. It is calcium sulphate (not sulphite) with half a molecule of water, so it is calcium sulphate hemihydrate.
3. Calcium sulphate dihydrate is gypsum.

Teacher's Note:
1. Hemi means half; di means two.
2. Mixed with water it sets back to gypsum.

Answer: (d) Nitrous acid

Explanation:
1. Acids ending in -ic give -ate salts: phosphate, sulphate, carbonate.
2. Acids ending in -ous give -ite salts. Nitrous acid \(\ce{HNO2}\) gives nitrites.
3. The answer is nitrous acid.

Teacher's Note:
1. -ic acid → -ate salt; -ous acid → -ite salt.
2. Nitric acid (HNO₃) gives nitrates.

Answer: (a) NaOH

Explanation:
1. Turmeric turns red in bases and stays yellow in acids and neutral solutions.
2. Only sodium hydroxide, a base, turns it red: NaOH.

Teacher's Note:
1. This is why soap stains on yellow turmeric-stained clothes turn red.
2. NaCl and sugar are neutral.

Answer: (b) \(\ce{H3PO4}\)

Explanation:
1. Configuration 2, 8, 5 is phosphorus.
2. The '-ic' acid is phosphoric acid \(\ce{H3PO4}\), while \(\ce{H3PO3}\) is phosphorous acid.
3. The answer is (b).

Teacher's Note:
1. Find the element first, then the acid.
2. Sulphur acids are for 2, 8, 6.

Answer: (c) ACB

Explanation:
1. Acidity is the number of OH groups.
2. NaOH has 1, Ca(OH)₂ has 2 and Al(OH)₃ has 3.
3. The increasing order is A, C, B: option (c).

Teacher's Note:
1. Acidity of a base is the number of hydroxide ions it gives per formula unit.
2. Do not confuse it with basicity, which is for acids.