CBSE Class 7 Science Advanced Chemistry Acids, Bases and Salts MCQs Set 02

Answer: (c) glass vessels or plastic vessels

Explanation:
1. Pickles contain organic acids, which react with metals such as tin and aluminium and spoil the pickle.
2. Glass and plastic do not react with these acids.
3. So pickles are stored in glass vessels or plastic vessels.

Teacher's Note:
1. Acids attack metal containers.
2. Choose the option in which neither container is a metal.

Answer: (b) the decomposition of HNO3 and formation of \(\ce{NO2}\)

Explanation:
1. Pure nitric acid is unstable and slowly decomposes: \(\ce{4HNO3 -> 4NO2 + 2H2O + O2}\).
2. The reddish-brown gas \(\ce{NO2}\) dissolves in the acid and gives it a yellow colour.
3. So the answer is decomposition and formation of \(\ce{NO2}\).

Teacher's Note:
1. It is a decomposition, not oxidation by air.
2. \(\ce{NO2}\) is brown and NO is colourless.

Answer: (a) formation of soluble salts

Explanation:
1. Sulphuric acid reacts with bases to form sulphate salts that dissolve in water.
2. These soluble salts can be taken up by plants, so the property exploited is the formation of soluble salts.

Teacher's Note:
1. Match each use with the property column of the uses table.
2. Car batteries use the electrolyte property instead.

Answer: (d) carbonic acid

Explanation:
1. Sulphuric, hydrochloric and nitric acids ionize completely, so they are strong.
2. Carbonic acid ionizes only partially, so it is the weak acid.

Teacher's Note:
1. The three strong mineral acids are the usual trio.
2. Carbonic acid is the common weak mineral acid.

Answer: (b) \(\ce{Ca(HCO3)2}\)

Explanation:
1. The configuration 2, 8, 8, 2 has 20 electrons, so the metal is calcium, \(\ce{Ca^2+}\).
2. The acid used in soft drinks is carbonic acid, so the negative radical is carbonate or bicarbonate.
3. The salt reacts with one molecule of base, so it has replaceable hydrogen: the acidic salt \(\ce{Ca(HCO3)2}\).
4. \(\ce{CaCO3}\) is a normal salt and does not react with a base.

Teacher's Note:
1. Count the electrons to find the metal.
2. A salt that reacts with a base must contain H.

Answer: (c) Aqueous solutions of \(\ce{SO3}\) and \(\ce{Na2O}\) turn blue litmus to red and red litmus to blue, respectively.

Explanation:
1. \(\ce{SO3 + H2O -> H2SO4}\) is acidic, so it turns blue litmus to red.
2. \(\ce{Na2O + H2O -> 2NaOH}\) is basic, so it turns red litmus to blue.
3. This matches option (c).

Teacher's Note:
1. Nonmetal oxide gives an acid; metal oxide gives a base.
2. Acid turns blue litmus red, base turns red litmus blue.

Answer: (a) hygroscopic

Explanation:
1. Cobalt chloride absorbs moisture from air and becomes pink hydrated \(\ce{CoCl2.2H2O}\).
2. It remains a solid and does not dissolve, so it is not deliquescent.
3. It gains water instead of losing it, so it is not efflorescent.
4. So it is hygroscopic.

Teacher's Note:
1. Absorbs moisture and stays solid means hygroscopic.
2. Cobalt chloride paper is used to test for moisture.

Answer: (b) aluminium sulphate, \(\ce{Al2(SO4)3}\)

Explanation:
1. \(\ce{2Al(OH)3 + 3H2SO4 -> Al2(SO4)3 + 6H2O}\)
2. The 'ic' acid is sulphuric acid, so the salt is a sulphate with the formula \(\ce{SO4^2-}\).
3. The salt is aluminium sulphate, \(\ce{Al2(SO4)3}\).
4. Options (c) and (d) mismatch the name and the formula.

Teacher's Note:
1. The name and formula must agree: sulphate with \(\ce{SO4}\), sulphite with \(\ce{SO3}\).
2. Balance the charges: 2 Al3+ and 3 \(\ce{SO4}\) 2-.

Answer: (d) \(\ce{H2SO3}\), \(\ce{H2CO3}\)

Explanation:
1. Gunpowder contains \(\ce{KNO3}\), carbon and sulphur. Oxygen from nitre burns carbon and sulphur.
2. The gases formed are \(\ce{CO2}\) and \(\ce{SO2}\).
3. \(\ce{CO2 + H2O -> H2CO3}\) and \(\ce{SO2 + H2O -> H2SO3}\), so the answer is \(\ce{H2SO3}\) and \(\ce{H2CO3}\).

Teacher's Note:
1. Burning carbon gives \(\ce{CO2}\) and burning sulphur gives \(\ce{SO2}\).
2. Nitre only supplies oxygen here.

Answer: (b) tin is having high reactivity towards food components

Explanation:
1. The organic acids in pickles react with tin and form harmful compounds.
2. Cost and density have nothing to do with it.
3. So the reason is the high reactivity of tin towards food components.

Teacher's Note:
1. Think of what the acids in the pickle do to a metal.
2. The harmful compounds make the pickle unsafe.

Answer: (b) \(\ce{O2}\), \(\ce{NO2}\) and \(\ce{H2O}\)

Explanation:
1. \(\ce{4HNO3 -> 4NO2 + 2H2O + O2}\)
2. The products are nitrogen dioxide, water and oxygen.
3. The brown \(\ce{NO2}\) dissolves and makes the acid yellow, so the answer is \(\ce{O2}\), \(\ce{NO2}\) and \(\ce{H2O}\).

Teacher's Note:
1. Balance the equation to see all three products.
2. \(\ce{NO2}\) is the coloured one.

Answer: (d) nitric acid

Explanation:
1. Nitric acid is a good oxidising agent.
2. It dissolves the impurities on the metal, while gold does not dissolve in it, so the answer is nitric acid.

Teacher's Note:
1. Nitric acid and purification of noble metals go together.
2. Remember the property: gold and platinum are insoluble in it.

Answer: (c) sulphuric acid

Explanation:
1. Phosphoric, carbonic and acetic acids are weak acids.
2. Sulphuric acid ionizes completely: \(\ce{H2SO4 -> 2H+ + SO4^2-}\).
3. So the strong acid is sulphuric acid.

Teacher's Note:
1. Learn the three strong mineral acids.
2. All organic acids are weak.

Answer: (c) calcium sulphate

Explanation:
1. \(\ce{Ca(OH)2 + H2SO4 -> CaSO4 + 2H2O}\)
2. Complete neutralization gives the normal salt, calcium sulphate.
3. Sulphite comes from sulphurous acid, and 'bi' salts keep a hydrogen.

Teacher's Note:
1. Sulphuric acid gives sulphates.
2. Complete neutralization leaves no H or OH.

Answer: (a) sodium nitrate and potassium nitrate

Explanation:
1. Chile saltpetre is the common name of sodium nitrate, \(\ce{NaNO3}\).
2. Nitre is the common name of potassium nitrate, \(\ce{KNO3}\).
3. So the answer is sodium nitrate and potassium nitrate.

Teacher's Note:
1. Chile is a country of sodium nitrate deposits; link Chile with Na.
2. Nitre and \(\ce{KNO3}\) go together.

Answer: (d) calcium carbonate

Explanation:
1. Magnesium hydroxy chloride, \(\ce{Mg(OH)Cl}\), has a replaceable OH.
2. Sodium bicarbonate, \(\ce{NaHCO3}\), and disodium hydrogen phosphate, \(\ce{Na2HPO4}\), have replaceable H.
3. Calcium carbonate, \(\ce{CaCO3}\), has neither, so it is the normal salt required.

Teacher's Note:
1. Look for H or OH in each formula.
2. A normal salt has none.

Answer: (c) acetic acid

Explanation:
1. Carbonic and oxalic acids are weak but dibasic, and nitric acid is monobasic but strong.
2. Acetic acid, \(\ce{CH3COOH}\), is both weak and monobasic, so the answer is acetic acid.

Teacher's Note:
1. Test two properties for each option: basicity and strength.
2. Acetic acid has just one replaceable hydrogen.

Answer: (c) sulphur trioxide

Explanation:
1. \(\ce{SO3 + H2O -> H2SO4}\) gives only the strong sulphuric acid.
2. Sulphur dioxide and carbon dioxide give weak acids, and nitrogen dioxide gives a mixture of nitrous and nitric acids.
3. So the answer is sulphur trioxide.

Teacher's Note:
1. The word 'only' rules out oxides that give two acids or a weak acid.
2. \(\ce{SO3}\) is the top acid former in the list.

Answer: (b) mobile ions

Explanation:
1. Salts dissociate into ions in water.
2. The ions are free to move and carry the current, so the answer is mobile ions.

Teacher's Note:
1. Electrolytes conduct by ions.
2. Metals conduct by free electrons.

Answer: (a) 25341

Explanation:
1. Acidic salt has replaceable H, so it reacts further with a base: statement 2.
2. Basic salt has replaceable OH, so it reacts further with an acid: statement 5.
3. Normal salt has one basic and one acidic radical and reacts with neither: statement 3.
4. Double salt is a combination of two simple salts: statement 4.
5. Mixed salt such as \(\ce{NaKCO3}\) has two basic radicals and one acidic radical: statement 1.
6. The order is 2, 5, 3, 4, 1.

Teacher's Note:
1. Match each salt type with its defining statement one at a time.
2. Mixed salts have two different basic or acidic radicals.

Answer: (a) 231

Explanation:
1. Replacement of hydrogen goes one step at a time.
2. First one H is replaced: \(\ce{H3PO4}\) becomes \(\ce{NaH2PO4}\) (2). Then the second: \(\ce{NaH2PO4}\) becomes \(\ce{Na2HPO4}\) (3). Then the third gives the normal salt \(\ce{Na3PO4}\) (1).
3. The sequence is 2, 3, 1.

Teacher's Note:
1. Count the sodium atoms: 1, 2, 3.
2. The last step gives the normal salt.

Answer: (d) All the above

Explanation:
1. Slaked lime neutralizes acidic soil.
2. Antacid tablets neutralize extra acid in the stomach.
3. Lithium hydroxide neutralizes exhaled \(\ce{CO2}\) in submarines.
4. All three apply neutralization, so the answer is all the above.

Teacher's Note:
1. In each case a base removes an acid.
2. When every statement is true, choose 'all the above'.

Answer: (b) A→d, B→c, C→a, D→b

Explanation:
1. \(\ce{CuSO4}\) has no water of crystallization, so it is an anhydrous salt (d).
2. \(\ce{Na2CO3.10H2O}\) loses water on exposure to air, so it is efflorescent (c).
3. Concentrated \(\ce{H2SO4}\) absorbs moisture and stays a liquid, so it is hygroscopic (a).
4. \(\ce{ZnCl2}\) absorbs moisture and dissolves, so it is deliquescent (b).
5. So A-d, B-c, C-a, D-b, which is option (b).

Teacher's Note:
1. Match the easy ones first and eliminate options.
2. Washing soda is the standard efflorescent example.

Answer: (a) to supply oxygen

Explanation:
1. Nitre, \(\ce{KNO3}\), gives \(\ce{KNO2}\) and oxygen on heating.
2. The oxygen supports the burning of carbon and sulphur, so the role of nitre is to supply oxygen.

Teacher's Note:
1. Nitre is an oxygen supplier for fast burning.
2. Gunpowder explodes because the burning is very fast.

Answer: (c) sodium

Explanation:
1. A base soluble in water is an alkali.
2. Aluminium, copper and manganese hydroxides are insoluble, but sodium hydroxide is freely soluble.
3. So the metal may be sodium.

Teacher's Note:
1. Alkali metals form soluble hydroxides.
2. Transition metal hydroxides are insoluble.

Answer: (d) carbonic acid

Explanation:
1. Sulphuric, hydrochloric and nitric acids are strong acids.
2. Carbonic acid ionizes only partially, so it is the weak acid.

Teacher's Note:
1. Strong acids ionize completely.
2. Carbonic acid is the weak mineral acid you must remember.

Answer: (b) \(\ce{Al2(SO4)3}\)

Explanation:
1. \(\ce{2Al(OH)3 + 3H2SO4 -> Al2(SO4)3 + 6H2O}\)
2. Complete neutralization gives the normal sulphate: \(\ce{Al2(SO4)3}\).
3. Options (c) and (d) contain hydrogen, so they are acidic salts.

Teacher's Note:
1. Sulphuric acid gives sulphate with \(\ce{SO4}\).
2. A hydrogen in the formula means partial neutralization.

Answer: (d) all the above

Explanation:
1. Both sodium nitrate and potassium nitrate are used to make gunpowder, as fertilizers and in glass making.
2. So the answer is all the above.

Teacher's Note:
1. The common uses are the ones that apply to both salts.
2. Nitrates also supply nitrogen to plants.

Answer: (b) sodium dihydrogen phosphate and Mohr’s salt

Explanation:
1. Sodium dihydrogen phosphate, \(\ce{NaH2PO4}\), still has replaceable H, so it is an acidic salt.
2. Mohr’s salt, \(\ce{FeSO4.(NH4)2SO4.6H2O}\), is a combination of two simple salts, so it is a double salt.
3. So the pair is (b).

Teacher's Note:
1. Potash alum is also a double salt, but sodium phosphate is a normal salt.
2. Calcium hydroxy chloride is a basic salt.

Answer: (d) sulphurous acid

Explanation:
1. Acetic acid is organic; sulphuric acid is strong; phosphoric acid is tribasic.
2. Sulphurous acid, \(\ce{H2SO3}\), is mineral, weak and dibasic, so the answer is sulphurous acid.

Teacher's Note:
1. Check three things: origin, strength and basicity.
2. Only \(\ce{H2SO3}\) matches all three.

Answer: (b) carbon

Explanation:
1. Carbon forms \(\ce{CO2}\) and CO.
2. \(\ce{CO2 + H2O -> H2CO3}\) gives an acid, whereas CO is neutral.
3. So the nonmetal is carbon.

Teacher's Note:
1. Only one of the two oxides gives an acid.
2. CO is a neutral oxide.

Answer: (d) all the above

Explanation:
1. Acids conduct only when ions are present.
2. Pure sulphuric acid, pure nitric acid and liquefied \(\ce{HCl}\) are not ionized without water.
3. So all of them are poor conductors: all the above.

Teacher's Note:
1. No water means no ions.
2. Add water and the acid ionizes and conducts.

Answer: (c) 4213

Explanation:
1. Complex salt: \(\ce{K4[Fe(CN)6]}\) has a complex ion (4).
2. Basic salt: \(\ce{Ca(OH)Cl}\) retains an OH (2).
3. Normal salt: \(\ce{Ca3(PO4)2}\) has no H or OH (1).
4. Mixed salt: bleaching powder \(\ce{CaOCl2}\) has two acid radicals (3).
5. The order is 4, 2, 1, 3.

Teacher's Note:
1. Identify each salt type one by one.
2. Bleaching powder is the standard mixed salt.

Answer: (b) 231

Explanation:
1. Each step replaces one more OH by Cl.
2. First \(\ce{Al(OH)3}\) becomes \(\ce{Al(OH)2Cl}\) (2), then \(\ce{Al(OH)Cl2}\) (3), and finally the normal salt \(\ce{AlCl3}\) (1).
3. The sequence is 2, 3, 1.

Teacher's Note:
1. Count the Cl atoms in the product: 1, 2, 3.
2. The normal salt comes last.

Answer: (b) \(\ce{Mg(OH)2}\), \(\ce{Al(OH)3}\)

Explanation:
1. Antacids are mild bases that neutralize excess stomach acid.
2. \(\ce{NaOH}\) and \(\ce{KOH}\) are corrosive and unsafe to swallow.
3. So the pair is \(\ce{Mg(OH)2}\) and \(\ce{Al(OH)3}\).

Teacher's Note:
1. Milk of magnesia is \(\ce{Mg(OH)2}\).
2. Strong alkalies are never used as antacids.

Answer: (a) A→d, B→a, C→b, D→c

Explanation:
1. CaO absorbs moisture and stays solid, so it is hygroscopic (d).
2. \(\ce{MgCl2.6H2O}\) absorbs more moisture and turns into a solution, so it is deliquescent (a).
3. \(\ce{FeSO4.7H2O}\) is hydrated but not deliquescent (b).
4. \(\ce{MgSO4}\) has no water of crystallization, so it is anhydrous (c).
5. So A-d, B-a, C-b, D-c, which is option (a).

Teacher's Note:
1. Eliminate options by fixing A and C first.
2. Magnesium chloride is a classic deliquescent salt.

Answer: (d) \(\ce{H2SO3}\), \(\ce{H2CO3}\)

Explanation:
1. Carbon and sulphur of gunpowder burn in the oxygen from nitre.
2. They form \(\ce{CO2}\) and \(\ce{SO2}\).
3. \(\ce{CO2 + H2O -> H2CO3}\) and \(\ce{SO2 + H2O -> H2SO3}\), so the answer is \(\ce{H2SO3}\) and \(\ce{H2CO3}\).

Teacher's Note:
1. Gunpowder gives \(\ce{CO2}\) and \(\ce{SO2}\) on burning.
2. Their acids are carbonic and sulphurous.

Answer: (c) sodium oxide, potassium oxide

Explanation:
1. Sodium oxide and potassium oxide dissolve in water to form \(\ce{NaOH}\) and \(\ce{KOH}\).
2. Copper oxide and aluminium oxide do not dissolve, so the pair is sodium oxide and potassium oxide.

Teacher's Note:
1. Both members of the pair must be soluble.
2. Oxides of Na and K are the safest soluble ones.

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

Explanation:
1. The assertion is true: pickles are kept in glass or plastic containers.
2. The reason is also true: tin reacts with the organic acids in pickles and forms harmful compounds.
3. The reason explains the assertion, so option (a) is correct.

Teacher's Note:
1. Always check whether R really explains A.
2. Here the chemical reaction of tin is exactly why it is avoided.

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

Explanation:
1. The assertion is true: old pure nitric acid becomes yellow.
2. The reason is true: \(\ce{4HNO3 -> 4NO2 + 2H2O + O2}\).
3. The brown \(\ce{NO2}\) dissolved in the acid gives the yellow colour, so R explains A: (a).

Teacher's Note:
1. Decomposition is the cause of the colour change.
2. \(\ce{NO2}\) is a reddish-brown gas.

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

Explanation:
1. The assertion is true: glass and plastic are the usual containers for pickles.
2. The reason is false: the components of pickles are inert towards glass and plastic. That is why they are safe to use.
3. So A is true and R is false: (c).

Teacher's Note:
1. Read the reason carefully; it says the opposite of what is true.
2. Glass and plastic do not react with the acids of pickles.

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

Explanation:
1. The assertion is true: pure sulphuric acid is colourless.
2. The reason is false: sulphuric acid is a stable acid and does not decompose on long standing.
3. So A is true and R is false: (c).

Teacher's Note:
1. Compare with nitric acid, which does decompose.
2. Sulphuric acid is stable, which is why it is used in batteries.