CBSE Class 7 Science Advanced Chemistry Acids, Bases and Salts MCQs Set 01

Answer: (d) ascorbic acid

Explanation:
1. Acetic acid is in vinegar, and oleic and stearic acids are found in oils and fats.
2. Lemons contain ascorbic acid (vitamin C) along with citric acid.

Teacher's Note:
1. Match each acid with its natural source: vinegar, lemon, curd, tamarind.
2. Learn the source table in the notes; it is asked often.

Answer: (c) carbonic acid

Explanation:
1. Palmitic acid and acetic acid are weak, but they are organic acids (obtained from living things).
2. Hydrochloric acid is a mineral acid, but it is strong.
3. Carbonic acid, \(\ce{H2CO3}\), is a mineral acid that ionizes only partly, so it is the weak mineral acid.

Teacher's Note:
1. An answer must satisfy both words: weak and mineral.
2. Carbonic acid is the usual example of a weak mineral acid.

Answer: (b) two

Explanation:
1. Sulphuric acid is dibasic, so one molecule gives 2 \(\ce{H+}\) ions.
2. Each \(\ce{NaOH}\) gives one \(\ce{OH-}\) ion, so two \(\ce{NaOH}\) are needed to neutralize both.
3. \(\ce{2NaOH + H2SO4 -> Na2SO4 + 2H2O}\), so the answer is two.

Teacher's Note:
1. The number of \(\ce{NaOH}\) molecules equals the basicity of the acid.
2. With only one \(\ce{NaOH}\), an acidic salt such as \(\ce{NaHSO4}\) would form.

Answer: (b) sodium hydroxide, potassium hydroxide

Explanation:
1. Hydroxides of sodium and potassium only melt on strong heating and do not decompose.
2. Calcium hydroxide and copper hydroxide break down into the oxide and water.
3. So the pair that does not decompose is sodium hydroxide and potassium hydroxide.

Teacher's Note:
1. \(\ce{NaOH}\) and \(\ce{KOH}\) are the heat-stable hydroxides.
2. Every pair that contains \(\ce{Ca(OH)2}\) or \(\ce{Cu(OH)2}\) can be rejected.

Answer: (d) nitric acid

Explanation:
1. Acetic, sulphurous and carbonic acids are weak because they ionize only partly.
2. Nitric acid ionizes completely: \(\ce{HNO3 -> H+ + NO3-}\).
3. So the strong acid is nitric acid.

Teacher's Note:
1. The three common strong mineral acids are \(\ce{HCl}\), \(\ce{H2SO4}\) and \(\ce{HNO3}\).
2. All organic acids are weak.

Answer: (b) lime water

Explanation:
1. Phenolphthalein turns pink only in a basic solution.
2. Lime water is a solution of calcium hydroxide, a base.
3. Soda water is acidic, while common salt and sugar solutions are neutral, so the answer is lime water.

Teacher's Note:
1. Phenolphthalein: colourless in acid, pink in base.
2. Soda water contains carbonic acid, so it stays colourless.

Answer: (a) hydrated sodium carbonate

Explanation:
1. Washing soda has the formula \(\ce{Na2CO3.10H2O}\).
2. The 10 molecules of water of crystallization make it a hydrated sodium carbonate.

Teacher's Note:
1. Hydrated means water of crystallization is present in the formula.
2. Magnesium sulphate is Epsom salt, not washing soda.

Answer: (c) basic

Explanation:
1. When only some of the \(\ce{OH-}\) ions of a base are replaced, at least one \(\ce{OH-}\) is left in the salt.
2. A salt that still has a replaceable \(\ce{OH-}\) is a basic salt, for example \(\ce{Ca(OH)Cl}\).

Teacher's Note:
1. Leftover H+ gives an acidic salt, and leftover OH- gives a basic salt.
2. Normal salts have neither left.

Answer: (c) green vitriol

Explanation:
1. Marble is \(\ce{CaCO3}\) and baking soda is \(\ce{NaHCO3}\); neither has water of crystallization.
2. Green vitriol is \(\ce{FeSO4.7H2O}\), which holds 7 water molecules.
3. So the hydrated salt is green vitriol.

Teacher's Note:
1. Look for a dot followed by a number of \(\ce{H2O}\) in the formula.
2. Vitriols (blue, green, white) are all hydrated sulphates.

Answer: (a) potash alum

Explanation:
1. Potash alum, \(\ce{K2SO4.Al2(SO4)3.24H2O}\), makes the dirt particles in water settle down.
2. Epsom salt is a laxative, green vitriol is used for ink and blue vitriol is a fungicide.
3. So the answer is potash alum.

Teacher's Note:
1. Alum is the salt that clears muddy water.
2. Learn the uses table of salts by heart.

Answer: (c) acetic acid

Explanation:
1. Carbonic acid and oxalic acid are dibasic and weak.
2. Nitric acid is monobasic but strong.
3. Acetic acid, \(\ce{CH3COOH}\), gives one \(\ce{H+}\) ion and ionizes partly, so it is the monobasic weak acid.

Teacher's Note:
1. Check basicity first, then strength.
2. Acetic acid is the standard example of a monobasic weak acid.

Answer: (c) sulphur trioxide

Explanation:
1. \(\ce{SO2 + H2O -> H2SO3}\) gives a weak acid, and \(\ce{CO2 + H2O -> H2CO3}\) also gives a weak acid.
2. Nitrogen oxide (NO) is a neutral oxide and gives no acid.
3. \(\ce{SO3 + H2O -> H2SO4}\) gives the strong acid, so the answer is sulphur trioxide.

Teacher's Note:
1. Sulphur trioxide leads to sulphuric acid.
2. Sulphur dioxide leads only to sulphurous acid.

Answer: (d) All the above.

Explanation:
1. Slaked lime neutralizes the acidity of soil.
2. Antacid tablets neutralize excess acid in the stomach.
3. Lithium hydroxide absorbs the acidic gas \(\ce{CO2}\) exhaled in a submarine.
4. All three are uses of neutralization, so the answer is all the above.

Teacher's Note:
1. Neutralization means acid + base giving salt and water.
2. Test each statement and see whether an acid and a base are involved.

Answer: (b) mobile ions

Explanation:
1. Salts split into positive and negative ions when dissolved in water.
2. Electric current is carried by these moving ions, not by electrons or neutral molecules.
3. So the answer is mobile ions.

Teacher's Note:
1. Solutions conduct by ions; metals conduct by electrons.
2. A substance whose solution conducts is called an electrolyte.

Answer: (d) calcium oxide

Explanation:
1. Calcium chloride, magnesium chloride and zinc chloride are deliquescent: they absorb moisture and turn into a solution.
2. Calcium oxide is hygroscopic: it absorbs moisture and stays a solid.
3. So the answer is calcium oxide.

Teacher's Note:
1. Hygroscopic substances stay solid, deliquescent ones dissolve.
2. The words 'without changing its physical state' decide the answer.

Answer: (c) magnesium

Explanation:
1. The hydroxides of aluminium, copper and manganese are insoluble in water.
2. Magnesium hydroxide dissolves slightly, and the book treats magnesium oxide as water soluble.
3. So the metal may be magnesium.

Teacher's Note:
1. Hydroxides of sodium and potassium are the freely soluble ones.
2. Among the four metals given, magnesium is the only one whose base dissolves at all.

Answer: (b) 4132

Explanation:
1. Sulphur burns in oxygen to give \(\ce{SO2}\): this is step 4.
2. \(\ce{SO2}\) is oxidised to \(\ce{SO3}\): step 1.
3. \(\ce{SO3}\) dissolves in water to give \(\ce{H2SO4}\), which turns blue litmus red: step 3.
4. Sulphuric acid fully reacts with \(\ce{KOH}\) to give neutral \(\ce{K2SO4}\): step 2.
5. The sequence is 4, 1, 3, 2.

Teacher's Note:
1. Follow the element: S, then \(\ce{SO2}\), \(\ce{SO3}\), \(\ce{H2SO4}\), and finally the salt.
2. A neutral salt gives no response to litmus.

Answer: (b) 321645

Explanation:
1. Phosphoric acid and aluminium hydroxide have basicity and acidity 3; oxalic acid and barium hydroxide have 2; acetic acid and ammonium hydroxide have 1.
2. Decreasing order of the pairs: 3, 2, 1.
3. Their normal salts in the same order: aluminium phosphate (6), barium oxalate (4), ammonium acetate (5).
4. The sequence is 3 2 1 6 4 5.

Teacher's Note:
1. First rank the pairs by the number 3, 2, 1.
2. Then write the salt of each pair in that same order.

Answer: (d) stearic acid

Explanation:
1. Hydroiodic, nitrous and carbonic acids are made from minerals, so they are mineral acids.
2. Stearic acid is obtained from living sources such as mustard oil, so it is an organic acid.

Teacher's Note:
1. Organic acids come from plants and animals.
2. Fatty acids such as stearic, oleic and palmitic acids are organic.

Answer: (c) \(\ce{HNO3}\) and \(\ce{Al(OH)3}\)

Explanation:
1. In (a), \(\ce{HCl}\) is a strong acid but \(\ce{KOH}\) is a strong base.
2. In (b), both are bases. In (d), carbonic acid is weak, so it fails the first test.
3. \(\ce{HNO3}\) is a strong acid and \(\ce{Al(OH)3}\) is a weak base, so the answer is (c).

Teacher's Note:
1. Strong acids: \(\ce{HCl}\), \(\ce{H2SO4}\), \(\ce{HNO3}\). Strong bases: \(\ce{NaOH}\), \(\ce{KOH}\).
2. Check the first member and then the second member of each pair.

Answer: (c) tribasic acid

Explanation:
1. \(\ce{H3PO4 <=> 3H+ + PO4^3-}\): one molecule gives three \(\ce{H+}\) ions.
2. So it is a tribasic acid.

Teacher's Note:
1. Basicity is the number of H+ ions given by one molecule.
2. Phosphoric acid is also a weak acid, but the question asks about basicity.

Answer: (b) diacidic base

Explanation:
1. \(\ce{Ca(OH)2 <=> Ca^2+ + 2OH-}\): one molecule gives two \(\ce{OH-}\) ions.
2. So it is a diacidic base.

Teacher's Note:
1. Acidity of a base is the number of OH- ions per molecule.
2. Calcium hydroxide is only slightly soluble, so it is not a strong base.

Answer: (a) normal salt

Explanation:
1. \(\ce{CaCl2}\) is formed by full neutralization: \(\ce{Ca(OH)2 + 2HCl -> CaCl2 + 2H2O}\).
2. It has no replaceable \(\ce{H+}\) or \(\ce{OH-}\), so it is a normal salt.

Teacher's Note:
1. Normal salts come from complete neutralization.
2. Check the formula for leftover H or OH.

Answer: (c) potassium hydroxide

Explanation:
1. Potassium hydroxide is a strong alkali that attacks the skin badly; it is called caustic potash.
2. The other three are weak or only slightly soluble.
3. So the answer is potassium hydroxide.

Teacher's Note:
1. \(\ce{NaOH}\) is caustic soda and \(\ce{KOH}\) is caustic potash.
2. Both are strongly corrosive.

Answer: (b) bases

Explanation:
1. Metal oxides are basic oxides.
2. When a soluble one dissolves in water, it forms the metal hydroxide, e.g. \(\ce{Na2O + H2O -> 2NaOH}\).
3. So the product is a base.

Teacher's Note:
1. Metal oxides give bases; nonmetal oxides give acids.
2. This is a frequently asked rule.

Answer: (a) calcium nitrate

Explanation:
1. \(\ce{CaO + 2HNO3 -> Ca(NO3)2 + H2O}\)
2. Salts of nitric acid are nitrates, and nitric acid is monobasic so no hydrogen remains.
3. The salt is calcium nitrate.

Teacher's Note:
1. '-ic' acids give '-ate' salts, '-ous' acids give '-ite' salts.
2. A monobasic acid cannot form a hydrogen salt.

Answer: (a) \(\ce{KHSO3}\)

Explanation:
1. Sulphite radical is \(\ce{SO3^2-}\) (from sulphurous acid), and 'hydrogen' means one H is retained.
2. With one K+ ion the formula is \(\ce{KHSO3}\).
3. \(\ce{K2SO3}\) is plain potassium sulphite and the sulphates come from sulphuric acid.

Teacher's Note:
1. Sulphite has 3 oxygen atoms and sulphate has 4.
2. Hydrogen in the name means H in the formula.

Answer: (d) ammonium carbonate

Explanation:
1. Smelling salt is \(\ce{(NH4)2CO3}\), which gives off a sharp smell of ammonia.
2. It gives relief in a common cold or fainting, so the answer is ammonium carbonate.

Teacher's Note:
1. Do not confuse it with sal ammoniac, which is ammonium chloride.
2. Smelling salt is named for its smell.

Answer: (d) sulphurous acid

Explanation:
1. Acetic acid is organic and monobasic; sulphuric acid is strong; phosphoric acid is tribasic.
2. Sulphurous acid, \(\ce{H2SO3}\), is mineral, weak and gives two \(\ce{H+}\) ions, so it is the dibasic weak mineral acid.

Teacher's Note:
1. Test three things: mineral, weak, dibasic.
2. Only \(\ce{H2SO3}\) passes all three.

Answer: (b) carbon

Explanation:
1. Carbon forms two oxides, \(\ce{CO2}\) and \(\ce{CO}\).
2. \(\ce{CO2 + H2O -> H2CO3}\) gives an acid, while CO is neutral and gives none.
3. Sulphur and phosphorus give acids from both of their oxides, and nitrogen has more than two oxides, so the answer is carbon.

Teacher's Note:
1. CO is a neutral oxide; remember it with NO.
2. Only one of the two oxides should give an acid.

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

Explanation:
1. An antacid must be a mild base, since it is taken to neutralize stomach acid.
2. \(\ce{NaOH}\) and \(\ce{KOH}\) are strongly corrosive and cannot be taken.
3. Magnesium hydroxide and aluminium hydroxide are mild, so the pair is \(\ce{Mg(OH)2}\) and \(\ce{Al(OH)3}\).

Teacher's Note:
1. Milk of magnesia is magnesium hydroxide.
2. Reject any pair that contains \(\ce{NaOH}\) or \(\ce{KOH}\).

Answer: (d) all the above

Explanation:
1. Acids conduct only when they ionize in water.
2. Pure concentrated sulphuric acid, pure nitric acid and liquid hydrogen chloride have no water and do not ionize.
3. So all three are poor conductors: all the above.

Teacher's Note:
1. Ions are needed for conduction.
2. Dissolving in water, not the acid itself, produces ions.

Answer: (a) calcium oxide

Explanation:
1. Calcium oxide absorbs moisture without changing its state.
2. Calcium chloride is deliquescent; it dissolves in the moisture it absorbs.
3. So the hygroscopic substance is calcium oxide.

Teacher's Note:
1. Hygroscopic: absorbs moisture, stays solid.
2. Examples: CaO, conc. \(\ce{H2SO4}\), \(\ce{CoCl2}\).

Answer: (c) magnesium oxide, potassium oxide

Explanation:
1. Copper oxide and aluminium oxide do not dissolve in water, which rules out (a), (b) and (d).
2. Magnesium oxide and potassium oxide form the hydroxides \(\ce{Mg(OH)2}\) and \(\ce{KOH}\) in water.
3. So the pair is (c).

Teacher's Note:
1. Oxides of copper and aluminium are insoluble.
2. Potassium and sodium oxides are the clearly soluble ones.

Answer: (c) 2413

Explanation:
1. The metal sodium burns to give \(\ce{Na2O}\): step 2. It dissolves to give the base \(\ce{NaOH}\) (A): step 4.
2. \(\ce{NaOH}\) reacts with \(\ce{H2SO4}\) (B) to give \(\ce{NaHSO4}\) (C), which is still acidic: step 1.
3. \(\ce{NaHSO4}\) reacts with more \(\ce{NaOH}\) to give neutral \(\ce{Na2SO4}\) (D): step 3.
4. The order is 2, 4, 1, 3.

Teacher's Note:
1. Find the base A first, since it turns red litmus blue.
2. An acidic salt C still turns blue litmus red, like the acid B.

Answer: (a) 426153

Explanation:
1. Acidity of the bases: ferric hydroxide 3, barium hydroxide 2, sodium hydroxide 1, so the decreasing order is 4, 2, 6. The basicity of the acids (\(\ce{HCl}\) 1, \(\ce{H2SO4}\) 2, \(\ce{H3PO4}\) 3) runs in the same order.
2. Salts with equal acidity and basicity in increasing order: sodium chloride (1, 1), barium sulphate (2, 2), ferric phosphate (3, 3), that is 1, 5, 3.
3. The sequence is 4 2 6 1 5 3.

Teacher's Note:
1. Work in two parts: pairs first, then salts.
2. The salt of a 1:1 pair comes first, then the 2:2 pair, then the 3:3 pair.

Answer: (c) sulphuric acid

Explanation:
1. Phosphoric acid, carbonic acid and acetic acid are all weak acids because they ionize only partly.
2. Sulphuric acid ionizes completely, so it is the strong one and the odd one out.

Teacher's Note:
1. Sort the options into strong and weak first.
2. Whichever one stands alone is the answer.

Answer: (c) \(\ce{KH2PO4}\)

Explanation:
1. A nonmetal with 5 valence electrons is phosphorus, and its ‘ic’ acid is phosphoric acid, \(\ce{H3PO4}\).
2. A metal with one valence electron forms a base of the type MOH, such as \(\ce{KOH}\) or \(\ce{NaOH}\).
3. One molecule of acid with one molecule of base gives \(\ce{MH2PO4}\), which still has two replaceable H atoms and so can react with two more molecules of base (1 : 2).
4. Among the options this is \(\ce{KH2PO4}\).

Teacher's Note:
1. Valence electrons tell you the element.
2. A salt that reacts further with a base is an acidic salt with replaceable hydrogen.

Answer: (a) \(\ce{SO3}\), \(\ce{MgO}\), \(\ce{NO}\)

Explanation:
1. \(\ce{SO3}\) is a gas that forms an acid with moisture and turns blue litmus red (A).
2. \(\ce{MgO}\) is a solid whose solution is basic and turns red litmus blue (B).
3. \(\ce{NO}\) is a neutral gas (C).
4. In (b), \(\ce{CO2}\) is acidic; in (c), CO is neutral and \(\ce{SO3}\) is acidic; in (d), \(\ce{Ca(OH)2}\) is not an oxide.
5. So the answer is (a).

Teacher's Note:
1. Identify the acidic gas, the basic solid and the neutral gas separately.
2. NO and CO are the neutral oxides.

Answer: (a) blue vitriol and limestone

Explanation:
1. Blue vitriol, \(\ce{CuSO4.5H2O}\), turns into white \(\ce{CuSO4}\) on heating, and adding water brings back the blue colour.
2. Limestone, \(\ce{CaCO3}\), gives quicklime \(\ce{CaO}\) on heating, and adding water to CaO releases so much heat that the water boils.
3. So X and Y are blue vitriol and limestone.

Teacher's Note:
1. Colour returning after adding water means a hydrated salt such as blue vitriol.
2. Boiling water points to quicklime reacting with water.

Answer: (c) calcium sulphate

Explanation:
1. The oxyacid of sulphur with four oxygen atoms is \(\ce{H2SO4}\).
2. \(\ce{H2SO4 + Ca(OH)2 -> CaSO4 + 2H2O}\)
3. Complete neutralization gives the normal salt calcium sulphate.

Teacher's Note:
1. Four oxygen atoms means the 'ic' acid, so the salt ends in 'ate'.
2. 'Bi' or hydrogen in a name means partial neutralization.

Answer: (a) to supply oxygen

Explanation:
1. Nitre is \(\ce{KNO3}\). On heating it gives \(\ce{KNO2}\) and oxygen.
2. The oxygen released lets carbon and sulphur in the gunpowder burn fiercely.
3. So its role is to supply oxygen.

Teacher's Note:
1. Gunpowder is nitre, carbon and sulphur.
2. Nitre is the oxygen source (an oxidiser).