CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter MCQs Set 04

Answer: (c) DCBA

Explanation:
1. (D) two substances join into one: combination.
2. (C) one substance splits into several: decomposition.
3. (B) zinc replaces iron: displacement.
4. (A) the ions of two compounds exchange partners: double displacement.
5. The order is D, C, B, A: option (c).

Teacher's Note:
1. Identify the pattern A + B → AB, AB → A + B, A + BC → AC + B, AB + CD → AD + CB.
2. Decomposition starts with a single reactant.

Answer: (c) Rusting of iron

Explanation:
1. Sublimation of camphor is a physical change.
2. Boiling of milk and the change of water into steam are also treated as physical changes.
3. Rusting forms iron oxide, a new substance: (c).

Teacher's Note:
1. Rusting is the standard chemical change example.
2. Check whether the process can be reversed by cooling.

Answer: (d) double decomposition

Explanation:
1. Two compounds exchange their ions to give two new compounds.
2. This is double displacement, which the book names double decomposition.

Teacher's Note:
1. Single displacement has an element and a compound.
2. Double displacement often forms a precipitate.

Answer: (a) 22,044

Explanation:
1. The atom has 12 protons.
2. One proton is about 1837 times as heavy as an electron.
3. So the mass of the positive charge = 12 × 1837 = 22,044 times.

Teacher's Note:
1. Positive charge in an atom means protons only.
2. Use atomic number as the number of protons.

Answer: (c) 14

Explanation:
1. Neutrons = mass number – atomic number.
2. 27 – 13 = 14.

Teacher's Note:
1. Mass number is the superscript and atomic number the subscript.
2. This element is aluminium.

Answer: (c) 2n²

Explanation:
1. The maximum number of electrons in the nth orbit is 2n².
2. For n = 1, 2, 3 this gives 2, 8, 18.

Teacher's Note:
1. Check the formula with n = 1: 2 electrons.
2. n² would give only 1 for the K shell.

Answer: (a) 3, 2

Explanation:
1. Balanced: \(\ce{3CuO + 2NH3 -> 3Cu + 3H2O + N2}\)
2. The reactant coefficients are 3 for CuO and 2 for NH₃: 3, 2.

Teacher's Note:
1. Give the coefficients in the same order as the reactants appear.
2. Check N: 2 = 2 and O: 3 = 3.

Answer: (a) MPO₄

Explanation:
1. In \(\ce{M2(SO4)3}\) two M ions balance 3 sulphate ions: 2x = 6, so M³⁺.
2. Phosphate is \(\ce{PO4^3-}\), so one M³⁺ balances one phosphate.
3. The formula is MPO₄.

Teacher's Note:
1. Work out the metal's valency first.
2. Equal charges cancel, so no subscripts are needed.

Answer: (d) 1

Explanation:
1. Balanced: \(\ce{2Pb3O4 -> 6PbO + O2}\)
2. Check: Pb 6 = 6, O 8 = 6 + 2.
3. The coefficient of O₂ is 1.

Teacher's Note:
1. Take 2Pb₃O₄ so that oxygen balances with whole numbers.
2. A coefficient of 1 is not written.

Answer: (a) zincate

Explanation:
1. Silicate \(\ce{SiO3^2-}\) has charge –2.
2. Zincate \(\ce{ZnO2^2-}\) also has –2.
3. Phosphate is –3 and nitrate and bisulphate are –1.
4. The answer is zincate.

Teacher's Note:
1. Compare charges of all four options.
2. Zincate is the radical of zinc as an anion.

Answer: (a) Zinc

Explanation:
1. Only metals above hydrogen in the reactivity series displace hydrogen from dilute acids.
2. Zinc is above hydrogen; copper, mercury and silver are below it.
3. The answer is zinc.

Teacher's Note:
1. Zinc with dilute HCl or H₂SO₄ is the standard lab method for hydrogen.
2. Metals below hydrogen do not react with dilute acids.

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

Explanation:
1. Ferric is Fe³⁺: (i) → (D).
2. Cuprous is Cu⁺: (ii) → (C).
3. Stannic is Sn⁴⁺: (iii) → (E).
4. Mercuric is Hg²⁺: (iv) → (A).
5. Uncombined potassium metal has zero charge: (v) → (B).
6. This is option (b).

Teacher's Note:
1. Ferric, cupric and stannic are the 'ic' forms with the higher charge.
2. A free element carries no charge.

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

Explanation:
1. A is true: calcium hydroxide is \(\ce{Ca(OH)2}\), with two OH radicals.
2. R is false: calcium has 20 electrons arranged as 2, 8, 8, 2.
3. So A is true and R is false: (c).

Teacher's Note:
1. Calcium has valency 2, so it needs two OH⁻ radicals.
2. Always fill the third shell with 8 before the fourth.

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

Explanation:
1. A is true: every element has its own atomic number, which identifies it.
2. R is true: isotopes of an element differ in mass number but have the same atomic number.
3. R does not explain why atomic number is fundamental, so the answer is (b).

Teacher's Note:
1. Different elements always have different atomic numbers.
2. Isotopes are a separate idea, so R is a true but unrelated fact.

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

Explanation:
1. A is true: ammonium is NH₄⁺ and carbonate is CO₃²⁻, so two NH₄⁺ balance one CO₃²⁻.
2. R is true: a compound is electrically neutral.
3. The formula is written to make the charges equal, so R explains A: (a).

Teacher's Note:
1. Check the charge: 2 × (+1) + (–2) = 0.
2. Cross the valencies to get formulas.

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

Explanation:
1. A is false: the atomic number, not the mass number, is the fundamental property because it identifies the element.
2. R is true: for example, calcium-40 and argon-40 have the same mass number.
3. So A is false and R is true: (d).

Teacher's Note:
1. Atoms with same mass number but different atomic numbers are called isobars.
2. Mass number can therefore not identify an element.

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

Explanation:
1. A is true: electrons have negligible mass.
2. R is true: the heavy protons and neutrons are in the nucleus.
3. So the mass is concentrated in the nucleus because of R: (a).

Teacher's Note:
1. A proton is about 1837 times as heavy as an electron.
2. Mass number counts only protons and neutrons.

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

Explanation:
1. A is true: curdling forms new substances, so it is a chemical change.
2. R is false: in a physical change the molecular composition does not change.
3. So A is true and R is false: (c).

Teacher's Note:
1. Molecular composition changes only in chemical changes.
2. Check the reason as well as the assertion.

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

Explanation:
1. A is true: electrons keep revolving in fixed orbits around the nucleus.
2. R is true: a shell can hold only 2n² electrons.
3. The fixed number of electrons in an orbit does not explain why electrons revolve, so R is not the explanation: (b).

Teacher's Note:
1. The electron moves fast to overcome the pull of the nucleus.
2. The capacity of a shell is a separate fact.

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

Explanation:
1. A is true: burning is a chemical change.
2. R is true: new substances, carbon dioxide and water, are formed.
3. The formation of new substances is the reason, so R explains A: (a).

Teacher's Note:
1. New substance formed means chemical change.
2. Combustion always forms oxides.