CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter MCQs Set 01

Answer: (c) 1 and 2, respectively

Explanation:
1. Hydrogen has Z = 1, so its only electron is in the K shell: 1.
2. Helium has Z = 2, so both its electrons are in the K shell: 2.
3. So the numbers are 1 and 2, respectively.

Teacher's Note:
1. The K shell holds at most 2 electrons.
2. Count electrons from the atomic number.

Answer: (b) bisulphate

Explanation:
1. Permanganate is \(\ce{MnO4-}\), with charge −1.
2. Bisulphate \(\ce{HSO4-}\) has charge −1; sulphate has −2; magnesium has +2 and potassium +1.
3. So the answer is bisulphate.

Teacher's Note:
1. Compare the sign as well as the size of the charge.
2. Potassium has size 1 but the opposite sign.

Answer: (b) 3 and 4

Explanation:
1. Chloride ion has valency 1.
2. In \(\ce{XCl3}\) three chloride ions balance X, so X has valency 3.
3. In \(\ce{YCl4}\) four chloride ions balance Y, so Y has valency 4.
4. So the valencies are 3 and 4.

Teacher's Note:
1. The number of Cl atoms equals the valency of the metal.
2. Cl always has valency 1 here.

Answer: (c) \(\ce{AB2}\)

Explanation:
1. Let the positive radical be A (valency 2) and the negative radical B (valency 1).
2. Criss-cross: the valency 1 of B goes with A and the valency 2 of A goes with B.
3. This gives \(\ce{A1B2}\), which is written \(\ce{AB2}\).

Teacher's Note:
1. Swap the valencies and drop the subscript 1.
2. The positive radical is written first.

Answer: (b) chemical change

Explanation:
1. Conc. sulphuric acid removes water from sugar and leaves black carbon.
2. A new substance (carbon) is formed and the change cannot be reversed.
3. So it is a chemical change.

Teacher's Note:
1. Charring means a new black substance is formed.
2. Irreversible change with a new substance is chemical.

Answer: (d) physical change

Explanation:
1. Wax changes from solid to liquid only.
2. Its molecules stay the same and it solidifies again on cooling.
3. So it is a physical change.

Teacher's Note:
1. Change of state alone is physical.
2. Burning the wax would be a chemical change instead.

Answer: (a) chemical composition

Explanation:
1. Corrosion (for example rusting) turns the metal into a new compound such as an oxide.
2. So the chemical composition changes: chemical composition.

Teacher's Note:
1. Corrosion forms a new substance on the metal.
2. Colour change is only a visible sign.

Answer: (b) physical change which involves condensation of water vapour

Explanation:
1. Clouds, mist and fog are tiny droplets of water formed when water vapour cools.
2. Vapour changes into liquid droplets, which is condensation, and no new substance is formed.
3. So the answer is physical change which involves condensation of water vapour.

Teacher's Note:
1. Vapour to liquid is condensation.
2. Sublimation is solid to gas directly.

Answer: (c) double displacement

Explanation:
1. Two compounds react and exchange their radicals: Ag goes with Cl and Na goes with \(\ce{NO3}\).
2. Two new compounds form, so it is a double displacement reaction.
3. Here AgCl is also a precipitate.

Teacher's Note:
1. Two compounds in and two new compounds out means double displacement.
2. Displacement uses an element and a compound.

Answer: (b) Neon

Explanation:
1. Neon has the electronic configuration 2, 8, which is a complete octet.
2. An octet is very stable, so neon does not react: neon.

Teacher's Note:
1. Noble gases are chemically inactive.
2. Others need to gain, lose or share electrons.

Answer: (d) 2

Explanation:
1. Chlorine has valency 1 and there are two Cl atoms.
2. The metal must balance a charge of −2, so its valency is 2.

Teacher's Note:
1. The subscript of Cl gives the valency of the metal.
2. Check by the criss-cross method.

Answer: (a) \(\ce{M2(SO4)3}\), \(\ce{M2(SO3)3}\)

Explanation:
1. The metal ion is \(\ce{M^3+}\), sulphate is \(\ce{SO4^2-}\) and sulphite is \(\ce{SO3^2-}\).
2. Criss-cross: sulphate gives \(\ce{M2(SO4)3}\) and sulphite gives \(\ce{M2(SO3)3}\).
3. So the answer is option (a).

Teacher's Note:
1. Put the radical in brackets when more than one is needed.
2. Swap 3 and 2.

Answer: (b) 19, 20

Explanation:
1. Elements with 8 electrons in both L and M have the configuration 2, 8, 8 or 2, 8, 8 followed by more electrons.
2. Z = 19 is 2, 8, 8, 1 and Z = 20 is 2, 8, 8, 2, so both have L = M = 8.
3. The other pairs include Z = 12 (2, 8, 2), 16 (2, 8, 6) or 17 (2, 8, 7), which do not satisfy this. So the answer is 19, 20.

Teacher's Note:
1. Write the configuration of each element.
2. Check L and M for both elements of the pair.

Answer: (c) Dissolution of concentrated acid in water

Explanation:
1. Lime in water forms slaked lime, and electrolysis splits water, so both are chemical changes.
2. Carbon dioxide dissolved under pressure partly reacts with water to form carbonic acid, so it is a chemical change.
3. Concentrated acid mixed with water only becomes dilute, so the change is physical.

Teacher's Note:
1. Dilution makes no new substance.
2. Check whether the solute reacts with the water.

Answer: (d) Absorption of moisture by \(\ce{CaCl2}\)

Explanation:
1. Steam on red hot coke forms new gases, and dilute nitric acid slowly decomposes in light, so these are chemical.
2. Phosphorus pentoxide reacts with moisture to form an acid, so that is chemical too.
3. Anhydrous \(\ce{CaCl2}\) only absorbs water and becomes moist without any new substance. This is not a chemical change.

Teacher's Note:
1. Deliquescence is a physical change.
2. Look for a new substance in each option.

Answer: (b) dacbe

Explanation:
1. First write the radicals with charges (d).
2. Then write the valencies below them (a).
3. Interchange the valencies as subscripts (c).
4. Divide by the highest common factor if there is one (b).
5. Finally put brackets around radicals needed more than once (e).
6. So the order is d, a, c, b, e.

Teacher's Note:
1. Remember the order: radicals, valencies, swap, simplify, brackets.
2. Brackets come last because they depend on the subscripts.

Answer: (c) protons and neutrons

Explanation:
1. The nucleus is the central part of the atom.
2. It holds protons and neutrons, which are called nucleons.
3. Electrons move outside it, so the answer is protons and neutrons.

Teacher's Note:
1. Electrons are never inside the nucleus.
2. Nucleons = protons + neutrons.

Answer: (b) 2n²

Explanation:
1. A shell numbered n holds at most \(\ce{2n^2}\) electrons.
2. For n = 1, 2, 3, 4 this gives 2, 8, 18, 32.
3. So the answer is 2n².

Teacher's Note:
1. Test the formula with n = 1: it must give 2.
2. Learn 2, 8, 18, 32.

Answer: (b) helium

Explanation:
1. Neon, argon and xenon all have 8 electrons in the outer shell.
2. Helium has only 2 electrons, so it has a duplet.
3. So the answer is helium.

Teacher's Note:
1. Helium has only the K shell.
2. The K shell is full with 2 electrons.

Answer: (c) potassium and phosphorus

Explanation:
1. A cation is formed by a metal, which loses electrons, and an anion by a nonmetal, which gains electrons.
2. Potassium (2, 8, 8, 1) loses 1 electron to form \(\ce{K+}\), and phosphorus (2, 8, 5) gains 3 electrons to form \(\ce{P^3-}\).
3. So the pair is potassium and phosphorus.

Teacher's Note:
1. Metal first (cation), nonmetal second (anion).
2. Neon forms no ion at all.

Answer: (a) silicate

Explanation:
1. Silicate is \(\ce{SiO3^2-}\), which is negative.
2. Ammonium is \(\ce{NH4+}\), ferrous is \(\ce{Fe^2+}\) and chromium is \(\ce{Cr^3+}\), all positive.
3. So the answer is silicate.

Teacher's Note:
1. Only ammonium and hydronium are positive radicals.
2. Names ending in -ate or -ite are usually negative radicals.

Answer: (b) \(\ce{2Fe(OH)3 + 4H2SO4 -> Fe2(SO4)3 + 6H2O}\)

Explanation:
1. In (b) the left side has 4 S (from 4 \(\ce{H2SO4}\)) but the right side has only 3 S in \(\ce{Fe2(SO4)3}\).
2. The correct form is \(\ce{2Fe(OH)3 + 3H2SO4 -> Fe2(SO4)3 + 6H2O}\).
3. The others have equal atoms on both sides. So (b) is not balanced.

Teacher's Note:
1. Count the atoms of each element on both sides.
2. Start with the element that appears the fewest times.

Answer: (b) \(\ce{C + O2 -> CO2}\)

Explanation:
1. A synthesis reaction is a combination of one element with another element.
2. In (b) carbon and oxygen, both elements, form \(\ce{CO2}\).
3. (a) is element with compound, (c) is compound with compound, (d) is decomposition. So the answer is (b).

Teacher's Note:
1. Synthesis means element + element.
2. Check what each reactant is.

Answer: (c) curdling of milk

Explanation:
1. Blackening of silver and rusting of iron damage the articles, so they are harmful.
2. Curdling of milk gives curd (and then paneer, butter, etc.), which is useful.
3. So the answer is curdling of milk.

Teacher's Note:
1. Ask whether the change helps us or spoils things.
2. Curd is a useful product.

Answer: (c) Molecular formula of a compound changes

Explanation:
1. In a physical change the molecular composition stays the same.
2. So the molecular formula cannot change in it.
3. The statement that does not fit is the molecular formula changes.

Teacher's Note:
1. Formula change belongs to chemical change.
2. Physical changes are temporary and make no new substance.

Answer: (b) chemical displacement

Explanation:
1. The reactivity series tells which metal can push another out of its compound.
2. That happens in chemical displacement reactions.

Teacher's Note:
1. More reactive displaces less reactive.
2. Use the series to decide whether a reaction occurs.

Answer: (b) \(\ce{XCl5}\)

Explanation:
1. In \(\ce{X2O5}\), oxygen has valency 2, so 5 oxygen atoms carry a total of 10 and each X has valency 5.
2. Chloride ion has valency 1, so X needs five of them.
3. The chloride is \(\ce{XCl5}\).

Teacher's Note:
1. Swap the subscripts to read off the valency.
2. Then criss-cross with chlorine.

Answer: (c) \(\ce{M(H2PO4)2}\)

Explanation:
1. Nitrite is \(\ce{NO2-}\) and two are needed, so the metal ion is \(\ce{M^2+}\).
2. Dihydrogen phosphate is \(\ce{H2PO4-}\), with charge −1.
3. Criss-cross gives \(\ce{M(H2PO4)2}\).

Teacher's Note:
1. Find the valency of the metal first.
2. Put the radical in brackets when it is repeated.

Answer: (d) 20 and 14

Explanation:
1. Neutrons = A − Z.
2. First element: 40 − 20 = 20.
3. Second element: 27 − 13 = 14.
4. So the numbers are 20 and 14.

Teacher's Note:
1. Use neutrons = mass number − atomic number.
2. Do the subtraction for each element separately.

Answer: (b) physical state as well as chemical composition

Explanation:
1. On heating, solid \(\ce{NH4Cl}\) turns into vapour (sublimation, a physical change).
2. On further heating the vapour breaks down: \(\ce{NH4Cl -> NH3 + HCl}\), a chemical change.
3. So both the state and the composition change.

Teacher's Note:
1. Two things happen one after the other.
2. Sublimation is physical; decomposition is chemical.

Answer: (b) formation of rust

Explanation:
1. Clouds form by condensation, milk powder by removing water, and the black coating by deposition of material: all physical.
2. Rust is a new compound, hydrated ferric oxide, formed from iron, so it is a chemical change.

Teacher's Note:
1. Rust is a new substance.
2. Others are changes of state or removal of water.

Answer: (c) bacd

Explanation:
1. Write the ions with charges first (b).
2. Write the valencies, 2 and 2, below them (a).
3. Both valencies are 2, so they have the common factor 2 and become 1 and 1 (c).
4. Then they are written as subscripts (d), giving \(\ce{ZnO}\).
5. So the order is b, a, c, d.

Teacher's Note:
1. Since both valencies are equal, they can be simplified before swapping.
2. The charges are written first, then the valencies.

Answer: (a) \(\ce{MX2}\), \(\ce{MY}\)

Explanation:
1. X has valency 1 (uninegative), so it needs 7 valence electrons: 2, 8, 7 (chlorine).
2. Y comes just before X, so its configuration is 2, 8, 6 and its valency is 2 (sulphur).
3. With the divalent metal M: X gives \(\ce{MX2}\) and Y gives \(\ce{MY}\).
4. So the answer is option (a).

Teacher's Note:
1. Find each nonmetal's valency from its valence electrons.
2. Then criss-cross with M of valency 2.

Answer: (c) \(\ce{XH3}\)

Explanation:
1. Gaining three electrons gives \(\ce{X^3-}\), so the valency of X is 3.
2. Hydrogen has valency 1.
3. The hydride is \(\ce{XH3}\).

Teacher's Note:
1. Valency equals the number of electrons gained.
2. Swap valencies with hydrogen.

Answer: (c) C

Explanation:
1. The positively charged particles are protons, so Z = 10, 18 and 8.
2. A is 2, 8 and B is 2, 8, 8: both have a complete octet and are inactive.
3. C is 2, 6, which is not complete, so C is chemically reactive: C.

Teacher's Note:
1. Write the configuration from Z.
2. Full outer shell means unreactive.

Answer: (b) 3 : 5

Explanation:
1. For X: protons 12, neutrons 12, so mass number = 24. \(\ce{X^2+}\) has 12 − 2 = 10 electrons.
2. For Y: \(\ce{Y^2+}\) has 18 electrons, so the atom has 20 electrons and 20 protons.
3. Neutrons of Y = 2 × 10 = 20, so its mass number = 20 + 20 = 40.
4. Ratio = 24 : 40 = 3 : 5.

Teacher's Note:
1. A dipositive ion has lost two electrons.
2. Mass number = protons + neutrons.

Answer: (b) 2, 7

Explanation:
1. The neutral atom has as many electrons as protons, which is 9.
2. Nine electrons are arranged as 2, 7 (the ion has 10 electrons, 2, 8).
3. So the configuration of the atom is 2, 7.

Teacher's Note:
1. Use the protons to get the atom's electrons.
2. The neutrons do not matter here.

Answer: (c) 2, 8, 18, 2

Explanation:
1. The positive charge is carried by protons, and each proton is 1837 times as heavy as an electron.
2. Number of protons = 55110 ÷ 1837 = 30, so Z = 30.
3. Thirty electrons are arranged as 2, 8, 18, 2 (zinc).

Teacher's Note:
1. Divide by 1837 to get the number of protons.
2. Then fill the shells in order.

Answer: (b) 2, 8, 8 and 2, 8, 6

Explanation:
1. X has a stable configuration with M as the valence shell: 2, 8, 8. The difference between M and K is 8 − 2 = 6.
2. Let Y have x electrons in M. Then (8 − 2) : (x − 2) = 3 : 2, so x − 2 = 4 and x = 6.
3. So X is 2, 8, 8 and Y is 2, 8, 6.

Teacher's Note:
1. Write 2, 8, 8 for X first.
2. Use the ratio to find the unknown M-shell electrons.

Answer: (a) 3, 5

Explanation:
1. Carbon: 10 \(\ce{CO2}\) needs 10 C, so \(\ce{H2C2O4}\) has coefficient 5.
2. Sulphur: the right side has 1 + 2 = 3 S, so \(\ce{H2SO4}\) has coefficient 3.
3. Check hydrogen: 6 + 10 = 16 on the left and 16 in 8 \(\ce{H2O}\) on the right.
4. So the coefficients are 3 and 5.

Teacher's Note:
1. Balance carbon and sulphur first; hydrogen and oxygen last.
2. Always verify with the hydrogen count.