CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. What is meant by synthesis reaction? Give an example.
Answer:
A synthesis reaction is a combination in which two elements join to form a compound.
Example: \(\ce{H2 + Cl2 -> 2HCl}\).
In simple words: Two elements simply join to make one compound.

Teacher's Note:
1. Element + element only.
2. Balance the example.

Question 2. \(\ce{A + BC -> AC + B}\). In the above equation, compare the reactivities of A and B.
Answer:
A displaces B from its compound BC, so A is more reactive than B.
In simple words: The element that pushes the other out is the more active one.

Teacher's Note:
1. Only a more reactive element can displace another.
2. Write A > B.

Question 3. What is meant by electrolytic decomposition? Give an example.
Answer:
Electrolytic decomposition is the breaking of a compound into simpler substances by passing electricity through it.
Example: \(\ce{2H2O -> 2H2 + O2}\).
In simple words: Electric current splits the compound.

Teacher's Note:
1. Water is the standard example.
2. Name the energy used: electricity.

Question 4. Give reason for the stability of inert gases.
Answer:
Inert gases have a complete outer shell, an octet (8 electrons) or, in helium, a duplet (2 electrons). Such an arrangement needs no gaining or losing of electrons, so they are stable and unreactive.
In simple words: A full outer shell leaves nothing to gain or lose.

Teacher's Note:
1. Quote both octet and duplet.
2. Helium has only the K shell.

Question 5. What is the valence shell? What is the maximum number of electrons it can have in an atom?
Answer:
The valence shell is the outermost shell of an atom. It can hold a maximum of 8 electrons.
In simple words: The last shell has at most 8 electrons.

Teacher's Note:
1. The valence shell decides the valency.
2. For helium it holds only 2.

Question 6. An atom is electrically neutral. Justify.
Answer:
An atom has equal numbers of protons (each +1) and electrons (each −1). The positive and negative charges are equal and cancel, so the atom is electrically neutral.
In simple words: Equal plus and minus charges cancel out.

Teacher's Note:
1. Neutrons carry no charge.
2. Protons = electrons.

Question 7. What is photolytic decomposition? Give an example.
Answer:
Photolytic decomposition is the breaking down of a compound by light.
Example: \(\ce{2HOCl -> 2HCl + O2}\).
In simple words: Light splits the compound.

Teacher's Note:
1. Photo means light.
2. Name the compound that breaks up.

Question 8. Identify physical and chemical changes among the following: (a) Boiling of milk (b) Ripening of fruits (c) Curdling of milk
Answer:
(a) Boiling of milk is a physical change.
(b) Ripening of fruits is a chemical change.
(c) Curdling of milk is a chemical change.
In simple words: Boiling only warms the milk, but ripening and curdling make new substances.

Teacher's Note:
1. Look for a new substance.
2. Curdling turns lactose into lactic acid.

Question 9. Why should a chemical equation be balanced?
Answer:
Atoms can neither be created nor destroyed in a chemical reaction. So the number of atoms of each element must be the same on both sides of the equation (law of conservation of mass).
In simple words: No atoms are lost or made, so both sides must match.

Teacher's Note:
1. Mention conservation of mass.
2. Only coefficients are adjusted.

Question 10. Dissolution of calcium oxide in water is which type of reaction?
Answer:
It is a combination reaction: \(\ce{CaO + H2O -> Ca(OH)2}\).
In simple words: Two compounds join to make one new compound.

Teacher's Note:
1. It is compound + compound.
2. The product is slaked lime.

Question 11. In a double decomposition reaction, one of the products is insoluble. What type of reaction is that?
Answer:
It is a precipitation reaction.
In simple words: An insoluble product settles as a solid.

Teacher's Note:
1. The solid is called a precipitate.
2. Example: NaCl and AgNO3 give AgCl.

Question 12. Potassium chlorate on strong heating gives potassium chloride and oxygen. What type of reaction is this?
Answer:
It is a thermal decomposition reaction.
\(\ce{2KClO3 -> 2KCl + 3O2}\)
In simple words: Heat breaks one compound into two substances.

Teacher's Note:
1. Heat is the energy supplied.
2. Balance O: 6 on each side.

Question 13. Balance the following chemical equations: (i) \(\ce{NH3 + Br2 -> N2 + NH4Br}\) (ii) \(\ce{FeS2 + O2 -> Fe2O3 + SO2}\)
Answer:
(i) \(\ce{8NH3 + 3Br2 -> N2 + 6NH4Br}\)
Check: N 8 = 2 + 6; H 24 = 24; Br 6 = 6.
(ii) \(\ce{4FeS2 + 11O2 -> 2Fe2O3 + 8SO2}\)
Check: Fe 4 = 4; S 8 = 8; O 22 = 6 + 16.
In simple words: Put numbers in front of formulae until every atom count matches.

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

Teacher's Note:
1. Hydrogen is the positive part of an acid.
2. Check that the charges cancel.

Question 15. State whether the following statement is true or false: The number of electrons in second shell of a metal with three valence electrons in the third shell is eight.
Answer:
True. A metal with 3 valence electrons in the M shell is 2, 8, 3 (aluminium), so its second (L) shell holds 8 electrons.
In simple words: The L shell is full with 8 before the M shell starts.

Teacher's Note:
1. Write the configuration 2, 8, 3.
2. The L shell can hold at most 8.

Question 16. State whether the following statement is true or false: The valency of helium is zero.
Answer:
True. Helium has 2 electrons in its only shell, which is a stable duplet. It neither gains nor loses electrons, so its valency is zero.
In simple words: Helium is already stable, so it has no valency.

Teacher's Note:
1. Noble gases have valency zero.
2. Helium's duplet is complete.

Question 17. State whether the following statement is true or false: The name of \(\ce{CrO4^2-}\) radical is dichromate ion.
Answer:
False. The radical \(\ce{CrO4^2-}\) is called chromate. Dichromate is \(\ce{Cr2O7^2-}\).
In simple words: CrO4 is chromate; dichromate has two chromium atoms.

Teacher's Note:
1. Count the chromium atoms.
2. Learn both radicals together.

Question 18. State whether the following statement is true or false: Rusting of iron is an example of both physical and chemical change.
Answer:
False. Rusting is purely a chemical change: iron changes into hydrated ferric oxide, a new substance.
In simple words: Rust is a new substance, so the change is only chemical.

Teacher's Note:
1. Rusting is irreversible and forms a new compound.
2. Do not call it a physical change.

Question 19. State whether the following statement is true or false: Sublimation of camphor is a physical change.
Answer:
True. Camphor changes from solid to vapour directly and its composition does not change.
In simple words: Camphor only changes state when it sublimes.

Teacher's Note:
1. Sublimation is a change of state.
2. Camphor can be recovered as a solid.