CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter Short Answer Questions Set 01

Short Answer Questions

Teacher's Note:
1. Give the charge in both units and as +1, −1, 0.
2. Mention that p and n are about 1 amu each.

Question 2. Why do metals form cations and nonmetals form anions?
Answer:
Metals have 1 to 3 electrons in their valence shell. It is easier to lose them and reach the stable octet of the shell below. When electrons are lost, protons outnumber electrons and a positive ion (cation) forms.
Nonmetals have 4 to 7 valence electrons. It is easier to gain the few electrons needed to complete an octet. When electrons are gained, a negative ion (anion) forms.
In simple words: Metals give away a few electrons; nonmetals take in a few electrons.

Teacher's Note:
1. Compare number of valence electrons.
2. Sodium and chlorine make a good example.

Question 3. Calculate the number of neutrons in the following. (a) \(\ce{^{31}_{15}X}\) (b) \(\ce{^{235}_{92}U}\) (c) \(\ce{^{39}_{19}K}\)
Answer:
Neutrons = mass number − atomic number.
(a) 31 − 15 = 16
(b) 235 − 92 = 143
(c) 39 − 19 = 20
In simple words: Subtract the lower number from the upper number.

Teacher's Note:
1. The upper number is A and the lower is Z.
2. Write the formula first.

Question 4. For each of the following reactions identify the products formed and balance the reaction. (a) \(\ce{NaCl + AgNO3 ->}\) (b) \(\ce{Zn + HCl ->}\)
Answer:
(a) \(\ce{NaCl + AgNO3 -> NaNO3 + AgCl v}\) (double displacement; AgCl is a white precipitate)
(b) \(\ce{Zn + 2HCl -> ZnCl2 + H2 ^}\) (displacement; zinc is more reactive than hydrogen)
In simple words: NaCl and AgNO3 swap partners, and zinc pushes hydrogen out of the acid.

Teacher's Note:
1. Check the atoms on both sides.
2. Hydrogen gas bubbles out in (b).

Question 5. Define radical. Give some examples of bivalent negative radicals.
Answer:
A group of atoms carrying a positive or negative charge is called a radical.
Bivalent negative radicals: sulphate \(\ce{SO4^2-}\), sulphite \(\ce{SO3^2-}\), carbonate \(\ce{CO3^2-}\), chromate \(\ce{CrO4^2-}\), silicate \(\ce{SiO3^2-}\), zincate \(\ce{ZnO2^2-}\).
In simple words: A radical is a charged group of atoms; these examples all carry a −2 charge.

Teacher's Note:
1. Give at least three examples.
2. Write the charge with each.

Teacher's Note:
1. Use a table with an example for each.
2. Mention reversibility.

Question 7. Mention different types of combination reactions with examples.
Answer:
1. Element + element (synthesis): \(\ce{H2 + Cl2 -> 2HCl}\)
2. Element + compound: \(\ce{2SO2 + O2 -> 2SO3}\)
3. Compound + compound: \(\ce{CaO + H2O -> Ca(OH)2}\)
In simple words: In every combination reaction, two substances join to form one compound.

Teacher's Note:
1. The general form is X + Y → XY.
2. Balance each example.

Question 8. What is meant by thermal decomposition? Give an example.
Answer:
Thermal decomposition is the breaking of a compound into simpler substances by heat.
Example: \(\ce{2Pb(NO3)2 -> 2PbO + 4NO2 + O2}\)
In simple words: Heat splits the compound into two or more substances.

Teacher's Note:
1. Show the heat sign above the arrow if you can.
2. Other example: 2KClO3 → 2KCl + 3O2.

Question 9. Define double decomposition reaction. Give an example.
Answer:
A reaction in which two compounds exchange their radicals to form two new compounds is a double decomposition (double displacement) reaction. General form: \(\ce{AB + XY -> AY + XB}\).
Example: \(\ce{2KNO3 + FeSO4 -> K2SO4 + Fe(NO3)2}\)
In simple words: Two compounds swap their partners.

Teacher's Note:
1. Name the radicals that are exchanged.
2. If one product is insoluble, it is a precipitation reaction.

Question 10. What is a chemical displacement reaction? Mention the condition required for it.
Answer:
A reaction in which a more reactive element displaces a less reactive element from its compound is a displacement reaction. Example: \(\ce{Zn + FeSO4 -> ZnSO4 + Fe}\).
Condition: the element added must be more reactive than the element it displaces. Otherwise there is no reaction, for example \(\ce{Cu + FeSO4 -> no reaction}\).
In simple words: A stronger metal pushes a weaker metal out of its compound.

Teacher's Note:
1. Mention the reactivity series.
2. Give one reaction that happens and one that does not.

Teacher's Note:
1. Cat-ion is positive; an-ion is negative.
2. Give two examples of each.

Teacher's Note:
1. Cancel equal valencies (3 and 3 gives 1 and 1).
2. Name the metal first, then the negative radical.

Teacher's Note:
1. Use sodium or carbon as the example.
2. Write A = Z + n.

Question 14. Draw the geometrical representation of sulphur.
Answer:
Sulphur has Z = 16 and A = 32. Its protons = 16, electrons = 16 and neutrons = 32 − 16 = 16.
Electronic configuration: 2, 8, 6 (K, L and M shells).

In simple words: Draw the nucleus with 16p and 16n and three shells holding 2, 8 and 6 electrons.

Teacher's Note:
1. Compute the neutrons from the mass number.
2. Draw the shells as circles and mark each electron.