Question 3
- Explain briefly each of the following separation techniques:
(i) crystallization;
(ii) evaporation;
(iii) filtration. [6 marks] - (i) List three characteristics of halogens.
(ii) Write a balanced chemical equation to illustrate the reaction of cold dilute NaOH with
I. fluorine.
II. chlorine. - (i) Give one difference between a gs and a vapour.
(ii) State one condidtion under which a gas exhiits ideal behaviour. - Magnesium of mass 1.2 g reacts completely with chlorin.
(i) Write the equation for the reaction
. (ii) Calculate the volume of chlorine used at s.t.p.
[Molar volume of gas at s.t.p. = 22.4 dm3; Mg = 24.0; Cl = 35.5] [5 marks] - Define each of the following terms:
(i) atomic number;
(ii) mass number. [4 marks]
Observation
The question was a popular choice among the candidates.
In part (a), majority of the candidates could not satisfactorily explain crystallization, evaporation
and filtration as separation techniques. They defined the techniques, but did not state the steps involved.
In part (b), majority of the candidates responded to the question. They were familiar with
the characteristics of the halogens.
In part (c), majority of the candidates did not answer the question correctly. Only few of them were able to differentiate between a gas and a vapour. But, they could state the condition under which a gas exhibits ideal behaviour.
In part (d), majority of the candidates correctly calculated the volume of chlorine used at s.t.p in the given equation.
In part (e), majority of the candidates correctly defined atomic number and mass number.
The expected answers include:
- (i) Crystallization – Heat / concentrate the solution. Cool. Obtain crystals
Cooling must be stated before scoring for formation of crystals
(ii) Evaporation - Heat the solution. The solvent escapes into the air . The required solute is left behind .
(iii) Filtration – Pour the mixture into filter paper or porous material. The liquid passes through , the residue remains on the filter paper. - (i) - most electronegative elements in their respective periods
- forms ionic compounds with highly electropositive elements / metals
- they are diatomic
- discharge at the anode during electrolysis.
- all non metals
- they are coloured
- all oxidizing agents
- form covalent compound with non metals
- have Van der Waals forces
- form ionic compounds with metals
- they are reactive
(ii) I Cancelled
II. Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H2O(l) - (i) A gas refers to a substance that has a single (physical) state whereas a vapour refers to a substance that is a mixture of gas and liquid / solid at room temperature OR A vapour condenses at higher temperature than a gas
(ii) - high temperature
- low pressure
- large volume - (i) Mg(s) + Cl2(g) → MgCl2(s)
- (i) Atomic number refers to the number of protons / electrons present in an atom.
(ii) Mass number refers to the total number of protons and neutrons / nucleons in (the nucleus of ) an atom.