Observation

This question was compulsory for all the candidates. The performance of candidates in this question was not impressive.

In part (a), majority of the candidates could not explain standard hydrogen electrode.
In part (b), a good number of the candidates gave a reason why metals are generally good reducing agents.
In part (c), only few of the candidates could state why diamond is a non-conductor of electricity. However, majority of them stated why it is used as an abrasive.
In part (d), majority of the candidates explained the term chemical change satisfactorily.
In part (e), majority of the candidates correctly explained the term isotopes.
In part (f), majority of the candidates could not differentiate between a weak acid and a dilute acid.
In part (g), majority of the candidates did not define pH as the degree of acidity or alkalinity of a solution. Majority of them omitted an important term to score.
In part (h), some of the candidates stated two harmful effects that nitrogen (IV) oxide has on the environment.
In part (i), majority of the candidates wrote an equation for the reaction of chlorine with magnesium bromide.
In part (j), majority of the candidates could not state the method used to obtain pure ethanol from the aqueous mixture

The expected answers include:

1. It measures the tendency of an element to form ions in solution relative to the tendency of hydrogen atoms to form ions in solution at conditions of 298 K / 100KPa / standard conditions in 1 mol dm-3 H+ solution

   OR

   Consists of platinum metal block with surface covered with a tube of hydrogen at 1 atm pressure immersed in 1 mol dm-3 H+ solution at 25oC.
2. Metals (readily) donate electrons / they are electron donors
3. (i) Because it sparkles / shinny nature and reflects light in an attractive way / have high refractive index.
   (ii) Because it is very hard / it is the hardest of all natural substances.
   (iii) There are no free valence electrons in the diamond crystal / all the valence electrons are involved in bonding / bond formation
4. (i) A chemical change is one which is not (easily) reversible / in which new substances are formed.
   (ii) - rusting of iron
   - burning of substance
   - decomposition of organic matter
   - neutralization of an acid with a base
   - slaking of lime
   - fermentation
   - oxidation / reduction / redox reaction
   - decay
   - dissolution of a metal in acid
   - hydration of a compound
   - hydrolysis of a compound
5. Two or more atoms of the same element that have the same proton number but different number of neutrons / two or more atoms of the same element having the same atomic number but different mass numbers
6. Weak acid ionizes / dissociates partially in solution whereas dilute acid contains small amount of the acid in solution / contains large amount of solvent.
7. pH is a measure of the negative logarithm (to base 10) of the hydrogen / hydroxonium ion concentration ( of a solution )
   OR
   pH is a measure of the concentration of hydroxonium or hydrogen ions in a solution
   OR
   pH is the measure of the degree of acidity or alkalinity of a solution OR
   The logarithm to base 10 of the reciprocal of hydroxonium / hydrogen ion concentration(formula / equation is not acceptable)
8. - Acid rain / nutrient pollution in water bodies
   - Smog
   - Depletion of ozone layer
   - Breathing difficulties / respiratory diseases
9. Cl2(g) + MgBr2(s) → MgCl2(s) + Br2(l)
10. (i) Fractional distillation Accept distillation
    (ii) Exclude air / oxygen / absence of air / prevention of air