Observation

This was a compulsory question.  Majority of the candidates that responded to it
performed fairly. 

In part (a), majority of the candidates were not familiar with the term ‘nucleons’. They could not show that protons and neutrons that constitute the nucleons are located in the nucleus of an atom.
In part (b), majority of the candidates defined Graham law of diffusion correctly.  However,
a good number of them omitted constant temperature and pressure in their definitions.

 In part (c), majority of the candidates explained why aluminum does not corrode easily.
 
In part (d), majority of the candidates stated three examples of periodic properties correctly, however, a few of them stated one or two examples.

In part (e), majority of the candidates could not state two reasons for the deviation of real gases from ideal gas behaviour.  

In part (f), majority of the candidates did not understand the difference between fractional and simple distillation, and it reflected in their responses.

In part (g),majority of the candidates listed correctly the three factors that determine the
selective discharge of ions at the electrodes during electrolysis.

In part (h), majority of the candidates got the two reactions correctly as substitution and addition reactions respectively.

In part (i), majority of the candidates did not understand that butane burns in limited supply of air to produce carbon(II) oxide, and not carbon(iv) oxide.

In part (j), majority of the candidates listed correctly three methods of separating a solid from
a liquid.

The expected answers include:

(a)        Nucleons refer to the protons and neutrons that are found in the nucleus    
            OR Nucleons are nuclear particles which are protons and neutrons.

(b)       Graham’s law of diffusion states that, at constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density/ molar mass / vapour density.
OR Accept mathematical expression r     at constant temperature and pressure
             where r  = rate of diffusion of a gas;
                         d = density.

(c)       Aluminum oxide layer forms on the surface of the metal. The oxide layer forms a protective coating/prevents aluminum from further attack/ is impermeable to water/ air/ layer does not flake off .                                                      
(d)       -           atomic radius
            -           Ionic radius
            -           ionization energy/redox potentials
            -           electronegativity
            -           electron affinity/redox potentials
           -          metallic/non metallic properties/electrical and thermal conductivity
           -          melting point/boiling point

(e)       -           Molecules of real gases have significant attractive forces between them
            -           The volume of the molecules of real gases is not negligible
            -           Collision between the molecules is not perfectly elastic                                                                                   
(f)        -           Separation of crude oil into its components/petroleum refining/
            -           Purification of ethanol  Accept production of ethanol
            -           Separation of liquefied air / industrial preparation of nitrogen / oxygen

(g)       -           Position of the ions in the electrochemical series
            -           Concentration of the ions in the electrolyte.
            -           Nature of the electrodes used / Type of electrode used
                                                                                                                                   
(h)       (i)        Substitution reaction
            (ii)       Addition reaction  
                                                                                                                                   
(i)        -           carbon/soot                          
            -           carbon (II) oxide                 
            -           water                                     
                                                                                                                                   
(j)        -           Filtration                               
            -           Crystallization (and filtration)
            -           Evaporation                          
           -          decantation

           -           Centrifugation (and decantation)