Observation

This question was popular among the candidates and their performance in the question was above average.

In part (a), majority of the candidates were able to explain the term allotropy and named the substances that that exhibit allotropy.

In part (b), majority of the candidates were able to explain the term neutralization. However, they could not describe how crystals of

NaNO₃ could be prepared in the laboratory.

In part (c), majority of the candidates were able to identify the ions in solution during electrolysis of NaCl solution. However, majority of them could not write a balanced chemical equation for the reaction at each electrode.

In part (d), majority of the candidates were able to state the differences between evaporation and boiling.

The expected answers include:

1. 1. Is the existence of two or more forms of an element in the same physical
      
      

      state OR

   
   

   It is the ability or phenomenon of an element to exist in two or more forms in the

   
   

   same physical state

   
   
   2. • carbon
      • antimony
      • tellurium
      • Iron
      • cobalt
      • polonium
      • arsenic
      • phosphorus
      • sulphur
      • tin
      • silicon
      • oxygen
      • selenium
      • boron
      • germanium

   3. 

2. 1. neutralization is the reaction between H⁺ ions and OH^- ions to form water.
   
   OR
   
   

   neutralization is the reaction between an acid and a base/alkali to form salt and water

   
   
   2. Prepare 50 ^cm3 of 2.0 mol^dm-3 sodium hydroxide solution. Add the
      
      prepared solution to the given solution and stir.
   
   

   Evaporate the mixture to obtain saturated solution . Leave saturated solution for some time to crystallize .Filter the crystals and dry them between filter papers.

   
   

3. 1. Na⁺ and Cl^-₍₁₎ (from NaCl)
      
      

      H⁺ and OH^- ₍₁₎ (from H₂O)

      
      
   2. • at the anode - chlorine

      
      
      • at the cathode - hydrogen
      
      
   3. • at the anode - 2Cl-_(aq) → C _l2(g) + 2e^-
      
      
      • at the cathode - 2H⁺_(aq) + 2e^- → H_2(g)
      
      
   4. It will turn yellow
      
      

   5. • for the manufacture of soap / paper / rayon/fabric
      
      
      • purification of bauxite
      
      
      • refining of petroleum
      
      
      • water treatment
      
      
      • metal processing
      
      
      • oven and drain cleaners
      
      
4. 1. Because it dissolves most substances
   2. 
   
   
   3. Freezing/solidification