Observation

Few candidates responded to this question and their performance in the question was below average.

In part (a), majority of the candidates could not define the term structural formula. Also, they could not draw the structural isomers of pentane correctly.

In part (b), majority of the candidates could not state the observations made by Lord Rutherford in his α-particle scattering experiment.

In part (c), majority of the candidates were able to state the laws of chemical combination. However, many of them could not state the factors that determine solubility of an ionic solid in water.

In part (d), majority of the candidates displayed good knowledge of pH.

The expected answers include:

1. A ( graphic) representation of a molecule of a compound showing how the atoms are linked together / The arrangement of atoms within the molecule of a substance.
   1. 

   2. 

   
   
   3. pentane
      • 2 – methylbutane
      
      
      • 2, 2 – dimethylpropane
      
      
2. i most of the α – particles went straight through the foil
   
   • a few α – particles were deflected through small / fairly large angles
   
   
   • very few α – particles were repelled back towards the source
   
   
ii the atoms making up the foil contained a lot of empty spaces

• the positive part of the atom was concentrated in a very small region of
  space (the nucleus)


• the nucleus must be very heavy containing almost the entire mass of
  the atom



Law of Conservation of matter




3. 1. During the course of a chemical reaction, matter is neither created nor destroyed.

      
      

      Law of Definite proportions

      
      
      • All pure samples of the same chemical compound contain the same elements combined in the same proportion by mass.
      
      Law of Multiple proportions
      
      • When two elements A and B combine to form more than one different compounds, the different masses of B that combine separately

        with a fixed mass of A are in a simple ratio.

      
      
   2. • lattice energy
      
      
      • hydration energy
      
      
      • temperature
      
      
   3. forces between the units / molecules in CO₂ are weak van der Waal’s forces whereas forces between the units / atoms in SiO₂ are (strong) covalent bonds

4. 1. 1. Solution X
      
      
      2. Solution Y
      
      
   2. Solution Y > Solution Z > Solution X / Y, Z, X
   
   
   3. The lower the pH the stronger the acid hence, the greater the extent of dissociation.