This question was very popular and well attempted by many candidates.  Good scales were chosen and good graphs plotted.  Candidates also provided correct answers to the short response questions.  However, few candidates mixed up the recording of temperatures for the two tins thereby graphs could not be labeled.  Many candidates were unable to correctly deduce the time taken for the temperature to fall from 85 ^oC to 65 ^oC.  Few candidates could not specifically mention how heat loss by radiation is minimized in a vacuum flask.

 

 

In part (a) the tasks required of the candidates included:

1. Reading and recording values of temperature;

 (Trend; As t increasesθ_cand θ_Dincrease)

1. Recording their data in a composite table showing t, θ_c and θ_D;
2. Distinguishing graph axes, selecting reasonable scales, plotting at least four points correctly from each set of temperaturesθ_candθ_Don the same axes.;
3. Drawing two smooth curves and indicating on each graph temperature of 85 ^oC and 65 ^oC;
4. Reading and recording the time taken to cool from 85 ^oC and 65 ^oC on each graph;

1. Stating the precautions to ensure accurate results as follows:

-     Liquid gently stirred;
-     Parallax error avoided in reading the thermometer;
-     Splashing of water avoided.

The expected responses to part (b) were as follows:

(b)(i)    How heat losses are minimized in a vacuum flask.

-     Silvered walls minimize radiation                                                                   

1. Vacuum prevents convection and conduction OR
2. Insulating cork/stopper minimizes  convection

    (ii)   Factors which affect the rate of evaporation of a liquid in an open container:                      
    -     Temperature

• Draught/wind
• Nature of liquid
• Humidity
• Area of liquid surface exposed