Chemistry 9701/51 — October/November 2011
Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme
Topics Planning · Analysis, Conclusions and Evaluation
When potassium nitrate dissolves in water, the temperature of the solution goes down because the enthalpy of solution is endothermic.
You are to plan an experiment to investigate how the solubility of potassium nitrate varies with temperature. The units of solubility are grams per one hundred grams of water (g/100g water).
Predict how the solubility of potassium nitrate will change if the solution temperature is increased.
Explain your prediction using the fact that dissolving potassium nitrate is endothermic.
Display your prediction in the form of a sketch graph, labelling clearly the axes.
In the experiment you are about to plan, identify the following.
the independent variable
the dependent variable
Design a laboratory experiment to test your prediction in (a).
In addition to the standard apparatus present in a laboratory you are provided with the following materials,
- a boiling tube,
- a looped wire stirrer,
- a thermometer covering the temperature range 0 °C to 100 °C.
Describe how you would carry out the experiment. You should
- ensure a wide range of results suitable for analysis by graph,
- decide on the amounts of water and potassium nitrate to use,
- measure the amounts of the two reagents,
- heat the apparatus,
- decide at what point the temperature of the solution is to be taken.
State a hazard that must be considered when planning the experiment and describe precautions that should be taken to keep risks to a minimum.
Draw a table with appropriate headings to show the data you would record when carrying out your experiments and the values you would calculate in order to construct a graph to support or reject your prediction in (a). The headings must include the appropriate units.
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