Investigating Cooling Curves for Stearic Acid (Oxford AQA IGCSE Physics)

Required Practical: Investigating Cooling Curves for Stearic Acid

Aim of the experiment

• The aim of this experiment is to obtain a cooling curve for stearic acid as it is cooled and use the curve to find the melting point of stearic acid

Variables

• Independent variable = Time, t

• Dependent variable = Temperature, T

• Control variables:

  • Volume of acid, V

  • Initial mass of acid, m

Equipment

Equipment List

• Resolution of measuring equipment:

  • Thermometer = 1 °C

  • Stopwatch = 0.01 s

Method

Equipment for obtaining the cooling curve of stearic acid

1. Secure the boiling tube of boiling stearic acid in place with the clamp stand inside the beaker of boiling water. Position the thermometer and start your stopwatch

2. Record the temperature in degrees Celsius every minute until it reaches about 40 °C

3. For each reading state in the results table whether the stearic acid looks more like a solid or a liquid

Example Results Table

Analysis of results

1. Plot a graph of temperature against time

2. Draw a line for the cooling curve passing through all the indicated points

3. If the cooling curve shows a horizontal straight line then this indicates the time when the stearic acid is changing state

An example of a cooling curve for stearic acid

Evaluating the experiment

Systematic Errors:

• Make sure the measurements on the thermometer and stopwatch are taken at eye level to avoid parallax error

Random Errors:

• Make sure the thermometer is fully in contact with the stearic acid and not in contact with the glass boiling tube

• Repeat the experiment several times

• Make sure the thermometer and the boiling tube are in the same depth every time

Safety considerations

• Wear goggles during this experiment in case the stearic acid splatters

• Wear gloves during this experiment in case the stearic acid spills

• Stand up while carrying out the experiment making sure not to knock the clamp stand and boiling tube or beaker

• Place a mat or a soft material below the beaker to absorb any acid that might spill

• Use a G clamp to secure the clamp stand to the desk so that the boiling tube does not fall over