10.3 - Change of State (Cooling & Heating Curves)
The experiment
Aim
To follow the temperature of a substance as it changes state, and to show that its temperature does not change while the change of state is going on.
Background
- Heating a substance normally raises its temperature, because the energy makes its particles move or vibrate faster.
- During a change of state, the energy goes into changing how the particles are arranged instead, so the temperature stays the same until the change is complete.
- You can't see what the particles are doing, but you can measure the temperature of the substance over time.
- So in this practical you record the temperature of a substance as it cools or is heated, and plot it against time. A flat section on the graph shows a change of state, and its temperature is the melting point or the boiling point.
Variables
- Independent variable - The time since the start.
- Dependent variable - The temperature of the substance.
- Control variables - The substance and its amount, the rate of heating or the surroundings, and the position of the thermometer.
Method - Cooling a melted solid
- Melt the solid - Stand a boiling tube of stearic acid, with a thermometer in it, in a beaker of water on a tripod and gauze, and heat the water with a Bunsen burner until the stearic acid has melted.
- Start the cooling - Turn the Bunsen off, lift the tube out in its clamp to stand in the air, and start a stopwatch.
- Take readings - Record the temperature every 30 seconds, carrying on well after the stearic acid has turned solid.
- Plot the graph - Temperature against time.
Method - Heating ice through melting and boiling
- Set up - Half fill a small beaker with crushed ice, and stand a thermometer in the middle with its bulb surrounded by ice, not touching the glass.
- Heat gently - Stand the beaker on a tripod and gauze, heat it gently with a Bunsen burner, and start a stopwatch.
- Take readings - Every 20 seconds, stir gently, then record the temperature and whether the beaker holds ice, ice and water, water, or boiling water.
- Stop after boiling - Carry on until the water has boiled steadily for a minute or two.
- Plot the graph - Temperature against time.
Safety
- Eye protection - Wear it throughout.
- Hot apparatus - Keep the beaker on a heat-proof mat, and handle the boiling tube with a clamp or tongs.
- Set stearic acid - Don't pull the thermometer out once the stearic acid has set, or the glass will snap. Melt the sample in hot water first.
Worked example - Cooling curve of stearic acid
A student melted a sample of stearic acid, took it out of the hot water, and recorded its temperature every 30 seconds as it cooled in a room at about 20 °C.
| Time (s) | 0 | 30 | 60 | 90 | 120 | 150 | 180 | 210 | 240 |
|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) | 85 | 78 | 72 | 69 | 69 | 69 | 69 | 66 | 63 |
Describe the graph, find the melting point, explain the flat section, and explain why the curve is less steep at the end.
Step 1 - Plot the graph and describe its shape
Plot temperature against time. The temperature falls steeply for the first 90 s, stays at 69 °C from 90 s to 180 s, and then falls again more slowly.
Step 2 - Read off the melting point
The flat section is where the liquid is turning into a solid. It's at 69 °C, so the melting point is 69 °C.
Read it from the middle of the flat section, not from either end. The graph rounds off slightly where the flat part joins the sloping parts, because different parts of the sample solidify at slightly different moments, and the middle is where the whole sample is changing state together.
Step 3 - Explain why the temperature stops falling
As the liquid solidifies, bonds form between the particles, which releases energy. This energy balances the energy being lost to the room, so the temperature stays the same until the whole sample is solid.
Step 4 - Explain why the curve is less steep at the end
How fast the stearic acid cools depends on the temperature difference between it and the room.
- At the start, the difference is 85 − 20 = 65 °C, so energy is transferred quickly and the curve is steep.
- At 240 s, the difference is 63 − 20 = 43 °C, so energy is transferred more slowly and the curve is shallower.
Common exam questions
Question 1
Why is the ice stirred between readings?
So the temperature is the same throughout the beaker. Otherwise the part nearest the heat is hotter than the rest, and the thermometer doesn't show the temperature of the whole sample.
Question 2
Why is crushed ice used rather than one large block?
Crushed ice packs closely round the thermometer bulb and melts evenly, so the thermometer reads the temperature of the ice. A block leaves gaps of air or meltwater round the bulb.
Question 3
Why does an impure sample of stearic acid have a lower melting point than a pure one?
Anything mixed in gets in the way of the particles settling into their fixed arrangement, so the sample turns solid at a lower temperature, and over a small range rather than at one temperature.
Question 4
On a heating curve for water, name the change of state at each flat section, and explain why the second is longer.
The first is melting, at 0 °C, and the second is boiling, at 100 °C. Boiling water takes much more energy than melting the same mass of ice, so at a steady rate of heating it lasts longer.
Question 5
Why is the melted stearic acid left to cool in air rather than put into cold water?
Cold water would cool it so fast that few readings would fall in the flat section, and the melting point would be hard to read.