🔥 Heating (ice)
On the sloping parts, energy raises the kinetic energy of the particles, so the temperature rises. On the flat parts, the energy overcomes the forces between the particles, so the state changes without a change in temperature.
Change of State » Practical: Temperature-Time Graphs
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 5.11P
- How to obtain a temperature-time graph during a change of state
- The method for heating crushed ice and for cooling melted stearic acid
- How to plot the graph and find the flat sections
- How to explain why the temperature stays constant, and how to work safely
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseThe idea: watch the temperature during a change of state
This lesson is examined in Paper 2 only.
In the lesson Heating and Changes of State you saw that while a substance melts or boils, the energy goes into pulling particles apart, so the temperature stays constant. In this practical you show it by measuring. You record the temperature at regular times and plot a graph. The flat sections of the graph show the change of state.
Key terms:
- Temperature-time graph: a graph with time on the horizontal axis and temperature on the vertical axis.
- Flat section: a part of the graph where the temperature stays constant.
- Freezing: a liquid changing into a solid. It happens at the same temperature as melting.
- Water bath: a beaker of hot water used to heat something gently and evenly.
Method 1: heating crushed ice
Apparatus: a 250 cm3 beaker, crushed ice, a thermometer (or a temperature sensor and data logger), a stirrer, a stopwatch, a tripod and gauze, and a Bunsen burner or hot plate.
- Put on eye protection. Half fill the beaker with crushed ice and stand it on the gauze and tripod.
- Put the thermometer in the ice. Do not let the bulb touch the glass of the beaker.
- Record the starting temperature. Start the stopwatch and start heating.
- Stir gently and record the temperature every 30 seconds. Read the thermometer at eye level.
- Keep going while the ice melts, while the water warms up, and through boiling. Carry on for a few minutes of boiling.
- Turn the heater off and let everything cool before moving it.
What you should see: the temperature rises to 0°C and stays at 0°C while the ice melts. It then rises again to about 100°C and stays at about 100°C while the water boils. That is two flat sections. Water boils at about 100°C at normal air pressure.
Method 2: cooling melted stearic acid
Stearic acid (or wax) melts at a fairly low temperature, so you can melt it in a water bath and then watch it freeze.
- Put on eye protection. Half fill a boiling tube with solid stearic acid and stand it in a beaker of hot water, the water bath, until all of it has melted. Heat the water bath well above the melting point, to about 90°C.
- Put the thermometer into the liquid. Lift the boiling tube out of the water bath, using a holder, and stand it in a clean empty beaker or a rack to cool.
- Start the stopwatch and record the temperature every 30 seconds. Stir gently with a stirrer while the liquid is still runny.
- Keep recording until the temperature is well below the point where it went flat, for example for 10 minutes in total.
- Do not pull the thermometer out of solid stearic acid. Stand the tube back in hot water until it melts again.
Remember, a data logger records sensor readings automatically (see Practical: Investigating Motion). A temperature sensor and data logger do the same job. They take a reading at set times and draw the graph for you. You still need to be able to explain the graph.
Variables and accuracy
| Type | What it is |
|---|---|
| Independent variable | Time, in steps of 30 seconds |
| Dependent variable | Temperature, in °C |
| Control variables | Mass of substance, the same heating, the same stirring, thermometer in the same position |
- Stir so that every part of the substance is at the same temperature. Without stirring, the thermometer reads only the part next to it.
- Read the thermometer at eye level. Looking from above or below gives a wrong reading.
- Keep the bulb in the middle of the substance, not touching the glass. The glass can be hotter or colder than the substance.
- A water bath heats gently and evenly. A flame under a boiling tube could overheat the substance, and stearic acid and wax can catch fire. A water bath makes this much less likely.
Results, plotting and finding the flat sections
Worked example: stearic acid cooling
| Time (s) | 0 | 30 | 60 | 90 | 120 | 150 | 180 | 210 | 240 | 270 | 300 | 330 | 360 | 390 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) | 85 | 82 | 79 | 76 | 73 | 70 | 69 | 69 | 69 | 69 | 69 | 67 | 64 | 61 |
To plot it: put time on the horizontal axis, with a scale of 1 cm to 30 s, and temperature on the vertical axis, with a scale of 1 cm to 5°C starting at 55°C. Label both axes with units. Mark each point with a small cross. Draw a straight line through the points that fall steadily, then a horizontal line through the points at 69°C, then another straight line through the last points.
The flat section runs from 180 s to 300 s, so the temperature is constant at 69°C for 120 s. This is the temperature at which the stearic acid freezes, which is the same as its melting point.
Explaining the flat sections
❄️ Cooling (stearic acid)
On the sloping parts, the substance loses energy to the air and cools. On the flat part, the particles pull back together to form a solid, and the energy this gives out keeps the temperature constant until it is all solid.
In an exam, say that the energy is being used to change the state, not to change the temperature.
Working safely
- Wear eye protection, because hot water can spit.
- Hot liquids and hot glass cause burns. Use a holder for boiling tubes and let things cool before touching them.
- Keep long hair and loose clothes tied back near a Bunsen burner, and never leave a flame unattended.
- Stearic acid and wax are flammable. Heat them only in a water bath.
- A thermometer stuck in cold solid wax can snap if you pull it. Melt the wax again first.
Common mistakes
Common mistakes
- Saying the temperature rises during melting. It stays constant, even though heating continues.
- Drawing one smooth curve through everything. Draw straight lines through the sloping parts and a horizontal line through the flat part.
- Saying nothing is happening in the flat section. The state is changing.
- Not stirring, so the readings are uneven.
- Stopping too early, before the second flat section when heating ice.
Exam-style question
A student cools some melted wax in a boiling tube and records its temperature every 30 seconds. The wax is at 80°C at the start. The temperature falls steadily to 54°C. It then stays at 54°C for 3 minutes and falls again.
(a) Describe how the student should obtain the readings. [3 marks]
(b) State the freezing point of the wax. [1 mark]
(c) Explain why the temperature is constant for 3 minutes. [2 marks]
(d) Give one reason for melting the wax in a water bath. [1 mark]
Model answer
(a) Put a thermometer in the wax (1). Start a stopwatch and read the temperature every 30 seconds (1), at eye level and stirring gently, until well below 54°C (1).
(b) 54°C (1).
(c) The wax is changing from liquid to solid (1). The energy given out as the particles pull together keeps the temperature constant until it has all solidified (1).
(d) It heats the wax gently and evenly, so it is much less likely to overheat or catch fire (1).
Exam tip
For a flat section, always link it to the change of state: name the change (liquid to solid) and say that the energy is used for it, not for changing the temperature.