Investigating Heating and Cooling Curves of Water
Heat a beaker of ice and water steadily, take the temperature every ten seconds, and plot it. The graph you get has two flat sections in it, and explaining those is the whole point of the practical.
The temperature climbs, then stops dead while the ice melts, then climbs again, then stops dead again while the water boils. You are putting energy in the whole time. So where is it going?
The three states, and what actually differs
Ice, water and steam are all H2O. Changing state does not change the substance, it changes how the particles are arranged and how fast they move.
| State | Shape and volume | Where the particles are | What they are doing |
|---|---|---|---|
| Solid | Fixed volume, fixed shape | Close together, usually in a regular pattern | Vibrating on the spot |
| Liquid | Fixed volume, takes the container's shape | Close together, no regular arrangement | Moving about, sliding past each other |
| Gas | Fills the whole container | Far apart, no arrangement | Moving freely at high speed |
A liquid has a fixed volume but not a fixed shape. Pour 200 ml of water into a tall glass and a flat dish and you still have 200 ml. A gas has neither: it expands until it fills whatever it is in.
Temperature is a measure of how fast the particles are moving
Particles are never still. Temperature measures the average kinetic energy of that motion, so faster particles mean a higher temperature.
That one sentence is what makes the flat sections on the graph explainable, and it is the sentence learners skip.
Melting point, freezing point and boiling point
| Term | Definition |
|---|---|
| Melting point | The temperature at which a solid changes into a liquid |
| Freezing point | The temperature at which a liquid changes into a solid. For a given substance it is the same temperature as the melting point |
| Boiling point | The temperature at which the vapour pressure of the liquid equals the surrounding atmospheric pressure |
Melting and freezing point are the same number. Water melts at 0 °C and freezes at 0 °C. Which one is happening depends on whether energy is going in or coming out, not on the temperature.
Evaporation is not boiling
Liquids evaporate at every temperature, which is why washing dries on the line on a cold day and why a glass of water left out slowly empties.
Evaporation happens only at the surface. Boiling forms bubbles of vapour all through the liquid. That difference is examinable and it is regularly got wrong.
Why the graph goes flat: latent heat
This is the question the practical exists to raise.
Energy is going in at a steady rate the whole time. For most of the run the temperature rises steadily. Then it stops, sits still for a while, and starts rising again.
The energy is going into pulling the particles apart, not into speeding them up.
- At the melting plateau, the energy is freeing particles from their fixed positions in the ice lattice. They are being released, not accelerated
- At the boiling plateau, the energy is dragging particles right out of the liquid altogether and into the gas, which takes several times more again
Temperature measures average kinetic energy. While the energy is going into separation rather than motion, the kinetic energy does not change, so the thermometer does not move.
That hidden energy has a name: latent heat. Latent means concealed, and it is concealed in exactly the sense that you are pouring energy in and the thermometer is refusing to show it.
The boiling plateau is much longer than the melting one, and the class will see that on their own graph. Separating particles completely takes far more energy than merely loosening them.
Why water boils at 94 °C in Johannesburg
If your school is on the Highveld, your water will not boil at 100 °C, and your thermometer is not broken.
A liquid boils when its vapour pressure equals the air pressure pressing down on it. Go higher above sea level and there is less air above you, so less pressure, so the liquid needs less energy to reach that point.
| Where | Altitude | Water boils at |
|---|---|---|
| Durban, Cape Town, Gqeberha | Sea level | 100 °C |
| Johannesburg, Pretoria, Bloemfontein | about 1 700 m | about 94 °C |
| Top of Mount Everest | 8 849 m | 69 °C |
So a Gauteng class should read about 94 on the boiling plateau, and that is the right answer for where they are standing.
Every year classes across the Highveld measure 94, decide something has gone wrong, and write down 100 because that is the number they were taught. It is the most interesting number they will produce all lesson and it gets thrown away.
Melting point barely moves with pressure. Ice melts at 0 °C in Johannesburg and 0 °C in Durban. Only the boiling end of the graph shifts, which is worth asking a class to explain.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 1 |
| Topic | Matter and materials, states of matter and the kinetic molecular theory |
| Status | CAPS prescribed experiment |
| Marks | 40 on our worksheet. None is prescribed |
The practical
Apparatus
| Item | Qty |
|---|---|
| Beaker, 500 ml | 1 |
| Beaker, 250 ml, for the cooling curve | 1 |
| Thermometer, -10 to 110 °C | 2 |
| Spirit burner, or a hotplate | 1 |
| Tripod and gauze | 1 |
| Stopwatch | 1 |
| Goggles | 2 |
You supply the ice. Five cubes a group, out of the freezer at the last possible moment.
Two thermometers, not one. Both parts run in the same period, and moving a single thermometer from a boiling beaker into an ice bath wastes time and risks cracking it.
Part A: the heating curve
- 50 ml of water and five ice cubes in the 500 ml beaker. Leave it standing while the class rules up their tables
- Two columns: time in seconds, temperature in °C. Fill the time column in advance: 0, 10, 20, 30, up to 600
- Record the temperature at time zero. Bulb in the water, not touching the bottom of the beaker
- Light the burner, start the clock, and read every ten seconds. Heat at a steady rate
- Note the moment the last piece of ice disappears
- Keep heating and keep recording. Note when it starts to boil and carry on for at least another minute
- Plot temperature against time, with time on the x-axis
Ten minutes is the usual estimate and it is optimistic on a spirit burner. Plan for fifteen to twenty and rule extra rows.
Part B: the cooling curve
Runs at the same time as Part A, which is the other reason for two thermometers.
- Hot water from a kettle into the 250 ml beaker
- Stand it inside the 500 ml beaker packed with ice
- Record every thirty seconds until it stops falling
It will not reach a freezing plateau in a school period, and that is worth saying in advance rather than letting it look like a failure. Getting water to 0 °C and holding it there needs more ice and more time than a lesson has.
What you should see
| Section | What is happening | The graph |
|---|---|---|
| 1 | Ice warming up | Rising, and it starts below 0 °C if the ice came from a freezer |
| 2 | Ice melting | FLAT at 0 °C |
| 3 | Water warming | Rising steadily. The longest sloping part |
| 4 | Water boiling | FLAT at about 94 °C inland, 100 °C at the coast |
Two flat sections means the practical worked, whatever the exact numbers.
Rough timings on a spirit burner with 50 ml and five cubes: the melting plateau lasts 2 to 4 minutes, heating to boiling takes 8 to 14 minutes, then hold the boil for a minute or two.
The melting plateau is short and easy to miss. Watch the ice, not the clock, and mark the reading where the last piece goes.
If it does not work
| What you see | What caused it |
|---|---|
| The boiling plateau is at 94, not 100 | Correct, if you are inland. This is the right answer, not a fault |
| No flat section at 0 °C | Readings were missed, or the ice had already melted before heating started |
| The graph is a straight line with no flats | Readings were taken every minute instead of every ten seconds, so the plateaux got averaged away |
| The temperature climbs past boiling point | The bulb is touching the bottom of the beaker and reading the hot glass |
| The reading jumps around | The water is not stirred, so the bottom is hotter than the top. Stir with a rod, never the thermometer |
| The temperature drops when heating starts | The thermometer was warm from a pocket. Let it settle in the water first |
| It never boils | The burner is too small for the volume, or it has run dry |
| The melting plateau starts below 0 °C | Freezer ice is around -18 °C. The rise before the plateau is the ice warming, which is correct |
| Part B never flattens out | Expected. Not enough ice or time in one period |
The failure worth keeping
Somebody will stir with the thermometer and break it. It happens in almost every class.
Say it once at the start and then let the broken one be the lesson. A thermometer is R35 and the habit lasts a career.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Aim and variables | 4 |
| Heating curve results table | 8 |
| The graph | 10 |
| Reading melting and boiling point off the graph | 5 |
| Explaining the flat sections | 6 |
| Cooling curve | 4 |
| Conclusion | 3 |
No mark allocation is prescribed. The activity is set with nine numbered steps and no marks, so the worksheet and this split are ours.
The graph carries 10 on its own. Time on the x-axis as the method specifies, a sensible scale, all points plotted, and a smooth curve that shows the plateaux rather than a dot-to-dot zigzag.
The mark most often dropped is explaining the flat sections without mentioning kinetic energy. "It stays the same because it is melting" earns nothing, because it just restates the question. "The energy goes into separating the particles instead of speeding them up, and temperature measures how fast they move" earns all three.
If you have time
Put a lid on the beaker. The pressure rises slightly and the boiling plateau creeps up with it. That is a pressure cooker, and it is why one works.
Ask what a Durban school would measure. Then ask which class is right. Both are, and that is a genuinely uncomfortable idea for a Grade 10.
Add salt and run it again. The boiling plateau shifts up a degree or two. Same reason salt goes on icy roads, working the other way.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the full results table, a gridded page for the graph, the altitude question and the cooling curve
- Marking memorandum, with a worked sample set from a Gauteng school, altitude-adjusted marking, and the six places learners most often drop marks
Related practicals
- Physical and chemical change, Grade 10. The next chemistry topic
- Investigating solubility, Grade 10. Same glassware, same term
- Boyle's Law, Grade 11. Where gas behaviour goes next
Buy this experiment
We are putting together a Heating and Cooling Curves Kit for Grade 10 with both beakers, two thermometers, the burner, tripod, gauze, stopwatch and goggles in one box, plus a printed teacher guide and the marking memo. Coming shortly.
The thermometer is the item to check your store cupboard for. A red spirit thermometer reading -10 to 110 °C is R35 and it is the one piece of equipment this practical genuinely cannot be improvised without. Get two, so the heating and cooling runs happen side by side instead of one after the other.