What it shows
When liquid wax freezes it gives out latent heat, so the temperature stays nearly steady while it solidifies. The flat part of the cooling curve marks the freezing point.
The science
Heat is given out as wax cools. During freezing the particles settle into a solid and release latent heat, which replaces the heat lost to the room, so the temperature stays almost constant. A pure substance gives a long flat line. Candle wax is a mixture of hydrocarbons, so the plateau slopes a little and lies in the range of about 55 to 60 degrees C for common paraffin candles. After the wax has set the temperature falls again.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A lab thermometer reading to 110 degrees C
- Pieces of ordinary candle wax, about 20 g
- A test tube or a small steel tumbler
- A steel pan of hot water for a water bath
- A stopwatch or phone timer
- A clamp or a clothes peg on a stand to hold the tube
- Graph paper
Safety
For this project: An adult must supervise. Heat the wax only in a water bath, never over a flame, because wax can catch fire. Hot water scalds, so use tongs or a cloth, and keep the thermometer away from the flame or hot plate.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill the test tube one third with small wax pieces and stand it in a pan of water that is just below boiling, held upright by a clamp.
Wait until the wax melts completely and put the thermometer in the liquid, keeping its bulb off the bottom and sides.
When the wax reaches about 85 degrees C, lift the tube out of the hot water, dry the outside and stand it in a cup in the open air.
Start the timer and read the temperature every 30 seconds. Write each reading in a table.
Keep reading until the wax has set hard and the temperature has fallen to about 40 degrees C. A common mistake is to stir with the thermometer after the wax starts to set, which can break it.
Plot temperature on the vertical axis against time on the horizontal axis and join the points with a smooth line.
Mark the flat or slowly sloping part of the curve and read off the freezing range.
Repeat the experiment with the tube inside a larger glass of warm water so that it cools more slowly, and compare the lengths of the plateaus.
What you should see
A cooling curve with a steep fall, a nearly flat region near 55 to 60 degrees C where the wax solidifies, and then a second slow fall. The plateau is longer when the wax cools slowly.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the temperature stay almost constant while the wax freezes?
Latent heat released during freezing makes up for the heat lost to the surroundings.
2Is the freezing point the same as the melting point?
Yes, for a pure substance, both happen at the same temperature.
3Why is the plateau of candle wax not perfectly flat?
Candle wax is a mixture of many compounds, which freeze over a small range.
4Why is a water bath used for heating?
It cannot go above about 100 degrees C, so the wax is not overheated and cannot catch fire.
Take it further
Repeat with ice made from salty water and compare the freezing point and the shape of the curve with pure water.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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