What it shows
Ice at 0 °C takes a large amount of heat to melt, without any rise in temperature. Mixing a known mass of ice into warm water and noting the final temperature lets us calculate this latent heat as about 334 joules per gram.
The science
Heat lost by the warm water equals the heat gained by the ice, which first melts and then warms to the final temperature (method of mixtures). If the water has mass m1 and falls from T1 to Tf, and the ice has mass m2, then m1 c (T1 minus Tf) = m2 L + m2 c (Tf minus 0), where c is the specific heat of water, 4.2 J per g per °C. Solving gives L = [m1 c (T1 minus Tf) divided by m2] minus c Tf. The accepted value is about 334 J/g, and the measured value is usually a little different because of heat exchange with the cup and the air.
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 thermos flask or a well-insulated steel cup with a lid
- A thermometer with 0.5 or 1 °C marks
- A kitchen scale
- About 60 g of ice cubes made from clean water and dried with a cloth
- About 200 g of warm water at about 50 °C from the tap or flask
- A spoon for stirring
- A calculator and a notebook
Safety
For this project: An adult must handle the warm water, which should not be above 55 °C. Wipe spills and handle thermometers carefully because glass can break.
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.
Weigh the dry empty cup and then the cup with about 200 g of warm water, and find the mass of water m1 by subtraction.
Stir and record the temperature T1 of the water just before you add the ice.
Dry some ice cubes with a cloth to remove surface water, and add about 50 g of them. Weigh the ice in a second cup beforehand to be accurate.
Stir gently and put on the lid. Watch the thermometer fall and write the lowest steady temperature Tf, after all the ice melts.
Weigh the cup with the contents again. The increase in mass is the mass of ice m2.
Calculate L = [m1 times 4.2 times (T1 minus Tf) divided by m2] minus 4.2 times Tf, in J per gram.
Repeat the experiment twice with fresh ice and warm water and take the average of the three answers.
Compare with 334 J/g, work out the percentage error, and list where heat was lost or gained from the surroundings.
What you should see
A run with 200 g of water at 50 °C and 50 g of ice should end near 24 °C, giving a latent heat in the range of about 300 to 370 J/g.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is latent heat?
The heat absorbed or released during a change of state at a constant temperature.
2Why must the ice be dried?
Surface water would add to the mass of ice without needing latent heat.
3What is the principle of calorimetry?
Heat lost by hot bodies equals heat gained by cold bodies when there is no loss to the surroundings.
4Why does the temperature of melting ice stay at 0 °C?
All the heat goes into breaking the bonds between molecules and not into raising the temperature.
Take it further
Estimate the latent heat of evaporation using the heat taken from a cup of water by a wet cloth.
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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