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Science project idea

Latent heat of fusion of ice by mixing with warm water

Melting ice absorbs a lot of heat without changing temperature. Mixing dry ice at 0 degrees C into warm water and measuring the final temperature gives the latent heat of fusion, about 334 joules per gram.

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What it shows

Melting ice absorbs a lot of heat without changing temperature. Mixing dry ice at 0 degrees C into warm water and measuring the final temperature gives the latent heat of fusion, about 334 joules per gram.

The science

The heat lost by warm water as it cools to the final temperature T_f equals the heat needed to melt the ice, m_i L, plus the heat needed to warm the melted ice from 0 degrees to T_f: m_w c (T_w − T_f) = m_i L + m_i c (T_f − 0). So L = [m_w c (T_w − T_f) − m_i c T_f] / m_i. The ice must be dried before it is added, or the film of water on it adds to the error. The accepted value is 334 J/g (3.34 × 10⁵ J/kg).

Materials

Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A foam cup or a steel cup wrapped in cloth as a calorimeter
  • Ice cubes made from water, kept in a bowl of ice water
  • Warm tap water at about 40 degrees C
  • A thermometer reading to 0.5 degrees C
  • A kitchen scale
  • A paper towel to dry the ice
  • A spoon and a calculator

Safety

For this project: An adult should supervise the hot tap water. Handle the glass thermometer carefully.

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 rules

Step by step

7 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.

  1. Weigh the empty cup, add about 150 g of warm water, weigh again to get m_w and read the temperature T_w.

  2. Take two or three ice cubes from the ice-water bowl, dry them quickly with the paper towel and drop them in the cup.

  3. Stir gently until the ice has melted completely and note the lowest temperature T_f reached.

  4. Weigh the cup again; the gain is the mass of ice m_i.

  5. Compute L using the formula, taking c as 4.2 J/g K.

  6. Repeat the experiment three times and calculate the average.

  7. Compare with 334 J/g and find the percentage error, listing the causes.

What you should see

The experimental value of L lies between about 300 and 360 J/g, close to the accepted 334 J/g. The main errors are the water film on the ice and the heat gained from the room.

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 of fusion?

The heat needed to change 1 kg of a solid at its melting point completely to liquid at the same temperature.

2Why does the temperature not rise while the ice melts?

The heat goes into breaking bonds, not into raising the temperature.

3Why must the ice be dry?

Surface water would add mass that has already melted, giving a wrong value.

4Why is it better to start with the water a little above room temperature?

Then the final temperature is below room temperature and the errors from heat exchange partly cancel.

Free ebook: Viva Questions for Science Projects

Take it further

Repeat with crushed ice and with salt added and measure the lower temperature reached.

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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