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Science project idea

Cooking time against size: potato cubes in boiling water

Heat moves into a potato from the outside by conduction, so a bigger piece takes much longer to cook through. By boiling cubes of different sides and noting when a fork goes in, you find the link between size and cooking time.

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What it shows

Heat moves into a potato from the outside by conduction, so a bigger piece takes much longer to cook through. By boiling cubes of different sides and noting when a fork goes in, you find the link between size and cooking time.

The science

Heat enters the cube through its surface and must travel to the centre by conduction. The time needed rises with the square of the distance, so a cube with twice the side takes about four times as long. This is why big pieces cook slowly and why cooks cut vegetables small when they are in a hurry.

Materials

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

  • 2 potatoes of the same kind
  • A pan with boiling water (adult only)
  • A stopwatch
  • A fork or toothpick
  • A ruler
  • A slotted spoon
  • A notebook

Safety

For this project: An adult must cut the potato, handle the boiling pan and lift the cubes out with a slotted spoon. Children only time and record. Keep the handle of the pan turned inward.

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. Ask an adult to cut potato cubes with sides of 1 cm, 2 cm, 3 cm and 4 cm, three cubes of each size.

  2. Bring a pan of water to a rolling boil and keep it boiling.

  3. Drop in the three 1 cm cubes together and start the stopwatch.

  4. After a minute lift out one cube and test it with the fork; repeat every minute until it slides in easily.

  5. Record the time at which the fork first passes through easily.

  6. Repeat for the other sizes with fresh water, which is the same boiling temperature.

  7. Plot the cooking time against the side of the cube and then against the square of the side.

What you should see

The cooking time rises steeply with size, and a graph of time against side squared is close to a straight line, which supports the conduction idea.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1How does heat reach the centre?

By conduction through the potato.

2Why does a bigger cube take longer?

The heat has to travel farther and there is more mass to warm.

3Why keep the water boiling?

So the outside stays at a steady 100 C.

4How does a pressure cooker help?

It raises the water temperature above 100 C.

Free ebook: Viva Questions for Science Projects

Take it further

Try the same test with carrot, which has different water content, and compare the graphs.

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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