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Science project idea

Boiling water at room temperature in a syringe: the pressure cooker idea

Water boils when its vapour pressure matches the pressure around it. Pulling the plunger of a sealed syringe lowers the pressure and makes warm water bubble, which is the same relationship that makes a pressure cooker cook faster when the pressure is raised instead.

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What it shows

Water boils when its vapour pressure matches the pressure around it. Pulling the plunger of a sealed syringe lowers the pressure and makes warm water bubble, which is the same relationship that makes a pressure cooker cook faster when the pressure is raised instead.

The science

Boiling occurs when the vapour pressure of the liquid equals the pressure on it. A sealed syringe with the plunger pulled out cuts the pressure inside, so even 50 C water can boil. In a pressure cooker the opposite happens: sealing the pot raises the pressure to about twice the atmosphere, so water boils near 120 C and food cooks faster.

Materials

Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A 20 ml or 50 ml plastic syringe without needle
  • Warm tap water of about 50 C, measured with a thermometer (adult prepares)
  • A cap or a finger tip to seal the nozzle
  • A thermometer
  • A bowl
  • A notebook

Safety

For this project: An adult must heat the water and keep it at or below 50 C, and no real pressure cooker may be opened or experimented with by a child. Never push or pull so hard that the syringe breaks. Wear eye protection because plastic can pop off.

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. Draw 10 ml of warm water at about 50 C into the syringe, leaving the nozzle clear.

  2. Hold the syringe upright with the nozzle at the top and push the plunger up gently to remove most air.

  3. Seal the nozzle tightly with a cap or a fingertip.

  4. Pull the plunger back firmly and watch for tiny bubbles that stream up from the water.

  5. Release the plunger and see the bubbles stop as the pressure returns.

  6. Repeat with water at room temperature and with water at about 40 C to compare how hard you must pull.

  7. Write a short note on how raising the pressure in a cooker has the opposite effect.

What you should see

The warm water bubbles vigorously when the plunger is pulled and stops when released, showing that boiling depends on pressure as well as temperature. Colder water needs a stronger pull.

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 the boiling point of water at lower pressure?

It is lower than 100 C, as in the hills.

2Why does a pressure cooker cook faster?

The raised pressure lets water get hotter than 100 C.

3Why is this safe to do?

Only warm water is used and no heat is applied in the syringe.

4What happens to food in the hills?

It cooks slower because water boils below 100 C.

Free ebook: Viva Questions for Science Projects

Take it further

Look up the boiling point of water at your city altitude and compare it with Shimla and Leh.

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.

Free ebooks for this project

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