Acidic chill

What’s happening?

Water and indicatorAt room temperature and ‘normal’ pressure (1 atmosphere), carbon dioxide (CO2) is a gas. Pouring water over the dry ice made it quickly turn from a solid into a gas without passing through a liquid phase. The dry ice itself is so cold the cold CO2 gas it releases chills the water in the air, turning it into a foggy vapour.

As it bubbles through the water, however, a small amount recombines with the hydrogen and oxygen in the water molecules to form a new chemical called ‘carbonic acid’ (CO2 + H2O → H2CO3). The remainder of the carbon dioxide merely floats through the water as a gas or bubbles away into the atmosphere.

Carbonic acid lowers the pH of the water, making it more acidic and changing the chemicals in the indicator in a way that makes them reflect different colours.


Right now, as your body’s cells busily munch on your breakfast, carbon dioxide is being formed through a process called respiration. Fortunately it doesn’t bubble out of your cells as a gas – it dissolves into your blood as carbonic acid instead.

Before you panic at the thought of having acid for blood, it helps to know it’s a good thing. Not all of the CO2 becomes H2CO3; if the environment is already too acidic, the carbonic acid can turn back into carbon dioxide, raising the pH again. As it helps to balance the acidity of your blood, carbonic acid is referred to as a buffer.

To help speed this process up, your red blood cells have an enzyme that can steal hydrogen from the carbonic acid to make bicarbonate (HCO3–). In your lungs, this whole process is reversed, releasing a percentage of the carbon dioxide as a gas from your blood and allowing it to diffuse into the air.

This week’s activity uses frozen carbon dioxide – better known as ‘dry ice’. This material can be purchased through some party supply companies or direct from BOC Gases.

Warning: Dry ice is not like water ice. Do not allow it to touch your skin; the extreme cold can cause harm. Wear thick gloves and safety goggles when doing this activity. If using a bottle, do not fasten the lid when there is dry ice in the container under any circumstances.

You will need

  • Dry ice
  • Safety goggles
  • Cotton gardening gloves
  • Tall, clear plastic container (soft drink bottle or measuring cylinder works best)
  • pH indicator (From swimming pool supplies stores, or make your own)
  • Jug
  • Water

What to do

  1. Create a 5cm high layer of dry ice on the bottom of the container.
  2. Mix pH indicator with the water to get a strong colour.
  3. Pour the water into the container with the dry ice.
  4. What colour does the indicator turn? Can you work out the pH of the solution?
    Water and indicator
    You’ll need pH indicator and water.
    Green = plain water
    Mix the indicator and the water.
    Red = acidic
    Carefully add some dry ice and watch it change colour