Bubbles and Clouds in Ice

Ice Block melting
Ice from tap water has bubbles and clouds inside.

Have a look at an ice cube. You’ll see bubbles, clouds and cracks! Let’s find out why they are there and how to make a coloured ice castle inside a clear block of ice!

You need:

  • 4 Empty milk cartons (cut in half)
  • Tap water
  • Distilled water
  • Salt
  • Food colouring

What to do?

Ice cube with food colouring
The amazing food colouring ice cube!
  1. Fill each of your four half-cartons with a different mix so you will end up with four different ice cubes.
    A/ Ordinary tap water
    B/ Distilled water
    C/ Salty water (add a teaspoon of salt to tap water).
    D/ Water with food colouring.
  2. Put the cartons into the freezer.
  3. Check on them now and again. Where does the first ice form? You may need to leave the cartons in the freezer til tomorrow for the blocks to freeze right through.
  4. Tear off the cartons to reveal your huge ice blocks

Why are ice cubes full of clouds and bubbles?

The cloudy parts of an ice cube are salts and minerals. The bubbles you see are bubbles of air. They seem to appear out of nowhere don’t they? The air and the salts were dissolved in the water all along. But they are invisible while the water is liquid. As the water freezes and changes into ice, the dissolved things come out of solution. They form the bubbles and clouds that we can see!

Questions and Answers:

Which part of the ice freezes first?

Ice floats, so no matter where ice crystals form, they will rise to the top.

Why are some cubes clearer than others?

Salts make the cube cloudy. So the amount of cloudiness shows you which ice block has the most salt.

Why are the bubbles trapped in the middle? Why don’t they just get away before the ice forms?

The bubbles get trapped because the ice forms on the top first, and then on the sides and lastly in the middle. This makes a solid ice wall around the last bit of liquid water. The first ice crystals are almost pure water. Air stays dissolved in the liquid part of the water, until the last bit of liquid freezes. Once the bubbles form they are trapped by the ice around them.

Why are the clouds in the middle of the cube and not the top?

Like air, the salts stay dissolved until the last minute. Then they spring out of solution so we can see them as white cloudy puffs. All the coloring also becomes concentrated in the middle. Most impurities in water will do this too. When you eat a frozen orange juice you may have noticed that the sweetest orangiest part is in the centre or towards the bottom!
My big ice cube is rounder on the bottom!

As water changes to ice it gets a bit bigger. The top of the carton froze solid first so the rest of the ice had to expand downwards or outwards. This means the cube will end up stretched and rounded on the bottom or the sides.


Cloudy salty ice block
The saltiest ice cube is cloudy all through
and not as smooth as the other cubes.

Ancient bubbles of air are trapped in the ice at Antarctica? Thousands of years ago, air dissolved into water in rain and the oceans. When the water froze, the dissolved air came out of the solution and became bubbles of air. But the bubbles couldn’t go anywhere! Until the ice melts, the air is stuck. It’s like a time capsule with little samples of air thousands of years old.
So who wants old air?

Old air is very valuable. Scientists love to get air that could be 100,000 years old. We can find out what air used to be like before there were factories, cars and cities. Scientists can use air trapped in old pieces of ice to find out how much carbon dioxide there used to be. That way we know how much it is changing today!

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