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KS2 Science at Home: week 7

Key Stage 2 Experiment of the Week! – Cool Crystals

How does it work? 

Cool Crystals2

More salt can dissolve in hot water than cold, so when the hot water cools in the microwave, the Epsom salts create crystals on the side of the cup. The unique shape of Epsom salt molecules makes them form long crystals that almost look like needles. Different crystals have different shapes. If you look at sugar or salt crystals under a magnifying glass, you can see their cool crystal shapes, too!

Materials you need

  • ¼ cup Epsom salt
  • ¼ cup hot water
  • Food colouring
  • A cup
  • A plate
  • Optional: a small jar


1. In the cup, measure out ¼ cup Epsom salt and ¼ hot water from the sink. Stir them together.

2. If all the salt doesn’t dissolve, heat the cut in the microwave for 20-30 seconds.

3. When all the salt is dissolved, put a drop or two of food colouring in the cup and stir to mix.

4. Place the cup in the refrigerator. Check on it every half hour or hour. Within 4 hours, crystals should form in the bottom of the cup.

5. Scoop the crystals onto a plate using a fork.

6. If you want the crystals to last longer, put them in a jar with a lid (the small jars that baby food comes in work well).

Some research for next week: What is catastrophic crack propagation? 
We will answer this question with next week’s science experiment!