Hot and Cold Salt Crystal Race

What you need

• Salt, about 1 cup
• Water, 1 cup hot, plus 1 cup cold
• Paper, 1 sheet
• Crayons or markers, a few
• Glass jars, 2
• Spoon, 1
• Ruler, 1
• Pencil, 2
• String, 2 pieces, 20 cm (8 in) each
• Towel, 1, or a tray

How to do it

Grown-up prep: You will need to heat the water and pour it into the jar. Pick a still spot like a shelf where the jars can sit for a week without being bumped.

1. A grown-up heats one cup of water until hot but not boiling. A grown-up does this
2. A grown-up pours the hot water into the first jar. A grown-up does this
3. Fill the second jar with cold water.
4. Stir salt into each jar, one spoonful at a time.
5. Tie a string to the middle of each pencil.
6. Lay each pencil across a jar so the string hangs in the water.
   Ask: Which jar do you think will grow the biggest crystals, and why?
7. Move both jars somewhere still and leave them alone.
8. Check the jars every day for a week and draw what grows.
   Ask: What do you notice that is different between the two jars today?
9. After a week, measure the biggest crystal in each jar with the ruler.
   Ask: Was your prediction right? What do you think made the difference?
10. Now show your friends, or try again! What else could change how crystals grow?

Play it again

• Sunny spot: put one jar in a sunny window and one in a shady corner to see if light changes the crystals.
• Salt swap: try sugar instead of salt and compare the shapes.
• Crystal gallery: grow a jar each week and make a chart of crystal sizes.