Kids, you will need to have Mum, Dad or another responsible adult help you with this one.
Aims:
This scientific demonstration will show the development of crystals over time from a saturated solution.
Materials:
- Sugar, table salt, or a chemical salt such as copper sulphate or borax
- Boiling water
- A teatowel
- A clean jar
- A spoon
- A straw or paddle-pop stick
- A cotton thread or pipe cleaner
Method:
- Pour boiling water into a jar until it is nearly half-full.
- Wrap the jar in the teatowel to keep the water hot and your hands cool.
- Using the spoon, put a little bit (about half a teaspoon) of the sugar or salt into the water.
- Stir until it dissolves.
- Repeat steps 3 and 4 until the solution is saturated. This is when no more sugar or salt can be dissolved. You will be able to tell this when you cannot get rid of the last few grains of sugar or salt no matter how much you stir.
- Tie the cotton thread or pipe cleaner onto the straw or paddle-pop stick.
- Lower the cotton thread or pipe cleaner into the saturated solution. Try to make sure it does not touch the bottom or sides of the jar.
- Place the jar in a location where it will not be disturbed and it is not in direct sunlight.
- Wait. As time passes (and it may be days), crystals will form.
Results:
What happened? What can you see?
Discussion:
What is a crystal?
A crystal is a solid material made of atoms or molecules arranged in a regular repeating pattern.
Ice is a good example of a crystal. When water cools below zero degrees Celsius, it changes from a liquid into a solid (this is called a phase change). Ice crystals form inside the liquid and these grow and fuse to make a solid block. Likewise, snowflakes are crystals of frozen water that form in very cold air.
What is a saturated solution?
A saturated solution is a solution of water and a salt where the water molecules are bound to as much salt molecules as possible. We need a saturated solution to form crystals quickly. It would take a very long time to make crystals if the solution wasn’t saturated, and they would not be very big.
Imagine that you are a water molecule. Let’s pretend that basketballs are salt molecules (because it’s difficult to hold more than one basketball in each hand). You can only hold two basketballs at a time. In a playground, we have 15 children. Those fifteen children can hold 30 basketballs altogether. When all 15 children are holding two basketballs each, we have a playground of children which is saturated with basketballs.
If we try to put more basketballs in the playground they won’t be held by children. As the children leave the playground (just like water molecules evaporating from a solution), they leave behind the basketballs in one big, orderly, patterned pile.
More information on Wikipedia.
