## Hands-on Science: Surface tension

In these fascinating water experiments you will learn how many paperclips you can add to a glass full of water.

What do you need?

• Glass
• Water
• Box of paperclips

Instructions

1. Fill the glass with water until it is level full
2. Now make a guess how many paper clips can you drop in the glass without the water overflowing?
3. Count the paperclips as you drop each in the glass
4. Ask another family member to do the same and see if they could add more

What’s going on?

Believe it or not water is sticky – not sticky like glue – but water molecules are very attracted to each other. If you had a glass of water, then the water molecules in the centre of the glass would be attracted to the water molecules above them, below them and to the side of them. But the water molecules on the surface of the water do not have any molecules above them to be attracted to, so they become more attracted to the molecules to the side and below them – this causes surface tension.

Water molecules have a positive and negative charge like little magnets. the negative part of the water molecule is attracted to another water molecules opposite pole where they stick together.  The attraction of water molecules to other water molecules is called cohesion. The force of the cohesion allows water to form a small dome over the top of a glass as you add the paperclips without overflowing.

Bonus Question: Can you make the paper-clip float? See answer here.