Oily hubbub
If you’re looking for some entertainment whilst
trapped inside a tent in the pouring rain, then what better than this little
activity? You’ll have all the stuff to hand.
You will
need:
- A
clear glass jar or other container
- Vegetable
oil
- Water
- Food
colouring (if you like)
- Salt
What you
do
- Pour
water into the jar to about 10 cm depth.
- Add
2cm of oil. When it has settled see if the oil and water have mixed. Is
the oil on top of or underneath the water?
- If
you like, add a little drop of food colouring into the jar as well and see
what happens to the drop.
- Sprinkle
salt on top of the oil and watch carefully. What happens to the food
colouring and what happens to the salt?
- You
can add more salt to keep the action going.
What's going on?
The oil and the water don’t mix
– they are immiscible – and the oil is less dense than the water,
so when you add oil to the jar it just sits in a layer on top of the water.
The food colouring just sits there, too, until you add the salt…
The salt is insoluble in the oil – this means it doesn’t dissolve. It is
also more dense than the oil. Gravity pulls the salt down to the bottom of the
glass, and the salt drags some of the oil with
it in little blobs.
When the salt reaches the water it starts to dissolve, and the blobs of oil
rise up to join the layer on the top again.
Once all the salt you’ve added has dissolved, the mixture calms down –
until you add more salt!
More ideas
Lava lamps work in a similar
way. The liquids in the lamp are very close in density but are immiscible in
each other, so they stay separate. As the blobs of ‘lava’ are heated up at the
bottom of the lamp, they become less dense and they rise up to the top. But the
top of the lamp is cooler because it is further from the heater. So the blobs
cool down, become more dense and sink again.
This activity was taken from the Little
Book of Experiments.
Midges
Have you 
This week’s question comes from Robert
Hewis aged 7 who asks:
INTECH is a science centre
19-23 May is Noise Action Week
