Activity: Underwater smoke

This week, a look at where some of the currents in the oceans come from.

You will need

• Two glasses of water. The glasses need to be made of glass, not plastic.
• Food colouring
• Candle
• Matches/lighter
• Water
• Fire extinguisher
• An Adult

What to do

Caution: This activity involves fire. For any activity involving fire, you must have an adult present plus water, a fire extinguisher and any other safety equipment you or the adult feel is needed.

It's a good idea to do this activity in a sink, as there may be some water sloshed around.

1. We are going to put a few drops of food colouring at the bottom of one of the glasses, without mixing it with the water.
2. Suck a few drops of food colouring into the dropper. I found it worked best if I thendrew a tiny bit of air up into the dropper at the end, so there was a small bubble below the food colouring.
3. Dunk the dropper into one glass and give it a gentle stir, to wash off any food colouring on the outside.
4. Lower the dropper into the other glass until it is just above the bottom.
5. Gently squeeze the dropper to make a puddle of food colouring at the bottom of the glass. Try to avoid making any further bubbles after the colouring is on the bottom, as they will stir things up.
6. Gently lift the dropper out of the glass.
7. Light the candle and wait until it is burning brightly.
8. With your hand near the top of the glass, hold the glass above the flame.
9. After about ten seconds, the food colouring should start to rise up and swirl around the glass, like wisps of smoke.
10. Put the glass down gently on a surface that won't be harmed by hot objects. Make sure you don't touch the bottom of the glass when you are finished, as it will be hot.

This activity can be a bit tricky, especially placing the food colouring at the bottom of the glass, but with practice you can create a nice puddle with almost no colouring in the rest of the glass.

What's happening?

When you held the glass above the candle, it heated the water and food colouring at the bottom of the glass. When you heat water, it expands a gram of water at 60 degrees Centigrade takes up more space than a gram of water at 10 degrees C. Another way of looking at this is to say one cubic centimetre of cold water is heavier than a cubic centimetre of hot water.

Since the warm water at the bottom of the glass was now less dense than the cold water above it, it started to rise up, towards the surface of the cold water, and took the food colouring with it. This caused currents in the water, which made the water swirl around.

The movement in the glass is an example of a convection current. Convection currents are currents caused by a fluid rising or sinking as it is heated or cooled. Convection currents happen in water or in air. They are one of the most efficient ways of spreading heat around. Many heaters work by heating the air around them, which causes convection currents that spread the heat around the room.

Applications

• In the ocean, the currents that move water around on the surface are caused mostly by winds. Water in the oceans can also move up and down, as it is cooled and heated. When water is cooled (mostly near the poles), it sinks down below the warmer water and flows out below the warmer water. This causes huge convection currents called Thermohaline currents. Thermohaline currents tend to be slower than currents on the surface, but they can extend down several kilometres and be hundreds of kilometres long.
• Convection currents in the atmosphere are responsible for a lot of weather patterns, such as storms.
• You may have seen a light called a "Lava Lamp" ( images.google.com.au/images?q=lava%20lamp for images), which works in a similar way to the currents our glass. A lava lamp has a glass tube containing a water-based clear liquid and a coloured wax, with a light bulb underneath. Normally the wax is slightly denser than the water, so it sinks. When the bulb is turned on, the wax melts and expands, floating up into the water. The difference is that the liquid wax doesn't mix with the water, so you end up with blobs of wax floating up and down in the tube.