Science Experiments

Cartesian diver

A Cartesian demonstrates the relationship between volume, mass and density.

What you need

• Clear plastic drink bottle (1.5–2L size) - filled with water
• Bendy Straw
• Paper clip
• Modelling clay (such as Plasticine)
• Scissors
• Container of water

What to do

Bend the straw and trim it to about 3cm in length. (### Photo)

Seal the straw with a piece of modelling clay. Start with a small ball of clay - the straw needs to be able to float in a container of water, just peeking out of the water. This is the 'diver'. When doing this with a class, have some containers of water around for testing divers before they get put in the plastic bottles.

What's going on?

This experiment demonstrates the property of buoyancy. An object is buoyant in water due to the amount of water it displaces or 'pushes aside'. If the weight of water that is displaced by an object in water exceeds the weight of the object then the object will float. As you apply pressure to the bottle, you apply pressure to the air bubble in the dropper reducing its size. As the bubble's size reduces, the dropper becomes less buoyant and begins to sink. Release the pressure on the bottle and the dropper begins to rise back to the top.

This same principle is used to control the buoyancy of an Argo Float. By pumping oil between bladders inside and outside the float an Argo can be made to sink and rise.

Salt Water Circuit

Students investigate the conductivity of saltwater, and develop an understanding of how the amount of salt in a solution impacts how much electrical current flows through the circuit.

What you need:

• 1.5 volt battery
• 2.5 volt Light bulb and holder
• Wire (and dog clips),
• 2 ice block sticks,
• aluminium foil,
• water in a small container,
• salt ,
• a stirrer.

What to do:

Cover two ice block sticks with aluminium foil. These are now your probes. Connect one wire to each probe. Connect the opposite end of one wire to one terminal of the light bulb socket. Connect a wire to the opposite terminal of the light bulb socket to the battery. Connect the wire from the other probe to the battery

You can see if your tester is working by touching the metal together. This will complete the circuit and make the light bulb glow. If it doesn't glow, check your connections to make sure everything is taped together in the right way.

Now to use your saltwater tester, put just the tips of the metal in saltwater, about an inch apart. Make sure the two metal parts don't touch. The saltwater will act like a wire, connecting the metal sticks, completing the circuit, and making the light bulb glow.

How this works.

The light bulb glows in saltwater because the saltwater acts like an invisible wire to connect the circuit. That's because when you add salt to water, the salt molecules dissolve in the water and break into smaller parts called ions. The ions carry electricity through the water.

Fresh water doesn't have these ions. So it's harder for the electricity to move through the water. It doesn't complete the circuit, and the light bulb doesn't glow.

Argo Floats measure salinity in a very similar way. Water passes through a cell on the float that records the conductivity and then converts this into a salinity reading.