Experiments with water

We have been talking about water, water cycle, water pollution.

Today we tried two water experiments.

The first one has a fish that can dive to the bottom of the bottle and come back up again.

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What happens and why:

When you squeeze the bottle, the fish sinks.

When you release the bottle, the fish surfaces again.

When you put the fish  into the water so that it floats, the trapped air bubble inside the pen top makes the fish lighter than the water so it floats.

When you squeeze the bottle, water is pushed up into the pen top, squashing the air into a smaller bubble.

The pen top now has more water in it, making the fish heavier, so it sinks.

When you relax your grip on the bottle, the air in the pen expands again, the fish becomes lighter and floats again.

At home: Make a Submarine

Pierce a hole in the lid and bottom of a plastic bottle.  Push a plastic tube through the hole in the lid.  Put your finger over the hole in the bottom. In the bathtub fill the bottle to the top with water.  Screw on the lid.  Let the bottle sink to the bottom of the bath.  Take your finger away from the hole and blow into the tube.  Your mini-sub will rise to the surface.  This is the principle on which submarines work.

The next experiment shows air pressure at work:

What happens and why:

The water in the bottle stays in the bottle with the ball/card as a lid!

This is because the air in the room is pushing up against the ball/card.

This force of air is stronger than the force of water trying to get out of the bottle so the water stays in the bottle.