Teaching Sequence

Work through this resource material in the following sequence:

20 minutes

Tune in: What will change?

20 minutes

Find out: Experiment time!

20 minutes

Reflection: What did we learn?

Tune in: What will change?

Step 1.

Discuss how something floats on water, where others sink? Why?

Students might suggest that a variety of reasons:

lighter things might float more easily than heavier things
bigger/wider/flatter things might float whereas smaller/longer things will sink

Step 2.

Explain the experiment to your students.

Step 3.

Students write their predictions in the first section of the Student Worksheet.

Find out: Experiment time!

Step 1.

Half fill a small bowl with water.

Step 2.

Place the paper clip on the surface of the water point first. Watch how it sinks.

Step 3.

Now place the paper clip on the surface of the water lengthwise so that it floats.

Step 4.

Dip your toothpick into the dishwashing liquid, then dip the tip of the stick in the middle of the water.

Step 5.

As the washing-up liquid touches the water, the paperclip will suddenly sink. Why?

Step 6.

Half fill another small bowl with water.

Step 7.

Sprinkle pepper on the surface of the water, forming a small layer.

Step 8.

Dip your toothpick into the washing-up liquid, then dip the tip of the stick in the middle of the water.

Step 9.

As the washing-up liquid touches the water, watch what is happening to the pepper. The pepper should spread out and merge together forming patterns.

Reflection: What did we learn?

Surface tension basically means that there is a sort of skin on the surface of the water where the water molecules hold tight together.

These molecules are pushing up against anything that is placed on top of them, which allows the object to float.

Think of it like a trampoline. When you jump on a trampoline, the surface of the trampoline stretches and bounces you back up. The surface tension of the water works in a similar way.

When the paperclip is placed lengthwise, there is lots of surface for lots of water molecules to push back against.

This is why heavy things such as boats are able to float - their weight is distributed across a wide surface.

However, when you put a paperclip into water point first, it sinks because the pointy end of the paperclip breaks the surface tension of the water. It slips between the molecules of water.

Adding the dishwashing liquid breaks the water's surface tension.

When you add dishwashing liquid to water, the soap molecules start to move around and interact with the water molecules. This creates a layer of soap molecules on the surface of the water that breaks up the cohesive force between the water molecules, reducing the surface tension.

The pepper makes this more obvious. You can see how the water molecules supporting the pepper are split up by the dishwashing liquid in between.

So when you add dishwashing liquid, the water does not have enough strength to repel the paperclip anymore, so it sinks.

Why Do Some Things Float and Some Sink?

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Summary

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