Water Density Experiment

liquid density2 Here are some ideas for exploring the effect of temperature on water density. The first experiment is very simple and suitable for children of all ages. It might take several attempts and some adult assistance, however, to master the tricky art of balancing bottles on top of each other!

You will need the following materials to carry out these experiments:

  • Two clear plastic bottles (or glass jars) of exactly the same size
  • Food dye
  • Hot (not boiling!) water
  • Cold Water
  • Some card, index cards are perfect for this!
  • A deep tray or washing up bowl to catch water, this can get messy!
  • Make sure everyone has post-it notes!

Method For First Experiment:

Fill one bottle (or jar) with cold water and fill the other bottle with hot water.  To ensure a fair test fill them to the same level.

Allow the contents settle for a couple of minutes.

Place a small drop of food dye in each.

Watch carefully, can children describe what is happening in each jar?

Have they got questions or can they think of any explanations for what is happening?  Get them to scribble their questions on a post-it!

What happened here?

The food dye in the cold bottle will have made very pretty swirls suspended in the water before very slowly spreading the color throughout the bottle.  In the hot water, the dye will have dispersed evenly throughout the bottle almost immediately.

This happens because warm water is less dense than cold water, the molecules have greater freedom of movement and mix the dye in very quickly.  The molecules in higher density cold water are closer together and so can’t move as much, so it takes much longer to distribute the dye.

Method For Second Experiment:

waterdensityexperiement Empty the bottles (or jars) and rinse them out.  Again fill one bottle with hot water and one bottle with cold water.

Put a drop of food dye in the hot bottle and place it in your deep tray.

Now make sure the cold bottle with clear water is full to the very top and place your card over it.  This should form a seal.

Quickly turn the bottle of cold, clear liquid upside down and place it on top of the other bottle.

Carefully slide the card away, don’t worry if you spill a little.  Watch and observe for three or four minutes.

Now  you are going to repeat this process, but with a difference.  Empty the bottles and rinse them.  Fill both bottles with cold water.

Add  food dye to one and place it in the tray. Using the same process as before upturn the bottle with clear liquid on top.  Watch and observe.

Again give children a chance to make observations and ask questions.  Can they think of any reason for this happening?  Is there a way to do further tests to try their ideas out?  Have a go!

What happened here?

Children might already know about density from other experiments and know that materials with lesser density float on top of materials with heavier density.  (See below)

When you placed the cold water on top of the lower density hot water, the hot water rises up to float above the cold water and so the two become very quickly mixed.  In the second experiment using cold water in both bottles, the mixing process is much, much slower because the water in the bottom is the same density as that in the top.

A way to demonstrate density!

This is a great idea for demonstrating density!  Take a small, clear plastic container.  Put a few marbles in it and shake.  These marbles are molecules, they can move around freely because they are not closely packed together.  Increase density by adding more marbles, there will be less freedom of movement the more you put in.  Fill the container with as many marbles as you can.  This time when you shake they should hardly move at all, in much the same way as densely packed molecules in a solid material.

What did we learn?

Knowledge Base:

  • That the density of a material can be affected by temperature.
  • That low density liquids will rise above higher density liquids.

Scientific Enquiry:

  • Making observations and asking questions.
  • Applying existing knowledge base to offer explanations.
  • Proposing further experiments to test explanations.

Take It Further:

{ 4 comments… read them below or add one }

Amy October 6, 2011 at 3:08 pm

On this website I think that it could do with some more scientific words in and it needs some pictures so that children can see if they are doing the experiment correctly


Mandy October 6, 2011 at 3:15 pm

Thanks for your input Amy, we always welcome constructive criticism! We’ll get onto it as soon as possible. We are hoping to make some videos very soon, which I’m sure you will find helpful.


chaitanya October 21, 2011 at 12:55 pm

nice ones


Mandy October 21, 2011 at 2:16 pm

Thanks Chaitanya!


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