Apr'16

#Tinker: Water Density Experiment


by STEM Scouts



This post is an edited version of write-ups originally appearing on Raising Sparks and Exploratorium.

Watch our experiment on the STEM Scouts Facebook page.

Here are some ideas for exploring the effect of temperature on water density. The first experiment is simple and suitable for children of all ages. It might take several attempts and some assistance to balance the bottles on top of each other.

Materials:

  • Two clear plastic bottles (or glass jars) of exactly the same size
  • Food dye
  • Warm water (Caution: Don’t make the water too hot for your safety)
  • Cold Water
  • Index cards
  • A deep tray or bowl to catch water

First Experiment Procedure:

  1. Fill one bottle (or jar) with cold water and fill the other bottle with warm water. To ensure a fair test fill them to the same level
  2. Allow the contents settle for a couple of minutes
  3. Place a few drops of blue food dye in the cold bottle and a few drops of red in the warm bottle

How Does It Work?

The blue food dye in the cold bottle swirls, suspended in the water, before slowly spreading the color throughout the bottle. In the warm water, the red food dye will disperse 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 quickly. The molecules in the higher density cold water are closer together and can’t move as freely, so it takes much longer to distribute the dye.

Second Experiment Procedure:

  1. Fill one bottle with warm water and one bottle with cold water
  2. Allow the contents settle for a couple of minutes
  3. Put a few drops of red food dye in the warm bottle and place it in your deep tray, then put a few drops of blue food dye in the cold bottle
  4. Now make sure the cold bottle with blue water is completely filled and place your card over it, which should form a seal
  5. Being sure to hold the card in place, quickly turn the bottle of cold blue liquid upside down and place it on top of the other bottle
  6. Carefully slide the card away, then watch and observe for three or four minutes
  1. Fill one bottle with warm water and one bottle with cold water
  2. Put a drop of red food dye in the warm bottle and put a drop of blue food dye in the cold bottle then place cold bottle in your deep tray
  3. Now make sure the warm bottle with red water is completely filled and place your card over it, which should form a seal
  4. Being sure to hold the card in place, quickly turn the bottle of warm, red liquid upside down and place it on top of the other bottle
  5. Carefully slide the card away and watch and observe for three or four minutes

How Does It Work?

If you did our Density Column experiment, you know that liquids have different densities, that is, some liquids float on top of other liquids. When you heat up water, the water molecules start moving around faster and faster. They bounce off each other and move farther apart. Because there’s more space between the molecules, a volume of hot water has fewer molecules in it and weighs a little bit less than the same volume of cold water—hot water is less dense than cold water. When you put the two together with the hot water on the bottom, the hot water rises to the top, mixing with the cold water along the way and creating purple water. When the cold water is on the bottom, the hot water doesn’t have to rise because it’s already on top. The cold blue water stays on the bottom and the hot red water stays on top.

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