Water Density vs Temperature

(Chemistry for ages 8+)


Have you ever wondered how a hot air balloon works? What makes it rise up in the air? The science behind it is a little complicated, but put simply, hot air rises. When air is hot, it is less dense than cold air and the warmer molecules move upwards while the cooler molecules stay low.

This is hard to show with air, but luckily water acts very similarly! The video above shows a fun activity to see how hot and cold temperatures result in differing densities. Here is what you’ll need to do this at home:



Large clear container (a fishbowl would work)
Room temperature water
Blue food coloring
Ice tray
Red food coloring
Hot water
Small plastic bottle (an empty contact solution bottle would work well)
Adult supervision (Adult supervision at all times please)



**Day before preparations

  1. Fill your large clear container with water from the tap and place a lid or plastic wrap on top to keep any dust or dirt from getting in. Set this container aside somewhere so it is out of the way but will allow the water to reach room temperature by the time you are ready to finish the activity tomorrow.
  2. Fill your ice tray with water. You do not have to fill every reservoir, but you should fill enough to give yourself a few colorful ice cubes.
  3. Place 3-4 drops of blue food coloring into each water-filled ice tray reservoir. Stir in the food coloring
  4. Carefully set the ice tray in the freezer and allow the ice cubes to form overnight. This will give you blue ice cubes to represent cold water.


**Day of procedures

  1. Heat some water using a tea kettle or microwave. This water needs to be hotter than tap water, but does not need to be fully boiling. Steaming hot is a good temperature.
  2. Carefully pour some hot water into your small plastic bottle. Be very careful so as not to spill it and burn yourself.
  3. Add several drops of red food coloring to the small bottle of hot water. You want the water to be very red, so add a bunch and shake it up.
  4. Remove the lid or plastic wrap from your large clear container when you are ready.
  5. Place 1 or 2 blue ice cubes in the container of room temperature water. Try to place them on one side of the container.
  6. Now carefully place your small bottle of hot red water on the other side of the container or room temperature water. Make sure the small container does not have its lid on, and use your finger to block the opening while you set it in water.
  7. Watch what happens as the cold and hot water is slowly released in the room temperature water. What do you notice?



Temperature affects density, so when air, water, or other substances are hot, they tend to be less dense, while cold temperatures make them more dense. When you add both cold and hot water to room temperature water, the cold water sinks to the bottom and the hot water floats on top.

This is also what happens with different temperatures of air, but it’s much harder to dye air and watch it in action.