Hot and Cold Water Science

Water density science experiment

Learn how density of water can change when it is heated up. This hot and cold water science experiment requires basic household stuff and is definitely going to WOW your kids. Who doesn’t like a simple science experiment which are quick and fun to do ! So, let’s get started.


What you need

  • Two identical wide mouthed small clear glass jars
  • Food Coloring – Red and Blue ( You can buy them here )
  • Index or plastic card ( Old playing card can be used if it covers the mouth of the jar )
  • Shallow Dish/Plate or baking pan
  • Hot and Cold Water

What to do

Fill one jar with cold water and another one with hot water. Pour blue food coloring in the cold water and red in the hot water.

Make sure both the jars are filled with water to the fullest. Place them in the shallow plate.

Place the card on top of the hot water jar and gently tap. The card should cover the whole mouth of the jar. It would form a seal with the water and the jar.

Carefully, (you would need an adult to do this part ) pick up the hot water jar and turn it straight upside-down. If the jar is tilted but not turned over completely, the water will gush out and make a mess. So, just flip the jar over and don’t hesitate. You might not need to put your hand on the card as the vacuum formed inside the jar holds the card onto the surface. It is advisable to practice turning the jar upside-down with index card placed on top of it under the sink using tap water before trying it with hot water.

Put the upside down red jar on top of the blue jar. Make sure the edges of both the jars line up perfectly all around.

Have someone hold onto both the jars while you very slowly and patiently pull out the card in between the jars.

The Science behind the Hot and Cold Water

Why doesn’t the water mix ?

Empty and clean both the jars. Perform the above experiment where you turn the blue jar upside-down and put it on top of the red jar. What happens now ? Why does the water mix this time ?

The reason is that whenever you put together two liquids with different density, then the liquid with less density floats on top of the denser liquid. Hot water is less dense than cold water. When the water is heated, the water molecules start bouncing off each other and hence, move farther apart creating more space between the molecules. Eventually, the volume of hot water has less molecules in it and weighs less than the same volume of cold water. So, hot water is less dense than the cold water.

When you put the jar with hot water on top of the cold water, the cold water doesn’t have to rise, since it’s denser than the hot water and stays on the bottom. Whereas, when you put the jar with cold water on top of the hot water, hot water being less dense, rises to the top, mixing with the cold water along the way and creating a purple water.

More Water Density Experiment Ideas

How about trying to perform the same experiment with a jar of salted water and plain water. Which one is more dense, go figure it out and let us know in the comments section. And, don’t forget to like my facebook page.

If you want to spark your kids science curiosity, check out some of these great books on science experiments which are easy and fun to do at home.

While you are here, check out some more science experiments.

Interested in more, head over here.

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