Hot & Cold Water Science Experiment – Density Science Activity

This article bring you a science activity video about how differently the density of hot water and cold water.

Courtesy: HooplaKidz Lab (Now known as Lab 360)

Have you ever wondered why the top floors of a multi-story building feel warmer than the bottom floors or the basement? Do you know why oil always floats on water? Well, the answer lies in the difference in density. Hot air is less dense than cooler air, and so hot air rises and makes the upper floors warmer than the lower ones. Oil is also less dense than water, and so it floats. Surprisingly, the difference in density also exists between hot water and cold water, which you could see and confirm yourself by conducting this super fun experiment! 

Steps of the hot and cold water experiment

Word of caution: Make sure to do the experiment under the supervision of parents.

Materials Required 

1 Jar of cold water

1 Jar of hot water 

1 Plastic card 

1 Spoon 

1 Large dish or a baking pan

Red and blue food coloring 

Steps to follow

  1. Add a few drops of red food color into the hot water jar and stir it cautiously with a spoon. 
  2. Similarly, add a few drops of blue food color into the cold water jar and stir it with a spoon.
  3. Now you have a red-colored hot water jar and a blue colored cold water jar. Make sure that water is full up to the top in both the jars. 
  4. Now, carefully lift the blue colored cold water jar and place it in the dish. Add some more water if some water is spilled during lifting until you see a bulge of water at the rim. 
  5. Place the plastic or index card carefully onto the top of the blue-colored cold water jar. 
  6. Next, lift that jar from the dish and turn it upside down. You will notice that there is no need to hold the card with your finger to seal the mouth of the jar. The vacuum formed inside is holding the card in its place. Here, you may need some practice over the sink to do this step. 
  7. Now take the help of your parent to place the red colored hot water jar in the dish. Carefully place the blue-colored cold water jar on its top. 
  8. Ask an adult to hold both the jars. Now, gradually pull the card out from the middle. 


Hot and Cold Water - Science Experiment

You will observe that the water of both the jars will slowly mix with each other to form a purple colored mixture. 

  1. Now, again take a red-colored hot water jar and a blue colored cold water jar. This time, carefully put the index card on the red-colored hot water and turn it upside down just like in step 6. 
  2. Take the help of your parent to place the blue colored cold water jar in the dish and place the red colored hot water jar on the top of the cold water jar. Gently pull out the plastic card from in-between. 

You will observe that like last time, the reaction of the hot and cold water is different. Both the red and blue colored water is not mixing with each other.


Now from this fun-filled experiment, it is proved beyond any doubt that it is not because of the different colors. It is instead because the cold water is denser than the hot water. So, when the blue-colored cold water jar was at the top, the hot water from below rises upwards. However, the cold water sinks leaving the water of both the jars mixed evenly. During the second time, when the red colored hot water jar was at the top, the result was completely different! As hot water is less dense, it remains at the top, and as the cold water is denser, it stays at the bottom.

Interested in more density science activities? Check out..

Make Sugar Density Rainbow

Create Fireworks in a Jar / Shaving Cream Rain Clouds

DIY Lava Lamp

Now, do you know why hot water is less dense than cold water?

Well, When you heat water, it becomes hot. As a result, its molecules start to move at a faster pace. They move farther apart from each other resulting in more space between each of them. In short, when water is heated, the molecular motion increases, and the intermolecular space increases. When there is more space than cold water of the same volume, the weight becomes less because of the presence of a fewer number of molecules. This does not happen when water is cold. So, hot water is less dense than the same volume of cold water.

Now, you may be wondering that if this is the case,  why ice is less dense than water?

We all know that ice cubes float in a glass of water. Isn’t it? Well, it is because water has an unusual and rare property.  When water becomes ice, its molecules form a crystalline structure and are pushed apart from each other. This implies that the molecules of ice are not as close as they are in liquid water. In brief, the intermolecular space in ice is more than that of the liquid water, and this makes it less dense than water.

This amazing video of the hot and cold water experiment gives a deep insight into density and the way the hot water and cold water behave because of the difference in densities.  You can also try to do a six-second experiment in which all that you need to do is to quickly pour a glass of orange-colored hot water and blue-colored cold water of equal volume in a container at the same time. You will notice that while the blue-colored cold water quickly sank at the bottom, the orange-colored hot water rises up to be distinctly at the top!

Isn’t the experiment too simple to understand why do hot water rises and cold water sink? So, learn while having loads of fun!


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