Hot vs. Cold Water Experiment

Materials you'll need

• Clear glasses
• Hot water
• Cold water
• Red food coloring
• Blue food coloring
• Index card or playing card

 

Step-by-step tutorial

Step 1: Fill both glasses completely to the top — one with cold water, one with hot water. The glasses need to be totally full so no air gets trapped when you flip one upside down.

Step 2: Add a few drops of blue food coloring to the cold water and a few drops of red food coloring to the hot water. Stir each one gently.

Step 3: Place the card flat on top of the cold (blue) glass, flip it upside down, and set it on top of the hot (red) glass. Slowly slide the card out — and watch the two colors swirl together and mix!

Step 4: Now reverse it: place the card on top of the hot (red) glass, flip it upside down, and set it on top of the cold (blue) glass. Slowly slide the card out.

Step 5: Watch what happens. This time the layers stay put. Red water on top, blue on the bottom, with a clean line between them.

Learn more

This experiment is all about density — how tightly packed the molecules in water are. When water heats up, its molecules move faster and spread a little farther apart, making hot water less dense than cold water. Cold water molecules move more slowly and pack in closer together, making cold water denser and heavier. When hot water sits on top of cold, the lighter hot water stays up high and the heavier cold water stays down low, so the layers hold. Flip the order and put denser cold water on top, and it sinks right through the hot water below, mixing the colors instantly.