Is cold water denser than warm water?

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The answer to this question is a little trickier than you might think. Most substances get denser when they get cooler. If a substance is “denser” it means that its pieces are packed together more tightly than another substance.

All substances (water, air, mud, gold, granite, grape juice, tar, taco seasoning, every one of them!) are made of tiny pieces called molecules. Heat makes molecules excited. They jump around and move quickly! They need space to do this. Cold, on the other hand, makes molecules slow down and it can make them nearly stop. They don’t need as much room to move around and the space between them shrinks!

It would seem that cold water would have to be denser than warm water, but something strange happens to water when it cools to about 39° Fahrenheit. It starts to become ice, and it’s one of a few substances that actually take up more space as a solid than as a liquid. The molecules freeze in a shape with a lot of empty space between them. This is why ice floats on lakes and why ice cubes float in your drink cups. So cold liquid water is denser than warm water, until it becomes ice!

by   (whyzz writer)
  • Exploration

    Dabbling in Density

    Heat isn’t the only thing that affects water’s density. Sometimes you can add things to water to make it denser. One of those things is salt! A lot of times chefs will add salt to water when they are boiling it to cook pasta. They do this because the salt will join up with the water to make salt water. The salt fills in some of the empty space between the water molecules. The salt water is denser! It takes more heat to boil denser water, so when it does boil, that pasta is actually cooking at a hotter temperature.

    You can check out salt’s effect on water without having to turn on the heat, but you’ll still want to have a grown-up with you. Get two glasses and fill them to about two inches from the top with water. Add a few heaping tablespoons of salt to only one of the glasses and stir it up (the salt will seem to disappear into the water.)

    Gently put an egg into the unsalted water. What happens? Fish the egg out and put it in the salt water glass. What happens now? If you get the same result, try adding a little more salt. With salt you can get the water to become denser than the egg. What happens when something has a lesser density than water? 

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