Why Does Ice Float in Water?

After spending the whole afternoon playing outside, there’s nothing more enjoyable than sitting at the kitchen table with a tall glass of ice-cold water. The sound of the ice cubes hitting the bottom of the glass is satisfying. As you pour water over them, they make a hissing and crackling sound while slowly rising to the top of the glass.

Have you ever wondered about the interesting combination of ice and water? It’s essentially solid water floating in liquid water. How amazing is that? (Pun totally intended.) But why does ice float in water?

Scientists will explain that it has to do with density, which measures mass per unit of volume. Ice floats because it is less dense than water.

Objects denser than water, like rocks, will sink to the bottom. To float, an object must displace fluid with a weight equal to its own weight.

The fact that ice floats in water is peculiar because most substances become denser when they turn into solids. However, water reaches its maximum density at 40º F (4.4º C). As water cools and freezes, it becomes less dense due to the unique hydrogen bonding nature.

Each water molecule consists of one oxygen atom bonded to two hydrogen atoms through covalent bonds. This is reflected in the chemical formula for water: H2O.

Water molecules are attracted to each other through weaker hydrogen bonds. These bonds form between the positively-charged hydrogen atoms and the negatively-charged oxygen atoms in nearby water molecules.

As the temperature of water decreases, the weaker hydrogen bonds start to separate the negatively-charged oxygen atoms, creating a rigid crystal honeycomb structure known as ice. The water molecules in ice take up approximately 9% more space than liquid water, making ice about 9% less dense than water.

If you have a gallon of ice and a gallon of water, the ice will weigh less than the water. When you place the ice in the water, the denser water pushes the ice to the top, causing it to float.

This unique property of water is particularly beneficial for fish living in bodies of water that freeze in winter. Because ice floats, bodies of water freeze from the top to the bottom. This allows fish to survive deep underwater even when the surface is frozen!

Try It Out

Feeling a bit chilly after learning about this Wonder of the Day? Get a friend or family member to join you and stay warm while trying out the following fun activities:

Looking to conduct a science experiment on the density of different substances? Give the Mixing & Separating Experiment – Three Layer Float a try. Before starting, make some predictions about how you think the substances will behave. Afterward, compare your predictions to the actual results. Were you surprised by any of the outcomes?

If you’re having trouble visualizing hydrogen bonds between water molecules, don’t worry! Check out the Hydrogen Bonds image online, which shows how these bonds form. For an extra challenge, use simple household supplies to create models of water molecules.

Did you know that you can do more with ice than just use it in drinks? Discover 23 Ice Crafts, Activities & DIY Decorations for Winter Fun with the help of a friend or family member. Choose a few projects to make and enjoy!

For more information, check out these sources:

– http://chemistry.about.com/od/chemistryfaqs/f/icefloats.htm

– http://www.foxnews.com/science/2013/12/12/ask-science-teacher-what-makes-ice-float.html

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