Whenever you put ice cubes into your glass of water, do you ever wonder what makes them float, instead of sinking to the bottom of your glass? This doesn’t just occur with small ice cubes - giant icebergs float atop oceans and lakes! Let’s check in with our friends at Science ABC to understand why the frozen (i.e. solid) form of water always floats on its liquid form.
Sinking vs. floating
The density of an object is what determines whether that object will sink or float. If an object or substance is less dense (having less weight) than other components in a mixture, it will float. When an object floats, it displaces a weight of fluid equal to its own weight. Science ABC uses a bucket of water and some rocks to explain this concept: When tossing rocks into a bucket of water, the rocks will sink. This is because the rocks are denser than the water, so they displace the water - or push it out of the way.
Why does ice float?
Since it’s known that solid objects are denser and have more weight than liquids - and ice is a solid - one would automatically think that ice would sink in water. But it doesn’t! What’s so special about ice that causes it to float? Believe it or not, ice is actually about 9% less dense than water. Since the water is heavier, it displaces the lighter ice, causing the ice to float to the top.
How is ice less dense than water?
When a liquid is cooled, more molecules are brought closer together and need to be accommodated in a smaller area. This results in most solids having a greater density than liquids. Not so with ice. Water consists of positively-charged hydrogen atoms and negatively-charged oxygen atoms. When water cools, the hydrogen bonds adjust to hold the negatively-charged oxygen atoms apart, which prevents the ice from becoming any denser. So for water, the density actually decreases along with a decrease in temperature - causing ice to be less dense than water!
A gift to nature
When looking at this concept in nature, we see how important it is: Lakes and rivers freeze from top to bottom, enabling fish to survive even after the surface of the body of water they live in has frozen over. This winter season, when you’re out in nature walking, sledding, or ice skating, if there’s a river or lake nearby, take some time to closely observe it. Has the top frozen over? If so, look through the frozen layer and see if you can spy any fish happily swimming around.
Looking for more Never Stop Asking "Why?" questions? Catch up on all of the past "Why's" on the blog!