A resource of Biophilia: Pittsburgh, #bi oPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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Across the Earth, water is our most precious resource. Our planet is covered in 70% water, and the living things on Earth need water every day—even when it’s freezing cold outside and the cold traps our water in solid ice. What is fascinating about water, though, it that it has some important properties that keep us thriving whether water is frozen in the winter or free-flowing in the summer. Let’s explore a few of these properties!

Note: throughout this post, we will be talking about water molecules. A “molecule” is the smallest possible unit of a chemical compound—like a single apple out of a bushel of apples. A water molecule is made up of two hydrogen atoms and one oxygen atom. Those two hydrogens, as we will explore together, have quite an impact on the properties of this vital chemical!  

Water Has Lower Density as a Solid Than as a Liquid
Have you ever noticed that ice cubes float in your water glass? Ice is about 9% less dense than liquid water, meaning that the individual water molecules are more spread out in ice than they are as a liquid. When frozen, individual water molecules (which are made up of two hydrogens and one oxygen) form an organized crystal structure because the hydrogens in each water molecule bond with other water molecules. As ice melts because of heat, which is a form of energy, the energy powers individual water molecules to break their hydrogen bonds with their neighbor water molecules and begin moving and flowing past each other. If you continue to heat the water, adding more and more energy, eventually the water is powered to spread out so much that it can leap away from the liquid water and form steam in the air.

This time of year, we should take a moment to note that this particular density comparison is an unusual property for a solid—most other elements and compounds are more dense as a solid than they are as a liquid. The quirky properties of hydrogen bonds are what make the difference in water, and it’s a good thing that they do! If you imagine a pond in the winter, where is the ice? On top! If ice froze at the bottom of ponds and lakes, the fish and other creatures in the body of water wouldn’t stand a chance against freezing in winter. Since it freezes on the top, though, aquatic inhabitants are able to live safely under the ice.

Water is Polar and Attracted to Other Polar Molecules
“Polarity” refers to molecules having both a positive and negative charge to them. You can think of a water molecule almost like a tiny magnet: one part of it has a positive end and one part of has a negative end (those hydrogens bonds—they affect everything!) Because water is polar itself, it is attracted to other polar molecules, including other water molecules. It is also attracted to compounds like table salt and sugars like sucrose and glucose, which are all polar and dissolve readily in water. Plants and animals take advantage of this when they produce their own “antifreeze,” and we notice uses of this trait in the winter when we salt the roads. Salts and sugar lower the freezing point of that water, which in turn keeps tree cells from freezing and keeps our roads clearer in colder temperature (though not without a trade-off).

High Specific Heat Capacity
Another important property of water is what is called “high specific heat”—it takes quite a bit of energy (heat) to change the temperature of water. This means that water can absorb heat without automatically increasing in temperature. Eventually, of course, water will heat up, but as the USGS points out, it takes 1000 calories of energy for the temperature of one kilogram of water to increase 1 degree Celsius (°C) but only 92 calories of energy to raise 1 kilogram of copper 1°C (in colloquial dietary terms, we refer to calories as the amount of energy we get from our food but, they are actually kilocalories.) This property is important for any creature that lives in water, or any of us creatures whose bodies are made of 55-60% water: we can be exposed to high temperatures without our bodies quickly baking us.

In the end, water has quite a number of properties that make it essential to life on Earth; we just scratched the proverbial surface here today. But the next time you see ice on a pond, see a tree growing leaves just after winter, you can smile and know that even water is a wonder.

Connecting to the Outdoors Tip: Family experiment! Would you like to test how well a tree’s sugary “antifreeze” protects the tree’s cells? Take two old plastic bags of some sort to be your “cells” (reuse! these can be old bread bags, bagel bags, or previously used ziplock bags) and add 2.5 tablespoons of table sugar to one of the bags. Now add half cup of water to each of the bags. Once you’ve tied off the bags, be sure shake up the sugar-water bag to mix well. Now, leave both bags outside overnight. Did either freeze or turn to slush?  What was the temperature that night? Do you think salt or sugar is better at preventing ice formation? What happens when you eat too much salty food? Does it make you thirsty? Why might a plant not want to use salt as anti-freeze?

Continue the Conversation: Share your nature discoveries with our community by posting to Twitter and Instagram with hashtag #bioPGH, and R.S.V.P. to attend our next Biophilia: Pittsburgh meeting.

Resources

USGS—Water Density

USGS—The Water in You

Boundless Biology—Water