One of the things that makes our planet special is the presence of liquid water. Water is fundamental for all life; without it every living thing would die. It covers about 70% of Earth's surface and it makes up 65-75% of our bodies (82% of our blood is water). Even though water seems boring – no color, taste, or smell – it has amazing properties that make it necessary for supporting life.
The chemical composition of water is H2O – two hydrogen atoms and one oxygen atom. Water has special properties because of the way these atoms bond together to form a water molecule, and the way the molecules interact with each other.
When the two hydrogen atoms bond with the oxygen, they attach to the top of the molecule rather like Mickey Mouse ears. This molecular structure gives the water molecule polarity, or a lopsided electrical charge that attracts other atoms. The end of the molecule with the two hydrogen atoms is positively charged. The other end, with the oxygen, is negatively charged. Just like in a magnet, where north poles are attracted to south poles ('opposites attract'), the positive end of the water molecule will connect with the negative end of other molecules.
What does this mean for us? Water's polarity allows it to dissolve other polar substances very easily. When a polar substance is put in water, the positive ends of its molecules are attracted to the negative ends of the water molecules, and vice versa. The attractions cause the molecules of the new substance to be mixed uniformly with the water molecules. Water dissolves more substances than any other liquid – even the strongest acid! Because of this, it is often called the 'universal solvent.' The dissolving power of water is very important for life on Earth. Wherever water goes, it carries dissolved chemicals, minerals, and nutrients that are used to support living things.
Because of their polarity, water molecules are strongly attracted to one another, which gives water a high surface tension. The molecules at the surface of the water “stick together” to form a type of 'skin' on the water, strong enough to support very light objects. Insects that walk on water are taking advantage of this surface tension. Surface tension causes water to clump in drops rather than spreading out in a thin layer. It also allows water to move through plant roots and stems and the smallest blood vessels in your body – as one molecule moves up the tree root or through the capillary, it 'pulls' the others with it.
Water is the only natural substance that can exist in all three states of matter – solid, liquid, and gas – at the temperatures normally found on Earth. Many other substances have to be super-heated or -cooled to change states. The gaseous state of water is present continually in our atmosphere as water vapor. The liquid state is found everywhere in rivers, lakes, and oceans. The solid state of water, ice, is unique. Most liquids contract as they are cooled, because the molecules move slower and have less energy to resist attraction to each other. When they freeze into solids they form tightly-packed crystals that are much denser than the liquid was originally. Water doesn't act this way. When it freezes, it expands: the molecules line up to form a very 'open' crystalline structure that is less dense than liquid water. This is why ice floats. And it's a good thing it does! If water acted like most other liquids, lakes and rivers would freeze solid and all life in them would die.
The Science of Olympic Swimming
If you've ever watched the summer Olympics, you've probably seen a lot of water! Swimming is one of the most popular Olympic sports, and yet it's also a favorite summer activity of people of all ages. Here are a few questions to ponder, inspired by the sport of swimming.
Why do Olympic swimmers wear swim caps and body suits?
Swimmers wear caps to reduce drag. Drag is the force that resists the motion of a solid object through liquid or air. It's caused by friction between the swimmer's body and the water. Try dragging a sweater across the carpet, then try dragging a plastic bag. Which one moves easier? The plastic bag is smoother and so has less friction, letting it slide across the carpet easier. Swimmers want the smoothest, most streamlined shape so that they can move through the water easier and faster. Caps and high-tech body suits cover skin and hair that can create friction and slow them down.
Does it matter how swimmers start their race and how they turn in the pool?
The principles of hydrodynamics (the motion of fluids and the forces acting on solid bodies immersed in fluids) help determine how fast people can move through the water. Swimmers use certain body positions and techniques to help reduce drag and allow them to swim faster. When starting, they want to enter the water with force and with their bodies in a straight line so they can slice through the water's surface like an arrow. Because of water's surface tension, hitting the water with a large area of the body will create too much friction and slow the swimmer down. (If you've ever belly-flopped, you know that it also hurts when a lot of your body hits the water at once!) This is why swimmers want to enter with just their hands and have their body follow in a straight line.
In most competitions, swimmers are allowed to remain underwater after their starts and turns for up to 15 meters. During this time they have their body in a tight, straight, streamlined position and kick with their feet together, which helps them move through the water very quickly. While swimming the freestyle or the backstroke, they usually perform a “tumble turn” or “flip turn” which allows them to take their momentum toward the wall and convert it into speed in the other direction.
Why is swimming pool water blue or blue-green?
Swimming pools are sanitized with chemicals like chlorine to help kill bacteria, viruses, algae, etc. that could spread diseases between swimmers. The chlorine is part of what makes the water a light blue color, although the way water reflects light is also a factor.
For more fun with water, check out these science projects: