Learn how water works with this video from HowStuffWorks.

Discover more about water and quiz yourself at the USGS's water science page. 

Find out about the global water crisis and how you can help.

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Splitting Water: Electrolysis Experiment

Is it possible to break water? In a sense, that's what electrolysis does. Electrolysis uses electricity to split water into its two ingredients: hydrogen and oxygen. Try it out with a battery and a couple pencils!

Materials

• 6-volt or 9-volt battery
• Two alligator clip leads or insulated wire
• Beaker or glass
• Piece of thin cardboard or cardstock
• Two #2 pencils 

What to do:

1. Fill the beaker or glass with warm water.
2. Carefully remove the erasers and metal sleeves so you can sharpen both ends of each pencil.
3. Cut a piece of the cardboard to fit over the beaker, then punch two holes in the center of the cardboard about an inch apart. Push the pencils through the holes and set them in the glass. They should extend into the water, but not touch the bottom of the glass. The cardboard will hold them in place.
4. Connect each pencil to the battery with an alligator clip lead attached to the exposed graphite (pencil lead). If you don't have alligator clip leads, use two lengths of wire and strip an inch of insulation off each end. Wrap the wire around the graphite of each pencil and connect the wires to the battery. You may need to use tape to hold the wires in place.

What's happening?

As soon as you connect the wires to the battery, you will see bubbles appearing around each of the pencil tips in the water and floating upward. Those bubbles are the components of water—hydrogen and oxygen gas—that have been split apart by the electricity as it travels through the water from one pencil to the other. The pencil attached to the negative terminal of the battery collects hydrogen gas while the one connected to the positive terminal collects oxygen. Does one pencil collect more bubbles than the other? Which one? Why do you think this is? (Hint: Water's chemical name is H₂O because it has two hydrogen atoms to every one oxygen atom.)

With some electrolysis equipment you can collect the two gases and test their different reactions to a flame.

Make a Soap-Powered Boat

Can you make a boat speed through the water with just a drop of soap? Try this project to find out!

Materials

• Index card
• Liquid dish soap

What to do:

1. Cut the index card into the shape of a boat with a notch in the back, just like the diagram.
2. Fill a shallow pan, sink, or bathtub with water and set your boat on the surface.
3. Pour a few drops of dish soap in the notch at the back of the boat.

What's happening?

Surface tension is the property that makes the surface of water appear to have a sort of elastic "skin," and is caused by the way water molecules are more attracted to each other than to the air. When your boat sits in the water the surface tension is the same on all sides. When you put the drop of soap near the back, however, the soap molecules break the water's surface tension. The force of the surface tension pulling on the front of the boat is now greater than the force pulling behind, so the boat moves forward. (Make sure you use clean water if you try again; it won't work if the water is already soapy.)

Learn more about surface tension with these fun surface tension experiments. 

Make a Water Thermometer

A thermometer shows the temperature when liquid inside it moves up or down on a scale. Find out how it works when you make your own in this project.

Materials

• Plastic water bottle
• Modeling clay
• Clear plastic straw
• Food coloring

What to do:

1. Put a few drops of food coloring into the water bottle and then fill it to the top with lukewarm water.
2. Insert the straw a couple inches into the bottle and mold the clay around it to seal the bottle and hold it in place. When you have a tight seal, water should go up into the straw.
3. Use a marker to mark the level of the water in the straw.
4. Set the bottle in a bowl of hot water. Watch the water level for awhile and then mark the level again.
5. Set the bottle in a bowl of ice and watch what happens, then mark the level.

What's happening?

 As water heats up, it expands and becomes less dense, rising to the surface. When it cools down, it contracts, becoming more dense and sinking down. This cycle is called convection. (Water is unique, however - when it gets cold enough to freeze, the molecules line up in an open crystalline structure that is actually less dense than the liquid form. This is why ice floats.) When the water in your bottle thermometer heated up, it expanded. But since the bottle was sealed, it had nowhere to go but up through the straw.

Real thermometers don't use water inside because it doesn't respond to temperature change very quickly. Try filling your bottle with 50% rubbing alcohol and 50% water. Does the liquid move up and down the straw faster? Why do you think this is?

With your homemade thermometer you aren't actually measuring temperature, just seeing temperature changes. If you have a real thermometer, you can use it to make a scale on your homemade thermometer: let your bottle get to room temperature and then mark the straw with what the actual room temperature is. Then set the bottle in the sun and do the same. Mark several different temperature levels and then watch your thermometer for a day and see how accurate it is.

For more fun with water, check out these projects:

• Liquid Density Experiments
• Make a Cartesian Diver
• Make a Water Wheel

Introducing Water

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. The way those atoms bond together to form a water molecule is what allows water's many special properties. The two hydrogen atoms form weak hydrogen bonds 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 been watching the Beijing 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.