Water: What makes water special?

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Investigation 1: Water on different surfaces

We learned that water is absorbed (soaked up) by porous surfaces, like paper towels, and it beads up on smooth surfaces, like wax paper.

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A dome is the shape a drop of water takes when it is on a flat surface.

Investigation 2: Surface tension

Surface tension is the skin-like surface on water (and other liquids) that pulls it together into the smallest possible area (sphere). It allows certain insects to walk on water.

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Some chemicals, like soap, reduce the surface tension of water. The camphor beetle can walk on water because of surface tension, and when it wants to shoot forward, it releases a chemical that breaks the surface tension behind it.


When water heats up, it expands, or gets bigger. When water cools, it contracts, or gets smaller.

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The colored alcohol in a thermometer also expands with heat, and contracts with cold. The benefit of using alcohol is that it will only freeze at a much lower temperature than water.

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You can see water expand when you heat it in a pot.

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If air temperature increases, so does the ocean temperature, which causes the water to expand, and the sea level to rise.

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When it is hot, the water in pipes can expand, and sometimes can make the pipes burst.

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The liquid of an egg expanding in the microwave (which is hotter than the stove) causes it to explode.

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Gases also expand when heated:

Hot air balloons are heated by a fire, making the air inside the balloon hotter than the air around it.

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Bread rises partly because yeast produces a gas, CO2, that expands when heated.

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The steam engine works because of expanding gases.

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Solids also expand when heated:

Popcorn expands when heated.

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Copper wire pipes expand when hot water passes through them, making a clanging sound when they rub against wood or other metal near the pipes.

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Bridges are built with expansion joints, which are spaces for when they expand in the summer.

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Railroad tracks are built with spaces for when the steel expands in the summer.

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When you run a bottle under hot water, it expands the plastic or metal cap, making it easier to open it.

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However, when water's temperature falls below 4 degrees Celsius, it expands again. When water expands, it is less dense (the molecules are further apart), and that's why ice floats.

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Density of Different Liquids

In general, if something floats above something else, it is less dense than the bottom liquid. As you can see below, water is less dense than milk, but more dense than oil. That's why when you see an oil spill, it is floating on top of the ocean. When you swim in a lake, notice that the surface water is warmer than the deeper water.

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Water cycle

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When a puddle dries, water evaporates (turns into vapor, which is a gas). Factors that make water evaporate faster include hot temperature, large surface area, and wind. Condensation is when vapor becomes liquid water again. When vapor touches a colder surface, it turns into water droplets. There are little cold dust particles in the sky, and when the vapor touches those colder particles, it turns into water droplets. These droplets on particles form clouds. When the droplets become heavy enough, gravity pulls them down, and it rains, snows, or hails. This is called precipitation.

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We see condensation on the inside of a pot lid when cooking, or on the outside of a cold glass of water on a hot day. Using condensation in solar still, we can even make fresh water out of salt water!

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Water has been used for thousands of years to help us do work. Waterwheels serve as a good example. external image WaterWheel.jpgexternal image image002.jpg

Hydroelectric plants use flowing water to move wheels, and converts that energy into electricity.

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Water quality describes the level of purity of a sample of water. It determines the use of that water, such as drinking bathing, irrigation, and so forth.

Different materials dissolve in water to affect its quality. When we filter water, we help purify it for drinking.

Homemade water filter

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1. Water Unit Summary

2. Water Unit Glossary

2. Video: Waterwheels

3. How to build a water filter

4. Video: Thermal Expansion and Oceans

5. Video on Properties of Water

6. Video on Water Striders