Properties of Water - Buford High School Biology Page



Properties of 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. 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. A solvent is only one portion of a solution, the solvent has the ability to dissolve solutes such as salt to form solutions such as salt water.

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.

Properties of Water Reading Name: _____________________

Use the provided reading on properties of water to answer the following questions.

1. Water covers about how much of Earth’s surface?

2. Water makes up what percentage of our body?

3. What is the chemical formula for water?

How many atoms is this?

4. Sketch a water molecule and label the

positive and negative ends.

5. The term polarity means that this molecule has oppositely charge ends (lopsided electrical charge), it is considered a __________ molecule.

6. Water’s polarity allows it to dissolve other __________ substances.

7. Based on its ability to dissolve many other substances it is often referred to as the _____________ solvent.

8. A solution is made up of a solvent and a solute….in the example of salt water what is the solvent and what is the solute.

Solvent: ________________ Solute: ______________

9. What is surface tension?

10. What organisms are noted in this paragraph as being helped by surface tension? How

is each of these helped?

Organism A: ___________________ -

Organism B: ___________________ -

11. What states of matter can water be found in?

12. What happens to water when it freezes?

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