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Notes/Study Guide

Water: A Polar-Covalent Molecule

Most elements are found combined with other elements forming COMPOUNDS.

Scientists will write out these compounds using a special shorthand called a chemical formula.

Water's chemical formula is H2O that means a water molecule contains 2 hydrogen atoms and 1 oxygen atom.

The atoms in compounds are held together by chemical bonds.

COVALENT BONDS: bonds that are formed when electrons are shared by atoms (co = share).

When atoms are joined together by covalent bonds a MOLECULE is formed.

Water is a molecule in which each hydrogen atom shares two electrons with one oxygen atom.

A molecule in which the charges are unevenly distributed is called a POLAR MOLECULE because the molecule is like a magnet with oppositely charged poles!

A water molecule is polar because there is an uneven distribution of electrons between the oxygen and hydrogen atoms.

The negative pole (-) is near the oxygen atom, and the positive pole (+) is near the hydrogen atom.

Because of their partial positive and negative charges, polar molecules such as water can attract each other and other polar substances.

The attraction between a hydrogen atom on one water molecule and the oxygen atom on another water molecule is an example of a HYDROGEN BOND.

Water is very different from almost any other small molecule.

It is a COVALENT molecule because of hydrogen and oxygen share electrons.

It is POLAR because the electrons are not shared evenly causing the oxygen end to be slightly negative and the hydrogen end to be slightly positive.

Water's unusual properties are a direct result of the polar nature of the water molecule!

Recap on the TWO Types of Bonds involved with Water

Covalent Bonds are WITHIN the water molecule

Hydrogen Bonds CONNECT one molecule to another

PROPERTIES OF WATER

Water Behavior Video

Good Solvent

Water is a great solvent! It can dissolve substance that are polar or ionic (charged). Since it dissolves more substances than any other solvent known we call it the UNIVERSAL SOLVENT!!

Many other molecules are polar which attracts them to water which is also polar. This attraction between molecules is similar to the way magnets attract each other. Water attaches to the particles such as salts, sugars, proteins, and hormones and dissolves them. Any POLAR or IONIC (charged) compound can be dissolved in water!

Hydrogen Bonding and Surface Tension

Hydrogen bonds hold water molecules together so tightly that the water’s surface acts like a membrane.

The insect known as the water strider is actually able to walk on the surface without breaking through. Hydrogen bonds form because water is polar, with one end of the molecule being negative and the other end positive. Opposites attract like magnets and hold water molecules close together. This is known as a hydrogen bond.

Expansion on Freezing

Most things contract when cooled, and water is no exception, up to a point. That point is 0 degrees Celsius. When cooled below that temperature, water molecules slow down and spread out. This structure is completed when the water freezes. What is most unusual is that ice takes up more space than the same amount of water, so ice is less dense that water, and it floats. Since ice is a good insulator, lakes and ponds can freeze over in the winter without freezing all the living things in the water below. If ice didn’t float, lakes would freeze from the bottom up, and many ponds would freeze solid, killing all life in them.

Cohesion and Adhesion

COHESION

Cohesion is an attraction between molecules of the SAME substance.

Remember that 'co' means share. Cohesion is when two water molecules share a bond, making them stick together. You can see this when a bead of water forms on a surface. You are seeing many water molecules sticking together with cohesion.

ADHESION

Adhesion is an attraction between molecules of different substances.

Think about adhesion as 'adding' in another substance for water to stick to. You can see this when water sticks to your hair or mirror after a shower.

Cohesion = water + water

Adhesion = water + something else

Capillary Action

A capillary is any tiny tube. If a capillary tube is made of glass or any other substance that is polar, water will spontaneously climb up inside it without having to be pumped in any way. The smaller the tube, the higher the water climbs. The attraction is so great between the water molecules and the molecules of the tube that water will climb in defiance of gravity. This is termed capillary action. This is extremely important for plants because they do not have to use energy to get water to their highest points.

Here's a way to remember capillary action:

CA = C + A

Capillary action = cohesion + adhesion

Heat of Vaporization & Heat of Fusion

Heat of vaporization is the amount of heat added to evaporate a certain amount of liquid.

Water has a very high heat of vaporization, meaning it takes a lot of heat to evaporate just a little water. This keeps lakes and ponds more full during the summer than they would be if water had a lower heat of vaporization.

Heat of fusion is the heat that must be removed from water in order to freeze it.

Water’s relatively high heat of fusion means that it takes much longer for lakes and streams to freeze in the winter, allowing living things more time to adjust to the change.

High Heat Capacity/Specific Heat

Water is able to absorb a lot of heat energy without having its temperature increase much. Because of this, water is said to have high heat capacity.

An amount of heat that would raise the temperature of water by 10 degrees will raise the temperature of an equal weight of iron by 94 degrees. This means that an empty frying pan will heat up faster than a pan of water because water absorbs much heat before the temperature rises. Water molecules are held together so strongly by their hydrogen bonds than an amount of heat that will get other molecules moving much faster will not speed up water molecules much at all. This property of water helps to reduce temperature changes in the animal body or plant, and it also minimizes temperature changes close to large bodies of water.

MIXTURES

What is a mixture? I'm glad you asked!

A MIXTURE is a material composed of two or more elements or compounds that are physically mixed together but not chemically combined.

Thus the components of a mixture can be retrieved by physical means.

Two types of mixtures that can be made with water are SOLUTIONS and SUSPENSIONS.

SOLUTE: The substance that is dissolved

SOLVENT: the substance in which the solute dissolves

Solutions

All the components of a SOLUTION are evenly distributed throughout the solution.

In a sugar-water solution like the picture to the left. Sugar is the SOLUTE, the substance that is dissolved. And the water is the SOLVENT, the substance in which the solute dissolves.

Waters polarity gives it the ability to dissolve both ionic compounds and polar molecules like sugar. Thus water is the UNIVERSAL SOLVENT.

Suspensions

Some materials do not dissolve when placed in water but separate into pieces so small that they do not settle out. Such mixtures of water and non-dissolved materials are known as SUSPENSIONS.

The movement of water molecules keeps the small particles suspended.