Properties of Water

Water molecules are polar, they have oppositely charged sides, and they form hydrogen bonds with other polar or charged molecules. This quality, being polar, causes water to have certain properties that affect living things.

Cohesion

Cohesion (Co- together, -hesion to stick) is the property of water molecules sticking to each other.

Water molecules form drops on a surface instead of spreading out in a thin film
Bodies of water exhibit Surface Tension on their at the top.
- An individual hydrogen bond is weak, but many bonds together can support a small amount of weight.
- Some insects can walk on water because the many hydrogen bonds cause the surface to bend, but not break.

Adhesion

Adhesion is the property of water that allows it to stick to other molecules besides water.

Many molecules have charged or polar regions (+ or -) that attract water.
Water is absorbed through a paper towel because it is attracted to the molecules of the paper
Capillary action is a process that plants use to pull water from the ground, through the roots, up the stems and to the leaves.
- The attraction between water and the molecules in the veins of a plant can help draw water against gravity to the top of a 100 foot tree.

Solubility

Solubility is the ability to dissolve other substances. Water is known as the Universal Solvent.

A Solution is a mixture in which a solid or a gas is dissolved in a liquid.
- Solvent - the liquid
- Solute - the substance dissolved in the liquid
The polarity of water (being polar) allows it to dissolve most non-polar molecules.
This makes it the perfect medium for transporting molecules in a cell or an entire body
- Cytosol - the liquid that fills cells is mostly water so that molecules can easily move around
- Blood is made of mostly water that transports nutrients, wastes, hormones and other molecules throughout the body.

Density

Water behaves differently than other molecules when it freezes with respect to its density.

Density is how much stuff (mass) you have in a given space (volume).
- A pound of gold and a pound of feathers weigh the same amount, but the gold takes up less space so it is more dense.
- Gravity tends to separate objects by their densities - objects with higher density sink; those with lower densities float.
- When substances are heated, they become less dense (hot air rises); when they are cooled, they become more dense.
Water behaves like normal substances until it is frozen.
- Really hot water turns to steam and rises further (normal)
- Warm water is less dense and it rises (normal)
- Cool water is more dense so it sinks (normal)
- Frozen water becomes less dense and it floats (not normal)

Polar molecules and hydrogen bonds are the reason ice is less dense than liquid water.
- Water molecules in a liquid are constantly moving (kinetic energy) and bumping into each other forming and breaking hydrogen bonds.
- Molecules slow down when they are cooled (less kinetic energy).
- Ice forms when the strength of the hydrogen bonds is greater than the kinetic energy and it holds the molecules in place (forms a solid)
- Hydrogen bonds hold water molecules farther apart than they are normally in a liquid, so ice is less dense than liquid water.
In the winter, water freezes in bodies of water and it floats on the surface.
- This creates an insulating barrier to the liquid water below allowing fish and other aquatic life to survive.
- If ice was more dense it would sink and push up more water to freeze and sink, thus lakes would freeze solid.

Specific Heat

Water has a high specific heat - the amount of energy it takes to raise the temperature 1 degree Celsius.

Hydrogen bonds absorb a lot of energy before they break and allow molecules to evaporate. They also release a lot of energy when they form.
Water tends to heat or cool much more slowly than the surrounding air or other substances.
- Temperatures near oceans tend to be more mild than those further inland - summer temps are cooler, winter temps are warmer.
- Sweat, or any other form of water, absorbs a lot of heat from the surface of the body before it evaporates and takes the heat energy with it thus cooling the body.