COOL Classroom 2.1

Discover Water

Water is considered common place to most of us. Many times we don’t even think about it unless we are hot or thirsty. The oceans are full of water and because of this we never think about a world without water. However, earth is the only known planet that has liquid water. This liquid water is what allows life on earth to exist!

Water, like all matter, is made up of tiny particles called atoms. One oxygen atom and two hydrogen atoms form a water molecule. The oxygen atom is much larger than the hydrogen atoms. Water is a polar molecule, meaning it is electrically charged. This happens because the negatively-charged electrons spend more time orbiting around the bigger oxygen atom giving that side of the atom its negative charge. This leaves the hydrogen atoms fewer electrons so it has a positive charge. You know the saying “opposites attract”, well the same holds true for electrical charges. Because of their polarity, or opposite electrical charges, the positive side of the water molecules are attracted to the negative side other molecules, including other water molecules. Hydrogen bonds are the attraction between liquid water molecules. They are not very strong bonds but they are very important and are responsible for many of the unique properties of water.

Water, Water Everywhere

Any substance can exist in three different physical forms; solid, liquid and gas. Water is the only substance on earth that naturally occurs in all three states. Temperature reflects the amount of kinetic energy of molecules or more simply, the motion of the molecules. The faster the molecules are moving the higher their temperature. This is similar to how the faster you move, the hotter you get! When moving, the hydrogen bonds between the molecules of water can break. The water molecules in the liquid phase can move so fast that they break their hydrogen bonds and go from a liquid to a gas phase. If temperatures get hot enough, the molecules all move fast enough to break their hydrogen bonds and move into the gas phase. We see this happening when water boils!

When liquid cools the molecules have less kinetic energy, slowing down and packing closer together. Volume is decreasing without mass changing so the water gets more dense. As seawater gets more dense, it sinks to the bottom of the ocean. This sinking of cold water at the poles creates global circulation of seawater.

Credit NASA

As water continues to cool the hydrogen bonds form and hold the water molecules in place as a solid. This solid form of water is ice. Ever notice that ice cubes float in your drink? That is because fresh water gets denser as it cools until it reaches about 4°C, (39°F). Below that temperature ice gets less dense as it cools. This happens because water expands as it freezes. If you have ever made ice cubes you have seen this happen. Since the mass remains the same, but the volume of the solid is greater, ice is less dense than liquid water ~ so it floats! Because of this, bodies of water such as lakes and bays freeze at the surface. This property allows fish living in lakes and ponds to survive the winter. The ice that freezes on the surface insulates the water below so it stays warmer! In general, it is extremely unusual for the solid phase of a substance to be less dense than its liquid phase, but lucky for aquatic life it is!

Melting Ice

A large amount of heat energy is necessary to melt ice. Water molecules in the ice begin to vibrate as heat is added, and eventually break their hydrogen bonds and move around. Ice requires a much warmer temperature to melt due to its hydrogen bonds.

Without the strength of the hydrogen bonds, ice would melt at -90°C (-130°F) instead of 0°C (32°F). Ice works well to cool your drinks because any heat that enters the liquid goes into breaking the hydrogen bonds of the ice, not into increasing the drink molecules’ velocity, which would raise the temperature of your beverage. Given that all of the energy does not go into increasing the speed of the water molecules but also into breaking the hydrogen bonds, it requires a lot of heat to raise the temperature of water.

The heat capacity of a substance is the amount of heat that must be added to raise the temperature of a substance a given amount. Water has one of the highest heat capacities of any natural substance. This reflects on the amount of heat water can store. Have you ever heard of water being used as a coolant in car engines? This is because it can absorb a lot of heat without increasing much in temperature. You experience the effects of water’s high heat capacity when you visit the shore. When you go to the beach in June, even though it has been warm for a few months, the water is still really cold. And if you return in November, even though the air temperatures have been cool for a few months, the water is still relatively warm. This is all because of water’s high heat capacity!

Water as a Solution

Water is called the universal solvent because it can dissolve more things than any other natural substance. Water is especially good at dissolving salts. Salts are a combination of particles with opposite charges, which are called ions.

Table salt consists of a positively charged sodium ion (Na+) and a negatively charged chlorine ion (Cl-) to form NaCl. When placed in water the strongly charged ions that make up the salt attract the weak charges of the water. Water molecules surround each of the ions and this weakens the electrical charges that were holding the salt molecules together. The salt crystals breaking apart is the process of the salt dissolving in water. The salt dissolved in water increases the mass of the water solution without increasing the volume by much, it increases the density of water (density = mass / volume ).

What do you know about water?

1. Which phase of water allows life to exist on earth?
a. Liquid
b. Solid
c. Gas
d. Vapor
e. They all do

2. ( T / F ) Hydrogen bonds are the extremely strong bonds that hold water molecules together.

3. Temperature reflects on the _______________________ of molecules.

4. What property of water allows aquatic life to live through the winter in lakes and ponds?
a. Water gets denser as it cools
b. Water has a high heat capacity
c. Water gets denser as it cool until it reaches 4oC and then is becomes less dense
d. Water is a universal solvent

5. Water works well as an engine coolant because:
a. It can absorb a lot of heat
b. It is a clear substance
c. It is the universal solvent
d. It can absorb a lot of heat without increasing in temperature