Science: An Elementary Teacher’s Guide/Water, Solutions, and pH

Unique Properties of Water

Water is the most important liquid in the universe. When NASA looks for life they look for water. Water is a big deal. Water is precious and it has been described by many as coolness in the desert due to its ice cold feeling when entering a heated persons' stomach. Humans and most mammals cannot survive more than a few days without water. The ultimate Thirst Quencher.

Water is a polar molecule and what makes it polar? Polar like the North and South Pole of the Earth? Yeah, similar to those concepts. Polar means to have a double nature nature. And when a molecule is polar it has an uneven distribution of electrons. Some atoms are more "greedy" or more electronegative meaning they attract and want more electrons than other atoms. An uneven distribution of electrons between bonds means one atom will get more electrons than the other atom and this results in a bond that is more positive on one end and more negative on the other. . . A dipole (2 poles)

See on the right how Oxygen pulls the electron density towards itself? Because Oxygen is one of the most electron-loving elements on Earth!! A partial positive forms on one end of the molecule (around the Hydrogens) and a partial negative is formed on the other side (around the oxygen). Since every single water molecule has this polarity, they are attracted to each other (the positive end of one molecule being attracted to the negative end of another). This creates a "stickiness" between the molecules, and the effect of these small attractions happening millions of times gives water some unique properties. Keep in mind, each molecule is jiggling and moving, so the bonds between any two molecules may be quickly broken, but new bonds form with other molecules. Some of the properties of water that are a product of this weak bonding (which, by the way, is called hydrogen bonding) are as follows:

• Water is a liquid at room temperature. Compare water to Carbon-dioxide to see why this is unusual: CO₂ is a much heavier molecule than H₂O (an atomic mass of 48 for one carbon and two oxygens, compared to an atomic mass of 18 for water's single oxygen and two hydrogens), and yet CO₂ is a gas at room temperature, and stays a gas until far, far below room temperature. Because the CO₂ molecules are not attracted to each other, they have to be slowed down considerably before staying together as a solid. While water will become solid at 0 degrees Celsius (32 degrees Fahrenheit), CO₂ will not freeze into a solid until -78.5 degrees Celsius (-109.3 degrees Fahrenheit). This solid is called "Dry Ice" because as it melts the CO₂ molecules go instantly from solid to gas (this is called sublimation). In contrast, as water warms from a solid it becomes liquid, because the molecules are still attracted to each other and keep one another from breaking away into the atmosphere. If you heat up the water molecules, they will move faster and faster, until there is enough energy to break the bonds between the water molecules and create steam (a gas) at 100 deg C (212 deg F).
• Water is less dense as a solid than it is as a liquid. To be less dense means that the molecules are spaced further apart. Typically a solid is always more dense than a liquid, but with water as you cool the molecules the attraction between them makes them self-arrange into a grid, where any one molecule has 6 other molecules around it in a crystal-like pattern. The molecules freeze in place in this crystalline arrangement, with more space between them than the randomly-bouncing molecules of liquid water. We are lucky that this property exists--otherwise, in winter a lake would freeze solid as ice piled up from the bottom to the top. By floating, the top layer of ice insulates the rest of the lake beneath, allowing fish to survive the winter.
• Water molecules are attracted to each other (cohesion), and are also attracted to other molecules (adhesion). You can observe this by looking at a drop of water hanging from your fingertip--the molecules hold on to each other and to your skin, allowing the drop to form. You can see adhesion by putting water onto a piece of paper towel and watching how it spreads. This property is important for plants that must get water from the ground up to their leaves. Plants do not have hearts to pump the liquid up--instead they have microscopic channels that the water molecules travel up. Many microscopically-thin columns allow the water to travel up to the leaves, where the molecules eventually leave into the air through tiny pores on the leaves. Plants need water for photosynthesis, and if it were not for adhesion plants would never have been able to grow taller than just a few inches.
• Water is a great solvent. Because of its polarity, water grabs onto all kinds of molecules, allowing most things to dissolve easily in water. There are, of course, types of molecules that do not dissolve well in water, like grease or oil. Those molecules are non-polar (no charge anywhere on the molecule). When we wash dishes we want to get the grease off, so we use soap--soap consists of rather large molecules that have one end that is polar and the other end that is non-polar. The polar end of the soap molecule grabs onto water molecules, and the non-polar end grabs onto oil and grease molecules. The result is that the oil is pulled apart into tiny droplets, each surrounded by several soap molecules, and carried away from each other into the water. The oil never actually dissolves in the water, however, and if you leave the dishwater overnight those tiny oil droplets will bump into each other and get back together, making for a yucky mess in the morning!