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Lesson summary: Water and life

Key terms

Polar moleculeA neutral, or uncharged molecule that has an asymmetric internal distribution of charge, leading to partially positive and partially negative regions
CohesionThe attraction of molecules for other molecules of the same kind
AdhesionThe attraction of molecules for other molecules of a different kind
DensityThe mass per unit volume of a substance
Specific heat capacityThe amount of heat needed to raise the temperature of one gram of a substance by one degree Celsius
Heat of vaporizationThe amount of energy needed to change one gram of a liquid substance to a gas at constant temperature

Unique properties of water

  1. Water is polar. Water molecules are polar, with partial positive charges on the hydrogens, a partial negative charge on the oxygen, and a bent overall structure. This is because oxygen is more electronegative, meaning that it is better than hydrogen at attracting electrons.
  2. Water is an excellent solvent. Water has the unique ability to dissolve many polar and ionic substances. This is important to all living things because, as water travels through the water cycle, it takes many valuable nutrients along with it!
  3. Water has high heat capacity. It takes a lot of energy to raise the temperature of a certain amount of water by a degree, so water helps with regulating temperature in the environment. For example, this property allows the temperature of water in a pond to stay relatively constant from day to night, regardless of the changing atmospheric temperature.
  4. Water has high heat of vaporization. Humans (and other animals that sweat) use water’s high heat of vaporization to cool off. Water is converted from its liquid form to steam when the heat of vaporization is reached. Since sweat is made mostly of water, the evaporating water absorbs excess body heat, which is released into the atmosphere. This is known as evaporative cooling.
  5. Water has cohesive and adhesive properties. Water molecules have strong cohesive forces due to their ability to form hydrogen bonds with one another. Cohesive forces are responsible for surface tension, the tendency of a liquid’s surface to resist rupture when placed under tension or stress. Water also has adhesive properties that allow it to stick to substances other than itself.
    These cohesive and adhesive properties are essential for fluid transport in many forms of life. For example, they allow nutrients to be transported to the top of a tree against the force of gravity.
  6. Water is less dense as a solid than as a liquid. As water freezes, the molecules form a crystalline structure that spaces the molecules further apart than in liquid water. This means that ice is less dense than liquid water, which is why it floats.
    This property is important, as it keeps ponds, lakes, and oceans from freezing solid and allows life to continue to thrive under the icy surface.

Common mistakes and misconceptions

  • Water dissolves everything because it is the “universal solvent." Water has the ability to dissolve many substances but the term “universal solvent" is misleading. Water is able to dissolve other polar molecules and ions, such as sugars and salts. However, nonpolar molecules like oils lack partial positive or partial negative charges, so they are not attracted to water molecules. This is why nonpolar substances like oil remain separate when added to water.

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