Properties of Water

Water, Why do we really need it? The structure of water molecules explains many of water’s chemical properties. Water is a polar molecule, which is a molecule in which part of the molecule is positively charged and part of the molecule is negatively charged. Opposites attract one another. Therefore the positive (hydrogen) part of one water molecule is attracted to the negative (oxygen) parts of other water molecules. This attraction creates weak bonds, known as hydrogen bonds, between adjacent water molecules. Each molecule of water consists of one atom of oxygen and two atoms of hydrogen, so it has the chemical formula H2O. The arrangement of atoms in a water molecule explains many of water’s chemical properties. In each water molecule, the oxygen atom attracts electrons much more strongly than do the two hydrogen. As a result, the oxygen atom has a slight negative charge, and the hydrogen atoms have a slightly positive charge. A difference in electrical charge between different parts of the same molecule is called polarity, making water a polar molecule. A polar molecule is a molecule in which part of the molecule is positively charged and part of the molecule is negatively charged.

Chemical Structure of Water Hydrogen bonds always involve a hydrogen atom. They are bonds between molecules and they are not as strong as bonds within molecules. Water has some unusual properties due to its hydrogen bonds.

Cohesion & Surface Tension Cohesion is the tendency for water molecules to stick together. Surface tension is when water molecules form an invisible layer which can hold weight. This phenomenon is due to cohesion.

Adhesion & Capillary Action Adhesion is when water molecules stick to other substances. This sticking of water to other molecules is also seen in nature when water is taken up by the roots in plants. This phenomenon is called capillary action.

Density Hydrogen bonds cause water molecules to line up with more space between each molecule in ice than in liquid water. As a result, water molecules are spaced farther apart in ice, giving ice a lower density than liquid water. Unlike most chemical substances, water in a solid state has a lower density than water in a liquid state. This is because water expands when it freezes. A substance with lower density floats on a substance with higher density. The floating ice acts as an insulator keeping the deeper water from freezing. This allows marine life to keep their habitat and live in the water.

Universal Solvent The two parts of a solution are solvent and solute. The solute is the substance that dissolves in the solvent. Water is a strongly polar solvent, and polar solvents are better at dissolving polar solutes. Many organic compounds and other important biochemical are polar, so they dissolve well in water. Can you think of a substance that water can’t dissolve? What chemical property can be used to describe this substance?

Heat Capacity Water has a high heat capacity because it takes a lot of energy to raise or lower the temperature of water. As a result, water plays a very important role in temperature regulation. Since cells are made up of water, this property helps to maintain homeostasis.

Properties of Water LAb There are six stations set up around the room. You will form groups of 3-4. Each station will take about 10-15 minutes. Move out of the way when you have finished at your station and wait for another to become available. The directions for each station are above each experiment. This may take a day or two to complete.

My Desert Sponge In this activity, you will take care of a sponge for 48 hours. You must make adaptations for the creature to stay hydrated. You will need to make observations over the course of this time period. Your sponge must be exposed to the environment for 4 hours, every 24 hours, so 8 hours total. This represents the time the animal must leave its shelter to feed and obtain energy required for survival.