Water: essential to life Chapter 11:

Where there is no water, there is no life. Water: is a special chemical, both common and unique Is the most abundant liquid on Earth, covering over 70% of our planet. It moderates our weather, shapes our lands and is essential for the existence of life.

The Water Cycle The water on Earth exists in solid, liquid and gaseous states and readily changes from one state into another. The water cycle could be said to begin with evaporation from: Oceans Lakes (solar energy being the major energy source) Rivers Human activities (combustion of fossil fuels) produce steam which also contributes to the water cycle Water vapour in the air is transported around the globe until it condenses to form clouds It returns to the ground as rain water or ice crystals in hail or snow The Cycle begins again

Water and Living things Water is involved in the reactions of life: photosynthesis and respiration It is also important as: Water provides a function in plants and animals where it transports nutrients and soluble wastes. The fact that water is a solvent and can dissolve many things explains why it is a very useful system to transport materials to where they are needed. Water can store large amounts of heat energy and transfer it away from cells where it is produced, to the body’s surface where it is transferred into the atmosphere. As living things contain large amounts of water this makes for a very efficient heat transferring system. Water on the skin of plants and animals can also act in cooling the body. Heat energy is transferred up into the water on the skin which then evaporates, taking away with it the heat energy that it absorbs.

Water provides a transport system. Transports nutrients and soluble wastes in the body. Food is consumed in a liquid environment.

Water provides a heat transfer system. It transfers heat energy from the cells (respiration) to the body’s surface, where it is lost to the surroundings. It acts as a thermal regulator, reducing temperature changes.

Water provides a cooling system. Humans lose 1 litre of water a day from perspiration and the surface of the lungs. Evaporation of water from a surface is the major cooling mechanism available to living organisms.

Water provides an environment. For aquatic animals and plants. Is the main source of their nutrients. Aquatic organisms depend on the buoyancy of water for physical support and their environment.

Water shapes the environment Oceans, rivers, lakes etc. Rainfall impacts on landscape Water erosion can alter a region over time. Water can freeze in the cracks of rocks causing them to split. The distribution of water influence the type and abundance of vegetation and animals. Coastal towns usually have a milder climate.

Water as a chemical reactant Water reacts in many naturally occurring process. Eg photosynthesis. Water is produced in many process. Eg. Combustion.

Water is important for reproductive process. Some water plants achieve fertilization by the release of spores into water. Developing embryo’s require an aqueous solution.

The properties of water The key to all the understanding is in the bonding between water molecules. Covalent bonding Strongly polar Extensive hydrogen bonding Hydrogen Bonding occurs when a hydrogen atom bonds with a very highly electronegative atom. A very obvious example is the Hydrogen bonding that occurs in water between the Hydrogen atoms in one molecule and the Oxygen atom in another neighboring molecule. This explains how water molecules attract to each other and stay together. (The Term HFON refers to the elements that can take part in Hydrogen bonding, Hydrogen, Fluorine, Oxygen and Nitrogen.)

The properties of water The key to all the understanding is in the bonding between water molecules. Covalent bonding Strongly polar Extensive hydrogen bonding Hydrogen Bonding occurs when a hydrogen atom bonds with a very highly electronegative atom. A very obvious example is the Hydrogen bonding that occurs in water between the Hydrogen atoms in one molecule and the Oxygen atom in another neighboring molecule. This explains how water molecules attract to each other and stay together. (The Term HFON refers to the elements that can take part in Hydrogen bonding, Hydrogen, Fluorine, Oxygen and Nitrogen.)

Melting and boiling points Water consists of very small molecules. Other substances of similar size are often gas at room temp, yet water is a liquid. Water freezes at 0.C and boils at 100.C These values are higher than those for other substances consisting of similar sized molecules. The reason for these high values is the strong hydrogen bonding that occurs between molecules in water.

Thermal Properties Water is good at storing heat energy. The specific heat capacity is 4.2 J per gram per degree. This is the energy required to increase the temperature by one degree. The Latent heat value measures the energy required to change the state of a substance eg. From ice to liquid, or liquid to water vapour. These two values of water are quite high compared to other substances and make water ideal in heating and cooling systems.

Density Water expands on freezing. Ice (lower density) floats on liquid water. Water is the only natural substance that expands on freezing. The density of liquid water at 3 degrees is 1 g/mL, whilst the density of ice is g/mL.

Ice In liquid water, the hydrogen bonds continually break and form as the molecules move around. However, in a solid state, the hydrogen bonds are permanent. In ice, each water molecule becomes hydrogen bonded to four others in a tetrahedral arrangement.

Ice continued This causes the water molecules to remain slightly further apart than they are in liquid water. Hence, ice is less dense than water and it floats. N.B. Snap freezing occurs too quickly for the hydrogen bonds to form, thus allowing no cell damage to occur.

A good solvent Water will dissolve a wide range of substances due to its polar nature. It will dissolve polar molecules and many ionic substances. However, it is a poor solvent for non-polar substances such at oil, fats and petrol.

Water

Specific Heat Calculations Chapter 10: Question 4, 5, 13, 15 Handout : Wonderful Water