Water Properties PDF
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The document explains the unique properties of water, including its density, specific heat capacity, high heat of vaporization, and its ability to act as a solvent. The document also covers water treatment, pollution and hardness. It is suitable study material for secondary schools.
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# C21 Water ## Objectives By the end of this topic, you will be able to: - Describe some of the unique properties of water - Explain why the properties of water can have useful and harmful effects on living systems - Discuss the consequences of the solvent properties of water - Distinguish betwee...
# C21 Water ## Objectives By the end of this topic, you will be able to: - Describe some of the unique properties of water - Explain why the properties of water can have useful and harmful effects on living systems - Discuss the consequences of the solvent properties of water - Distinguish between the two types of water hardness ## Water Water makes up between 60% and 70% of the human body and it covers 71% of the Earth's surface. Water is a liquid at room temperature and pressure, however it exists on Earth in all three states, solid (ice), liquid and gas (water vapour or steam). Water has certain unique properties that make it essential for life on Earth. These properties help us to understand such things as how water becomes polluted and why life can exist at the bottom of lakes when the top is frozen. ## C21.1 The Unique Properties of Water **Pure water**, i.e. water that does not contain any impurities, is a colorless, tasteless and odourless liquid that boils at 100 °C and freezes at 0°C at standard pressure. Water molecules are polar, meaning that each molecule has a side that has a partial positive charge and a side that has a partial negative charge. The two hydrogen atoms in each molecule have partial positive charges and the oxygen atom has a partial negative charge. This is shown in Figure 21.1.1.  The partial positive hydrogen atoms and partial negative oxygen atoms of the water molecules are attracted to each other. This attraction forms what is known as the hydrogen bond. Hydrogen bonds are usually stronger than other intermolecular forces which exist between molecules and this gives water several unique properties. We will now look at some of these properties and at their significance to living organisms. **The maximum density of water occurs at 4°C** Density is a measure of how compact a substance is. It is defined as the mass of a substance divided by its volume, i.e. $density = \frac{mass}{volume}$ If the volume of a fixed mass of a substance decreases, its density increases. If its volume increases, its density decreases. When most liquids are cooled and become solids they contract. The volume of the solid is less than the volume of the liquid from which it forms. The solid is, therefore, denser than the liquid and if it is placed in more of the liquid it sinks. Something slightly different happens in the case of water. When water is cooled down to 4°C it contracts and becomes denser, just like other liquids. However, if it is cooled below 4 °C it starts to expand and continues to expand until it freezes at 0 °C. This means that the volume of ice is greater than the volume of the water from which it forms. Solid ice is, therefore, less dense than the liquid water from which it formed and it floats on the liquid as it forms. This is why ice cubes float in a cold drink.  When a pond or lake freezes, ice forms at the surface and the denser, warmer water remains below the ice. This enables aquatic organisms to survive under the ice.  **Water has a high specific heat capacity** In Unit 12.2 you learnt that the specific heat capacity of a substance is the amount of heat needed to raise the temperature of a unit mass of the substance, e.g. 1 g, by 1 °C. Because water has a high specific heat capacity, it requires a lot of heat energy to increase its temperature by 1 °C. This means that water can absorb a lot of heat energy without its temperature changing very much or, in other words, when the temperature of the surroundings changes, the temperature of water does not change very much. This is of significance to living organisms for two main reasons. - The bodies of living organisms contain between 60% and 70% water. Because of this, living organisms can absorb a lot of heat energy without their body temperature changing very much.. This means they can survive in extremes of temperature. - As environmental temperatures change, e.g. from winter to summer, the temperature of large bodies of water such as lakes and seas does not change very much. This means that organisms living in aquatic environments do not experience extreme fluctuations in temperature. **Water has a high heat of vaporisation** The heat of vaporisation is the amount of heat energy required to change a liquid to a gas. A lot of heat energy is required to change liquid water to a gas due to the hydrogen bonds between the water molecules. This means that water is not very volatile. Because water has a high heat of vaporisation, when water evaporates from the surface of a living organism it removes a large amount of heat energy from the organism. This makes sweating and transpiration very effective methods of cooling organisms. **Water has a relatively high melting point and boiling point** The melting point of ice and the boiling point of water are much higher than other molecules of a similar size due to the hydrogen bonds between the molecules. Water exists as a liquid between 0 °C and 100°C, which means that at the temperatures experienced on Earth, most water is in the liquid state. Because of this, lakes, rivers and seas exist and provide an environment in which aquatic organisms can live. **Water dissolves a large number of substances** Water is sometimes referred to as a 'universal solvent' because it dissolves a large number of substances. Because water molecules are polar, water can dissolve both ionic and polar covalent substances. 
