2-1.pdf

Chemical reactions and their impact on water quality 1−2 Lesson 1-2 physical properties of water and their role in the distribution of living organisms Water has unique physical properties that distinguish it from other fluids (liquids and gases), such as the...

Chemical reactions and their impact on water quality 1−2 Lesson 1-2 physical properties of water and their role in the distribution of living organisms Water has unique physical properties that distinguish it from other fluids (liquids and gases), such as the decrease in its density when it reaches the freezing point and the high value of its specific heat, which affect many natural phenomena, and the distribution of living organisms in different environments. Density It is the mass of a unit volume of matter at a given temperature. Because matter is made up of molecules, the density of matter depends on the mass of the molecules and the distances between them. In case of pure water, the mass of 1 cm3 of it at a temperature of 4oC equals 1 g, that is, the density of water at 4oC equals 1 g/cm3, which is equivalent to 1000 kg / m3 in the international unit of density, and as the temperature of water decreases from 4oC to its freezing point, its density decreases as shown in the opposite graph. The ratio between the density of a given substance and the density of pure water at the same temperature is known as the relative density of the substance The density or the relative density of liquids is measured by hydrometer, which is a sealed hollow glass reservoir with a wider bottom for buoyancy, containing lead (or mercury) balls for vertical stabilization and connected to a long, small-diameter glass stem that is graduated in units of density so that the lower scale indicates the highest density measured by the hydrometer and the higher scale indicates the lowest density measured by the hydrometer. Practical activity: Measure the density of different samples of water Use a hydrometer to determine the density of water from different sources: (sea, river, canal, pond, lake, underground). Discuss how the hydrometer can be used to predict the presence of soluble pollutants in a sample of water. 13 Chapter 1 Aquatic ecosystem Water density and water currents in the oceans: The density of water in the oceans is affected by the pressure inside the oceans, the amount of salt dissolved in it, and its temperature. As the pressure increases with increasing depth, the water molecules get closer together, and therefore the density of the water increases. Density is also affected by the amount of dissolved salt (salinity) in the water. The higher the salinity of water, the higher its density. The normal salinity of ocean water is 35 grams per liter of water (or the equivalent of two teaspoons per cup of water). Finally, the temperature of the water affects its density. The lower the temperature of water (down to 4°C), the closer the molecules are to each other, the lower the volume they occupy and the higher the density of water. The differences in water density are one of the causes of water currents in oceans. These water currents carry heat and salt from the tropics to the poles, nutrients from the deep ocean to the surface, and fresh water from rivers or melting snow to different places when these currents travel around the globe. Density of water in Polar Regions The density of water changes as its temperature changes, generally the volume of a liquid increases as the temperature increases and the volume of a liquid decreases as the temperature decreases. Water is an exception to this rule. As the temperature of pure water increases from 0°C to 4°C, the water shrinks and as a result its density increases, and the density of water reaches its highest value (1000 kg/ m3) at 4°C. Water expands as the temperature rises above 4°C, so its density decreases. This helps to understand why a lake in polar regions starts to freeze at the surface rather than at the bottom. When the air temperature is between 4°C and 0°C, the surface water of the lake expands, becoming less dense than the water below it. Finally, the surface water freezes, and the ice remains on the surface as the density of the ice is less than the density of the water while the water remains near the bottom at 4 oC. If not, fish and other marine life would not survive. 14 Chemical reactions and their impact on water quality 1−2 Lesson Practical experiment The effect of the difference in density on the movement of water Prepare ice cubes and add food dyes to the water before it freezes, so that it is easy to observe the melting process of the ice cubes and the direction of water movement after it melts. Put one ice cube in a quantity of fresh water, and another ice cube in an equal quantity of salt water with the salt concentration equal to the salt concentration in ocean water at room temperature. -In which case does the ice cube dissolve at a faster rate? -What are your observations about the movement of water resulting from the melting of each cube? This is already happening in the ocean! If fresh water from melting icebergs enters the ocean, that fresh water spreads out on the surface of the ocean and does not sink. If the freshwater freezes, it forms an insulator between the deeper parts of the ocean and the cold atmospheric air above. Check your understanding (1) Analyze the opposite graph and conclude what happens to the density of water as the temperature changes. ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ (2) Give an example of how a change in temperature and density of water affects organisms in an aquatic environment. ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ 15