Water and Solubility

1. Water and Solubility Atmospheric water vapour Write a definition for these words: Solute: Solvent: Solution: Transpiration Run-off • Atmospheric water vapour • Evaporation • Ground water in water table • Movement over land • Organisms • Precipitation • Water in lakes, oceans, rivers etc. Use the words in the list to complete the diagram of thwe water cycle above

2. What dissolves in water? Many gases are soluble in water to some extent. However, some are impossible to dissolve because of the lack of attraction between these molecules and water molecules. We call the amount of solute that can dissolve in a certain amount of solvent the __________. This is measure in ________. The solubility of most solutes increases as temperature rises. A ________ solution is one where as much solute as possible has been dissolved. What would happen if we heat up this solution? Could we add more solute? What would happen if we then let it cool down? Solubility curves At 20°C, 200g of sucrose will dissolve in 100g of water. At 60°C, almost 300g of sucrose will dissolve in the same amount of water. How much sucrose would come out of solution and form crystals if we allowed a saturated solution at 80°C to cool to 20°C?

3. Hard water Hard water is water that contains dissolved ___________ and __________ ions. They get into the water by dissolving as rivers run over or through rocks containing these compounds. e.g. Soap scum These ions react with soap to make soap scum. Sodium stearate is a chemical found in the majority of soap. It is this that the calcium and magnesium react with. All of the calcium and magnesium have to react with the soap before the soap can start to actually wash your hands! scale This can block pipes, immersion heaters and kettles easily, making them much less efficient. It’s not all bad though, the dissolved compounds can be good for our health – calcium ions help our ______ and _____- develop! There is also some evidence that drinking hard water can reduce the incidence of heart disease in people who drink it. Sodium stearate Ca2+ and Mg2+ ions Calcium stearate and Magnesium stearate Na+ ions Ca2+ 2HCO3 CaCO3 CO2

4. Removing hardness ______ water does not contain dissolved substances that produce scum and scale. To soften water, the ______ and _________ ions need to be removed. This makes it cheaper to wash ourselves and out clothes, as well as heating our water. However, people are advised to drink hard water if they can, for the health benefits. Industrial businesses also require soft water, so that their machinery doesn’t scale up and make them less efficient. There are two important ways to soften hard water: Using washing soda Adding ________ ___________ to water will soften it. The washing soda will _______ out calcium and magnesium ions into insoluble salts, so they cannot react with the soap. Ca2+ + CO32- _________ Using an Ion Exchange Column Ion exchange works by simply swapping one ion for another – hence the name! Hard water gets passed through a column containing resin beads which contain Sodium. Calcium and Magnesium have a higher affinity for (are more attracted by) the beads than the Sodium, so the Sodium gets displaced. The Calcium and Magnesium ions remain in the column, and soft water containing Sodium leaves the column. Why do water softeners need a salt solution poured through them occasionally?

5. Water treatment Providing people with clean drinking water is vital. If the water comes from a borehole, it has usually been filtered by the rocks around it, so should be fairly clean: this just needs disinfecting with chlorine to make sure. However, when we take water from rivers and reservoirs we need to give it more treatment than this. This involves a number of physical and chemical processes. There are 5 stages to this. Cut out and stick the boxes in the correct order to produce a flow diagram of the treatment of water. The water now looks clean, but still contains bacteria which could be harmful. A small mount of chlorine is added to kill any harmful bacteria. The water then enters a settlement tank, where sand and soil fall to the bottom. The water is now stored in large tanks and service reservoirs to be pumped to homes, schools and businesses. The pH of the water is checked and corrected to ensure it is neutral. The water enters the treatment works, passing through a cage-like metal screen. This catches large objects such as sticks and leaves. Aluminium sulfate and lime are added to the water. This makes small particles of dirt clump together and sink. The sludge is collected and thrown on a landfill site where it forms mud. The water then passes through a fine sand filter, so no particles of dirt remain

7. Water treatment