At the end of this unit you should:

2. At the end of this unit you should: 1. Be able to explain why water is such a useful solvent and coolant. 2. Be able to explain what a polar molecule is. 3. Be able to describe the difference between a polar and non-polar solvent by investigation. 4. Be able to demonstrate and explain that water has surface tension. 5. Be able to explain why water has two different densities. 6. Be able to show water transport by osmosis and predict the effect of osmosis by investigating a number of substances. 7. Be able to explain what hard water is as well as its advantages and its disadvantages. 8. Be able to test for water hardness and rank the hardness in a water sample.

3. contaminated coolant deionising distillation Goldilocks Zone hard water ice impure insulator lather limescale molecule negative non-polar molecule osmosis polar molecule polluted positive pure soft water solute solution solvent specific heat capacity steam surface tension universal solvent water

4. LIGHTBULB QUESTION No. Water cannot dissolve every substance, but it can dissolve a huge number of substances and because it is a solvent that is easily found and used throughout the world it is called the ‘universal solvent’.

5. Equipment: Cold water (5°C), room temperature water (20°C), hot water (80°C–100°C), kettle, timer, teaspoon, tongue depressor/lollipop stick, three 100 ml beakers, 25 ml graduated cylinder and funnel (or 25 ml syringe), sugar, table salt, copper sulfate, powdered chalk, pepper, instant coffee, alcohol, cooking oil, flour, soap flakes, baking soda. Investigation 08.03.01: Testing the universal solvent

6. Instructions: 1. Place 1 000 cm3 of tap water in a fridge overnight, to cool to 50C. 2. Fill a kettle with tap water and boil. 3. Place 25 cm3 of the cold tap water in a 100 ml beaker. 4. Add three level teaspoons of solute and stir for five seconds. The tongue depressor can be used to ensure a level teaspoon of solute is used by drawing it across the rim of the spoon while holding it over the container of solute. 5. Allow the solution to settle for three minutes and record the solubility of the solute. 6. Repeat with 25 cm3 of room temperature tap water, using the second beaker. 7. Repeat with 25 cm3 of hot tap water, using the third beaker. 8. Test each solute in the same way at all three water temperatures.

7. 1. How many substances do you need to test for your results to be valid? • It is not possible to test an unlimited range of solutes, but a small number of very different solutes would help establish all the principle of water’s wide-ranging ability to dissolve substances, i.e. the solutes should not all be powders or granules of the same size but varying sizes, and possibly include some liquid solutes.

8. 2. What should you do to make sure that this is a fair comparison of all the substances? • If the substances are not all powders, then an equal mass needs to be used, mixed in an equal volume of water. If stirred or shaken, then this should be done for an equal amount of time and equally vigorously. Temperature should also be fixed. If using temperatures other than room temperature, maintaining a steady temperature may be difficult if the mixing process takes too much time.

9. 3. How would you carry out this test at home as a fair comparison? • Using a teaspoon – a simple kitchen utensil – fixes the mass and ensures students can extend their experimentation at home.

10. 4. What substances might you use? • Many of the substances that are used in cooking are good solutes of solvents e.g. sugar, salt, coffee granules, flour, milk, hot/cold water

11. 5. Based on your testing at home and in school, do you think it is still a reasonable idea to call water the ‘universal solvent’? • Yes, because … • OR • No, because …

12. (a) Water can be found all over the world. Can you list and explain two disadvantages of water being an excellent solvent? It is necessary to protect many substances to prevent them from dissolving (or clumping), e.g. sugar needs to be kept in a closed container; other substances also need to be coated with water-repelling chemicals.

13. (b) Teagasc – a government agency that advises farmers on agriculture science – recommends that farmers spread slurry on a damp day. Why? The nutrients from the slurry will dissolve into the soil better.

14. Insulator: A substance which does not easily allow the passage of heat (or sound). Specific Heat Capacity: The heat energy needed to increase the temperature of 1 kg of a substance by 10C. Coolant: A liquid or gas that is used to remove heat from something.

15. (a) Check the kW h rating of five electrical devices in your home. Does the physical size of the device or whether it heats water decide the kW h rating of the electrical device? The size of the device will not automatically decide its power requirements, the size of the heating element will. In a kettle the element is coiled upon itself so that there is a large surface area, meaning the water can be boiled quickly.

16. (a) Check the kW h rating of five electrical devices in your home. Does the physical size of the device or whether it heats water decide the kW h rating of the electrical device? Fig. 08.03.01 shows the heating elements in an electrically operated boiler; the smaller element heats a smaller volume of water for a shower rather than a bath.

17. (b) List and explain two disadvantages and two advantages of using water as a coolant. Advantages: Water is easily available, and absorbs a large amount of heat energy before it begins to change state. Disadvantages: When heated, water stays hot for a long time so can cause scalds and burns if not handled carefully; if used in a sealed container, steam pressure can build up so a pressure relief valve is often needed. If used in a metal container water can corrode the metal.

18. Equipment: Kettle, three 400 ml beakers, three thermometers, timer, graduated cylinder and funnel (or 100 ml syringe). Instructions: 1. Transfer 100 ml of boiled water to a beaker, and start the timer. 2. Place a thermometer into the water and take a reading after thirty seconds. 3. A temperature reading should be taken every thirty seconds for a total of five minutes. 4. Repeat the same procedure with 200 ml and 300 ml of water. Investigation 08.03.02: The cooling rate of water

19. 1. How can you make sure that this is a fair test? • An equal volume/mass of water is used each time; each beaker is of the same dimensions (height, diameter, glass thickness, design); each beaker is equally cold when the water samples are added, i.e. don’t use the same beaker each time as it will become increasingly warm, which will affect the rate.

20. 2. Based on your results, do you agree that water is a good coolant? • Generally, yes, because water absorbs a lot of heat energy and takes a long time to cool, as well as being easily available, but the fact that it takes so much time to heat and cool presents a possible danger or makes it inconvenient; an ideal coolant can heat and cool quickly as well as absorbing a lot of heat energy.

21. 3. Can you display your results in a way that is more convincing? • Numbers in a table may hide significant results so displaying the recorded temperatures as a three-series trend graph may be more convincing. The readings for each beaker would constitute a series each.

22. 4. Can you see any other patterns in your results? Explain. • As it is not possible to transfer the hot water to the beakers without some heat loss, each trial will have a slightly different starting temperature. This may mean that the same mass of water may show different rates of cooling; these rates will be lower when the difference between the initial temperature in the beaker and room temperature is smaller. This will only be seen if rate calculations are made for each trial.

23. Polar Molecule: A molecule which has a negative and positive end because electrons tend to sit at one end of a molecule.

24. Ethanol is also a very good solvent. Can you explain why? Like water it is polar, so becomes attracted to solutes that are polar, and so dissolves them well.

25. (b) Name a household liquid that does not dissolve in water. Based on the fact that this substance does not dissolve in water, what can you say about its molecules? Cooking oil. Its molecules are either non-polar or weakly polar, which means that they are not attracted to polar water molecules enough to dissolve.

26. Surface Tension: A layer formed on the surface of a liquid by the force of attraction between the molecules in that liquid.

27. Investigation 08.03.03: Testing surface tension Equipment: Three petri dishes (or wide-necked containers), talcum powder/custard powder/flour/chalk powder (or other insoluble powder of your choice), washing-up liquid. Instructions: 1. Half-fill the petri dish with water. 2. Gently sprinkle one of the powders over the water surface. 3. Dip one finger gently into the water and remove. 4. Rub a small amount of washing-up liquid onto your finger and dip your finger again. 5. Repeat with the other two powders.

28. 1. How can you ensure that this is a fair test? • Ensure the petri dishes (containers) are the same dimensions; the same amount of each powder is sprinkled evenly across the surface of the water; apply the same amount of washing-up liquid to the tip of your finger.

29. 2. Compare your observations for each powder. What patterns do you see? • The Marangoni Effect will apply to all insoluble powders when sprinkled as a light layer on the surface of water, confirming that surface tension applies in water and is relatively strong.

30. 3. Can surface tension alone explain your results? Justify your answer. • If all controllable factors/variables are fixed and the procedure was carried out consistently, you might tentatively answer ‘yes’. You can confirm this further by widening the variety of powders or liquids tested.

31. Investigation 08.03.04: Expansion and sinking Equipment: Two ice-cube trays, olive oil, tap water, two 200 ml beakers, 20 ml syringe, 50 ml syringe. Instructions: 1. Fill an ice-cube tray with tap water. 2. Fill a second ice-cube tray with olive oil. 3. Both should be allowed to freeze overnight. 4. Test a water cube by placing 100 ml of tap water into a beaker and adding the water cube. 5. Test an olive oil cube by placing 100 ml of olive oil into a beaker and adding the olive oil cube.

32. 1. How could you compare the density of these two types of ‘ice-cubes’ without calculating the exact figures? (See Unit 13.1 for more on density.) • Each cube can be tested in the liquid state of itself to see if the density as a solid is greater or not. • 2. What do you need to do to ensure that this is a fair test? • An equal volume of both liquids to be frozen, for the same length of time, and tested simultaneously so melting does not affect the results.

33. 3. Did both ‘ice-cubes’ behave in the same way? Explain the differences you observed. What evidence do you have for this? • The water cube floated and the olive oil cube sank because the density of water decreases as it cools and the density of olive oil increases as it cools. Repeating this with more than one cube validates the initial observation with one cube.

34. Osmosis: The movement of water from low solute concentration to high solute concentration through a semipermeable membrane.

35. (a) Salt can be used to kill slugs. How does sprinkling salt on a slug kill it? Salt causes rapid osmosis through the membranes of the slug, removing water from its body and causing it to die.

36. (b) Explain how osmosis causes the skin on our fingers to wrinkle if they are left in water too long. For many years it was believed that skin wrinkles were caused by osmosis if skin was in water for long periods, as the water moved into the outer layers of the skin. Recent research suggests that the wrinkling is caused by blood vessels constricting under the skin in response to water, in order to better grip wet objects in the same way that threads on a car tyre increase grip in wet weather.

37. Investigation 08.03.05: Osmosis Equipment: Table salt, table sugar, distilled/deionised water, four gummy bears (all the same colour), three 300 ml beakers, spatula, top-pan balance, two weigh-boats, graduated cylinder.

38. Instructions: 1. Place 15 g of salt into a clean beaker and add 100 ml of distilled/deionised water. Stir until thoroughly mixed. 2. Place 15 g of sugar into a clean beaker and add 100 ml of distilled/deionised water. Stir until thoroughly mixed. 3. Place 100 ml of distilled/deionised water into the third beaker. 4. Place one gummy bear into each beaker. The fourth gummy bear should act as a control. 5. Each beaker should be left overnight, and examined the following day.

39. 1. How can you ensure that your results are valid? • Validity can be ensured by repeating for consistency, or extending the investigation across a range of solution concentrations and types, and a variety of other sweets, fruits or vegetables. • 2. What conclusion can you make about the sugar or salt content of your food sample? Justify your answer. • In both cases the concentrations were higher than in the gummy bears, causing them to lose water and shrink.

40. 3. Would this conclusion be the same for all fruits, vegetables, or jelly sweets? Explain. • It should apply to any fruit or vegetable whose salt or sugar concentration was lower than the two solutions, or higher than the distilled/deionised water. Any fruit or vegetable without a semi-permeable outer membrane, e.g. a coconut, would not be affected.

41. Hard Water: Water that does not form a lather easily with soap. Soft Water: Water that forms a lather easily with soap.

42. (a) Decide which of these statements are True or False:

43. (b) Name two household appliances that heat water in a similar way to kettles. Why should you not use vinegar to get rid of limescale in these devices? A coffee maker and a washing machine. Vinegar could not be used for the coffee maker because it would affect the taste of the coffee made afterwards. In a washing machine it would cause the clothes to smell, may damage some delicate cloths, and could affect how washing powder works.

44. Investigation 08.03.06: Comparing water from different sources Equipment: Soap flakes (or grated hand soap/washing powder), three samples of tap water, mineral water, distilled/deionised water, five test tubes, test tube rack. Instructions: 1. Obtain samples of tap water from three different sources. 2. Place an equal amount of water into each of the test tubes from the collected samples, the mineral water and the distilled/deionised water. 3. Add five soap flakes to a test tube, stopper and shake for five seconds. 4. Compare and record the lather produced by each sample of water.

45. 1. Does the type of soap you use to test for hardness affect your results? • Some soap types will be stronger than others or may actually be detergents. There may also be differences between different brands of the same soap type. • 2. Describe how this investigation can be set up as a fair test. • By using the same volume of water from each sample at room temperature, and shaking equally vigorously. An equal mass of soap should also be used each time.

46. 3. What change in your investigation results could you expect if you boiled your water samples and tested them again for hardness? Justify your answer. • The mass of soap required should reduce or the lathers will have greater volume/last longer – this can be verified by carrying out the test.

47. Copy and Complete In this unit I learned that water is a polar molecule. This means that is has a positive end and a negative end. Because water molecules have positive and negativeends they attract each other which makes it difficult to get water to change its state of matter. A lot of energy is needed to do this so this makes water a good coolant. Water is also called the universal solvent because it can dissolve many solvents. Surface tension is a layer on the surface of a liquid caused by the force of attractionbetween the moleculesin that liquid. Osmosis is the movement of water from lowsolute concentration to high solute concentration. Pure water is made up of only watermolecules. Water that forms a lather easily with soap is softwater. Water that does not form a lather easily with soap is hard water.

48. 1. List and explain three properties of water. 1. Specific heat capacity – because it can absorb a lot of heat energy before changing state, water is a good coolant. 2. Polarity – the polarity of water means that it is a very good solvent. 3. Density – water expands as it cools, which means that its density decreases, unlike other substances which contract and increase their density.

49. 2. A science student investigated the solubility of two common substances – sugar and salt – in water for a range of temperatures. The data for sugar are given in Table 08.03.01. Salt maintained a constant solubility of 40 g/100 g of water for the temperature range investigated, 0˚C to 100˚C.

50. 2. (i) Draw a graph of the effect of temperature on the solubility of sugar.