General Chemistry Unit 2 Physical and chemical changes Name: __________________________________ Class: _____________ Change 1. Copper (II) sulphate and water (Demo) Watch while your teacher dissolves some copper (II) sulphate in water. 2. Sucrose and concentrated sulphuric acid (Demo) Observations Word equation for change Chemical or physical change? Why? Before Word equation: After Chemical or physical? Why? Before Word equation: After Chemical or physical? Why? Before Mass: ____________ Word equation: Watch while your teacher reacts sucrose (sugar) with concentrated sulphuric acid. 3. Magnesium and oxygen Weigh a ~5cm magnesium ribbon. Place your Bunsen on a heat mat. Light your Bunsen (air holes closed). Open the air holes, if necessary, to get a short flame that’s blue at the bottom and yellow at the top. Hold the magnesium ribbon at one end with the crucible tongs and place the other end in the purple part of the flame. Don’t look directly at the flame! Once all the magnesium has turned to ash, try to collect all the ashes and weigh them. Chemical or physical? Why? After Mass: ____________ Change 4. King Kong’s hand (Demo) Watch while your teacher solves a problem that has to do with their Halloween costume. 5. Copper (II) sulphate and iron Obtain the following materials: one pair of safety glasses per person, test tube rack, test tube, iron nail. Pour some copper(II)sulphate solution into a test tube. About half a test-tube should be enough. Carefully place an iron nail into the test tube and wait for the reaction to happen. It’s slow, so clean up a bit and do some post lab questions and then come back to have a look at it. Come back after a couple of days and have a look at it again. Observations Word equation for change. Chemical or physical change? Why? Before Word equation: After Chemical or physical? Why? Before Word equation: Chemical or physical? Why? After Questions (answer on a separate sheet of paper) 1. Which was your favourite change? Why? [2] 2. Explain what happened in change #2 [2] 3. The mass of magnesium oxide at the end of change #3 should have been greater than the mass of magnesium at the beginning. Why? [2] 4. Chances are some magnesium was lost in change #3. Suggest one way to improve this procedure so that little/no magnesium oxide is lost. This improvement must be specific and achievable in a school laboratory. [3] 5. State and try to explain what the brown precipitate was in change #5. (Hint: it’s not rust)[2] 6. A lit Bunsen burner is also an example of a chemical change. Explain why. [1] TEACHER INSTRUCTIONS 1. Copper sulphate + water This is simply a demonstration of how to make a standard solution. There is no need to mass the copper sulphate, just show them the steps and emphasise how to read the meniscus, as well as introducing volumetric flasks. 2. Dehydrating sugar with sulfuric acid - Best do this in an extraction hood. Safety goggles must be worn. Beaker gets very hot. Leave to cool or ask for a tray to move it around. Wear gloves The resulting black carbon “tower” should be neutralised prior to disposal. 3. Burning magnesium - Wear safety goggles Place the Bunsen on a heat mat Mass a piece of magnesium ribbon (~5cm) and get students to write down the mass Use crucible tongs to hold end of magnesium in the hottest part of the Bunsen flame (the violet/blue bit in the middle of the flame) Tell students squint their eyes (nearly shut) and not look at the magnesium directly as soon as it lights up. The resulting white powder is magnesium oxide Recover as much magnesium oxide as you can and mass it – it should have a greater mass than the Mg by itself because now it also contains oxygen atoms. Use this to introduce the idea of combination reactions where two elements react together to form a compound. Discard MgO in rubbish bin when cool 4. King Kong's Hand This is FUN. Good for reinforcing states of matter too. - Wear safety goggles Have students draw “hairs” onto the glove. Pour vinegar into two of the fingers in a glove, and sodium hydrogencarbonate in another. Pull together the top of the glove and mix the reactants together. Best do this over a sink, as the carbon dioxide produced inflates the glove and it might burst. Disposal: The contents may be safely poured down the drain. 5. Copper (II) sulphate and iron This is a displacement reaction – the sulphate “changes hands” and copper is deposited onto the nail. This happens because iron is more reactive than copper so it tends to be in a compound. copper sulphate + iron iron sulphate + copper Blue solution grey metal clear and colourless solution red metal Use the copper sulphate you made in 1. Make a point of explaining the red solid that appears is not rust. Rusting is a much much slower process than this. You will be able to see results nearly straight away and they will become really obvious within 30 mins or so. This is a good experiment to reinforce safety rules and expectations without much of a hazard so students can do it themselves. Ask them to pour the left over solution into a big waste beaker and to give the nail back to you. Place the nails on a paper towel. The reaction might stain the test tube.
