Physical and chemical changesIB MYP Chemistry: Subtopic test

1. 1
   Students in Nairobi observe four changes in a kitchen. Butter melts in a warm pan. Sugar dissolves in hot tea. Bread turns brown and crisp in a toaster. A cake mixture rises, turns golden brown and gives off a new smell in the oven.

   (a)

   Which of these changes is a chemical change?

   [1 mark]

   • AButter melting
   • BSugar dissolving in tea
   • CTea cooling down
   • DBread turning brown in the toaster

   (b)

   Why is the sugar dissolving in tea a physical change?

   [1 mark]

   • AThe sugar disappears so it has been destroyed
   • BNo new substance is made, and the sugar can be recovered by evaporating the water
   • CThe tea changes colour
   • DHeat is needed for it to happen

   (c)

   Give two pieces of evidence from the description that baking the cake involves a chemical change.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Cairo adds a 3 cm strip of magnesium ribbon to dilute hydrochloric acid in a test tube. The ribbon fizzes and gradually disappears, and the temperature of the mixture rises from 21 °C to 33 °C.

   (a)

   Which observation is evidence of a chemical change?

   [1 mark]

   • ABubbles of gas form where there were none before
   • BThe test tube is made of glass
   • CThe acid is a liquid
   • DThe ribbon is shiny at the start

   (b)

   A student says that the temperature rise on its own proves a chemical change has happened. Which statement is correct?

   [1 mark]

   • AShe is correct because all temperature changes are chemical changes
   • BShe is correct because physical changes never change the temperature
   • CShe is not correct because some physical changes, such as dissolving certain solids, also change the temperature
   • DShe is not correct because chemical changes never change the temperature

   (c)

   Explain why the changes in the test tube are chemical rather than physical.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Students in Manila heat 2.00 g of blue crystals of hydrated copper(II) sulfate in a test tube. The crystals turn white, droplets of liquid form on the cooler upper part of the tube, and the mass of solid left is 1.28 g. After the tube has cooled, the students add a few drops of water to the white powder. It turns blue again and the tube feels warm.

   (a)

   Describe three pieces of evidence from the investigation that show chemical changes took place.

   [3 marks]

   (b)

   Calculate the percentage decrease in mass when the crystals were heated, explain why the students describe the change as reversible, and state one safety precaution.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A family in Lagos uses candles during frequent power cuts. Most candles are made from paraffin wax, which comes from crude oil. When a candle is lit, the wax near the wick melts, the liquid wax rises up the wick and turns into a vapour, and the vapour burns with a yellow flame, giving out heat and light and making carbon dioxide and water vapour. The candle gets shorter. Wax that drips down the side cools and turns solid again.

   (a)

   Explain, using evidence, which changes in the candle are physical and which are chemical.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of the family using candles during power cuts, and reach a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions