O-LevelChemistryRates of Reaction5070/42 · May/June 2023 · Q46 Marks

Baking soda is used to make bread rise. When baking soda is heated, it decomposes and carbon dioxide gas is released. Baking soda also decomposes gradually when it is stored. The longer the baking soda is stored, the less carbon dioxide it releases when it is heated. Plan an investigation to show which of two different samples of baking soda has been stored for longer. Your plan should include the use of common laboratory apparatus and the two samples of baking soda. No other chemicals should be used. Your plan should include: • the apparatus needed • the method to use • the measurements to take • the variables to control • how to use the results to determine which sample has been stored for longer. You may draw a diagram to help you answer the question.

Plan an investigation to show which of two different samples of baking soda has been stored for longer. Your plan should include the use of common laboratory apparatus and the two samples of baking soda. No other chemicals should be used. Your plan should include: • the apparatus needed • the method to use • the measurements to take • the variables to control • how to use the results to determine which sample has been stored for longer. You may draw a diagram to help you answer the question.[6]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[1] (1) Apparatus: Balance
[1] (1) Apparatus: Bunsen or any named suitable heating equipment.
[1] (1) Apparatus: Test-tube / flask suitable to hold sample and suitable for heat
[1] (1) Apparatus: Equipment to measure volume of gas etc. e.g. measuring cylinder with water trough / (gas) syringe
[1] (2) Method and Measurements: Measures a mass / amount of a sample into a container
[1] (2) Method and Measurements: Heats
[1] (2) Method and Measurements: Reweighs or measures volume of gas produced / counts bubbles / measures time to turn limewater milky / (IGNORE one which goes milkier)
[1] (3) Control: Same mass / amount/volume of each sample OR calculate change in mass / volume produced per gram OR same surface area of soda.
[1] (3) Control: Heat for same length of time / heat to the same temperature / heat to constant mass / heat until fixed volume produced / heat until no further change in volume / IGNORE heat until reaction finished.
[1] (4) Use of results: Sample that produces more gas / loses more mass (weight)/has more bubbles has less time to turn milky / is milkier is stored for less time or vice versa.

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