1. 1
   A metalworking company heats a sample of magnesium ribbon in an open crucible over a Bunsen flame as part of a quality demonstration for visiting engineers, to show how the metal behaves when heated strongly in air.

   (a)

   As the magnesium burns, the total mass measured after the reaction is greater than the mass of magnesium ribbon they started with. Which statement correctly explains this observation in terms of the law of conservation of mass?

   [1 mark]

   • AOxygen from the air has combined with the magnesium and is now included in the solid product, adding to the measured mass
   • BMass has been created during the chemical reaction, breaking the law of conservation of mass
   • CThe magnesium ribbon gained extra atoms of magnesium from the crucible itself
   • DThe Bunsen flame added extra carbon atoms to the solid product, increasing its mass

   (b)

   A different reactant, such as a metal carbonate, is heated strongly in an open (non-enclosed) test tube, so that a gas produced by the reaction can escape into the air. What would you expect to happen to the measured mass of the solid remaining?

   [1 mark]

   • AThe mass of the remaining solid becomes exactly zero once any gas escapes
   • BIf a gas produced by the reaction escapes into the air, the mass of the remaining solid always increases
   • CEscaping gas has no effect on the measured mass of the remaining solid in any reaction
   • DIf a gas produced by the reaction escapes into the air, the mass of the remaining solid appears to decrease

   (c)

   Explain why the law of conservation of mass still applies to the magnesium and oxygen reaction in the open crucible, even though the measured mass of the solid product appears to increase compared with the starting magnesium ribbon.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student preparing for a school science fair project on fertiliser production balances several symbol equations related to the industrial synthesis of ammonia, to demonstrate the law of conservation of mass to the judges.

   (a)

   A student preparing for a school science fair balances the symbol equation for the reaction between hydrogen gas and nitrogen gas to form ammonia, starting from the unbalanced equation N2 + H2 -> NH3. Which of the following is the correctly balanced version of this equation?

   [1 mark]

   • A2N2 + 3H2 -> 2NH3
   • BN2 + H2 -> NH3
   • CN2 + 3H2 -> 2NH3
   • DN2 + 3H2 -> NH3

   (b)

   The student then checks the balanced equation N2 + 3H2 -> 2NH3 by counting atoms on each side to demonstrate the law of conservation of mass. Which statement correctly describes what this check should show?

   [1 mark]

   • AThe total number of molecules must be equal on both sides of the equation, regardless of atom counts
   • BThe total number of nitrogen atoms and the total number of hydrogen atoms must each be equal on both sides of the equation
   • COnly the number of nitrogen atoms needs to match; hydrogen atoms may differ between sides
   • DThe mass of nitrogen gas alone must equal the mass of ammonia produced

   (c)

   Using the balanced equation N2 + 3H2 -> 2NH3, explain how counting atoms of nitrogen and hydrogen on each side of the equation confirms that the law of conservation of mass is being obeyed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A construction company adds dilute hydrochloric acid to a sample of limestone (calcium carbonate) to test how quickly it reacts, as part of assessing whether the limestone is suitable for use as a building material in an area with acidic rainfall.

   (a)

   The unbalanced equation for this reaction is: CaCO3 + HCl -> CaCl2 + H2O + CO2. Balance this symbol equation, showing your reasoning.

   [3 marks]

   (b)

   During the reaction between limestone and hydrochloric acid in an open flask on a balance, the construction company's engineer notices that the reading on the balance decreases steadily as the reaction proceeds, even though no substance visibly leaves the flask. Explain, in terms of particles and gases, why the measured mass decreases, and state what would happen to the balance reading if the same reaction were carried out in a fully sealed (enclosed) flask instead.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A historical re-enactment society is recreating an 18th-century-style experiment to investigate whether burning wood changes the total mass of a system, comparing combustion in a sealed glass vessel with combustion in an open fireplace, to help museum visitors understand how the law of conservation of mass was originally discovered.

   (a)

   A historical re-enactment society is recreating an early experiment similar to those used by 18th-century chemists to investigate whether burning wood in a sealed glass vessel changes the total mass of the system, compared with burning the same wood in an open fireplace where combustion products can escape freely. Explain fully, using the law of conservation of mass and ideas about gases in enclosed and non-enclosed systems, what result the society should expect to observe in each case, and why these two experiments might have led early chemists to different, potentially incorrect, conclusions about mass changes during combustion.

   [6 marks]

   (b)

   A society member proposes an additional experiment: heating a sample of copper carbonate in a sealed test tube fitted with a delivery tube leading to a second sealed flask, so that any gas produced can be captured and weighed rather than allowed to escape into the room. Explain fully how this modified enclosed setup would allow the society to demonstrate the law of conservation of mass for the thermal decomposition of copper carbonate, and describe what measurements they would need to take before and after heating to prove that mass is conserved.

   [6 marks]

   Total for question 4: 12 marks

End of questions