1. 1
   A student places a coil of magnesium ribbon in a crucible with a lid, weighs the sealed crucible and its contents, then heats it strongly so the magnesium reacts with the oxygen already sealed inside, keeping the lid closed throughout, before reweighing the sealed crucible.

   (a)

   What would the student measure for the total mass of the sealed crucible and its contents after heating, compared with before?

   [1 mark]

   • AThe total mass stays the same
   • BThe total mass increases
   • CThe total mass decreases
   • DThe total mass becomes zero

   (b)

   Which state symbol should be used for the solid magnesium oxide product formed in the crucible?

   [1 mark]

   • A(l)
   • B(s)
   • C(g)
   • D(aq)

   (c)

   Write the balanced symbol equation, including state symbols, for the reaction between magnesium and oxygen occurring in the sealed crucible.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A different student places a sample of calcium carbonate in an open conical flask standing on a balance, then adds dilute hydrochloric acid. The reading on the balance decreases steadily as bubbles of gas are produced and the reaction proceeds.

   (a)

   A different student places calcium carbonate in an open conical flask on a balance and adds dilute hydrochloric acid. The reading on the balance decreases steadily as the reaction proceeds. Why does the measured mass decrease, even though no atoms are actually destroyed?

   [1 mark]

   • AThe balance is faulty
   • BSome of the calcium carbonate evaporates as a gas
   • CAtoms are destroyed during the reaction
   • DCarbon dioxide gas is produced and escapes from the open flask into the air

   (b)

   If the student had instead carried out the same reaction in a sealed flask, so that no gas could escape, what would be true of the total mass of reactants compared with the total mass of products?

   [1 mark]

   • AThe reactants would have a greater total mass
   • BThe products would have a greater total mass
   • CThey would be equal
   • DIt cannot be determined without more data

   (c)

   Explain why the mass measured on the balance decreases in the open flask, even though the law of conservation of mass is not being broken.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student mixes sodium carbonate solution with calcium chloride solution in a stoppered (sealed) flask standing on a balance. A white precipitate of calcium carbonate forms, and sodium chloride remains dissolved in the solution. The reading on the balance is checked before mixing and again after the reaction is complete.

   (a)

   Write the balanced symbol equation for this reaction, including state symbols.

   [3 marks]

   (b)

   Explain, with reference to the law of conservation of mass and the atoms present, why the reading on the balance stays the same before and after this reaction takes place in the sealed flask.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher demonstrates two experiments to a class, both carried out with the reaction vessel open to the air, standing on a balance. In the first, a coil of magnesium ribbon is burned and the balance reading increases. In the second, a sample of copper carbonate is heated strongly in an open test tube and the balance reading decreases.

   (a)

   A teacher demonstrates two experiments on an open balance. In the first, a coil of magnesium ribbon is burned in open air, and the balance reading increases. Explain why the measured mass increases in this experiment, without the law of conservation of mass being broken.

   [6 marks]

   (b)

   In the teacher's second experiment, copper carbonate is heated strongly in an open test tube and the balance reading decreases. Explain why the measured mass decreases in this experiment, without the law of conservation of mass being broken.

   [6 marks]

   Total for question 4: 12 marks

End of questions