Thermal stability and flame coloursEdexcel International A Level Chemistry: Subtopic test

1. 1
   A technician identifies the metal in unlabelled solid samples by flame tests. She dips a clean nichrome wire into each sample, holds it in the edge of a blue Bunsen flame and records the colour. The samples contain compounds of lithium, potassium, calcium and barium.

   (a)

   What colour is seen in the flame test on a barium compound?

   [1 mark]

   • ABrick-red
   • BApple green
   • CLilac
   • DYellow

   (b)

   Which sample gives a lilac flame?

   [1 mark]

   • APotassium chloride
   • BSodium chloride
   • CLithium chloride
   • DCalcium chloride

   (c)

   Explain, in terms of electrons, how the heated metal ions produce coloured light.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Samples of magnesium carbonate and barium carbonate are heated strongly in turn in a hard-glass test tube under identical conditions, and any gas given off is passed through limewater. Magnesium carbonate decomposes at a much lower temperature than barium carbonate.

   (a)

   What is seen when the gas from the heated carbonate is bubbled into limewater?

   [1 mark]

   • AA glowing splint is relit
   • BDamp blue litmus paper turns red
   • CThe limewater turns milky (cloudy)
   • DThe gas burns with a squeaky pop

   (b)

   Which equation represents the thermal decomposition of magnesium carbonate?

   [1 mark]

   • AMgCO₃ → Mg + CO₂ + ½O₂
   • BMgCO₃ → MgO + CO
   • CMgCO₃ → MgO + CO₂
   • D2MgCO₃ → 2MgO + 2CO + O₂

   (c)

   Explain why barium carbonate decomposes at a higher temperature than magnesium carbonate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats small samples of sodium nitrate and calcium nitrate in separate dry test tubes. Both tubes give off a colourless gas that relights a glowing splint, but only one of them also gives off a brown gas.

   (a)

   Write equations for the thermal decomposition of sodium nitrate and of calcium nitrate. State which tube gives the brown gas, naming it and giving its effect on damp blue litmus paper.

   [3 marks]

   (b)

   Explain why magnesium nitrate decomposes at a lower temperature than sodium nitrate, and why barium nitrate is more stable to heat than magnesium nitrate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Three white solids, X, Y and Z, are known to be sodium nitrate, potassium nitrate and calcium nitrate, in an unknown order. The laboratory has dry hard-glass test tubes, a Bunsen burner, wooden splints, damp blue and red litmus paper, concentrated hydrochloric acid and nichrome wire.

   (a)

   Describe how the chemist could use thermal decomposition and flame tests to identify X, Y and Z. Give the observations and the equations that lead to each identification.

   [6 marks]

   (b)

   Explain, in terms of the size and charge of the cations, why the Group 2 carbonates become more stable to heat down the group, and why sodium carbonate does not decompose at Bunsen temperatures although magnesium carbonate does.

   [6 marks]

   Total for question 4: 12 marks

End of questions