Fig. 1 shows a vacuum flask used to compare a dissolving process with a chemical reaction. A student adds 6.0 g of sodium chloride to water, seals the flask...
Question 1 Report
Fig. 1 shows a vacuum flask used to compare a dissolving process with a chemical reaction. A student adds 6.0 g of sodium chloride to water, seals the flask and shakes it until a clear solution forms. No new compound is made.
(a) Name the solute in the flask. [1]
(b) Explain why the flask has the same mass before and after the sodium chloride dissolves. [2]
(c) Use the masses to calculate the mass of water if the total mass of solution is 86.0 g. [1]
(d) Describe what happens to sodium chloride particles when they dissolve. [1]
This reaction is carried out in a copper recycling workshop. Fig. 1 shows copper oxide powder being heated in a tube while hydrogen passes over it. Water forms and is trapped in a cold tube. The technicians weigh all of the connected apparatus, rather than only the reaction tube, to test conservation of mass.
(a) Name the compound made in the cold trap. [1]
(b) Describe the colour change expected for copper oxide during the reaction. [1]
(c) Explain why weighing only the tube containing copper oxide could give an apparent decrease in mass. [2]
(d) Complete the word equation: copper oxide + hydrogen → copper + ________. [1]
Answer Details
Dissolving sodium chloride
- The solute is sodium chloride. [1]
- No particles are created or destroyed when the salt dissolves. All sodium chloride and water remain in the sealed flask, so its mass stays the same. [2]
- \[86.0\text{ g}-6.0\text{ g}=80.0\text{ g}\]
Mass of water = 80.0 g. [1] - Sodium chloride particles separate and spread between water particles. [1]
Reduction of copper oxide
- The compound made in the cold trap is water. [1]
- Copper oxide changes from black to pink-brown, or brown. [1]
- Weighing only the reaction tube can show an apparent mass decrease because water vapour leaves that tube. The oxygen from copper oxide is still present in the water collected elsewhere. [2]
- copper oxide + hydrogen → copper + water [1]
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