A beaker contains saturated aqueous sodium chloride solution in contact with undissolved solid sodium chloride. Sodium ions and chloride ions are constantly moving from solid to solution and from solution to solid.

a. i. Explain why this is a closed system.

ii. Explain why the concentration of the saturated sodium chloride solution does not change, even though ions are still moving into the solution from the solid.

b. Bromine is a reddish-brown liquid that vaporises at room temperature. Some liquid bromine is put in a closed jar. The colour of the bromine vapour above the liquid gets darker and darker until the depth of colour remains constant. Bromine liquid still remains in the jar. Explain what is happening in terms of changes in concentration of the bromine molecules in the vapour.