A student tries to make a pure, dry sample of hydrated cobalt(II) chloride crystals. She uses dilute hydrochloric acid and solid cobalt(II) carbonate. This is the student's method:
Her sample of cobalt(II) carbonate contains a small amount of solid impurity that dissolves in water, but does not react with the acid.
State why it is not necessary to have a precise measurement of the volume of hydrochloric acid in step 1.
State why the acid is warmed in step 2.
Suggest why a glass rod, rather than a metal spatula, is used to stir the mixture in step 3.
State how the student will know when the cobalt(II) carbonate stops reacting in step 4.
State why the method used in step 6 will not produce a pure sample of hydrated cobalt(II) chloride crystals.
Describe how the student could produce a pure, dry sample of crystals from the filtrate in step 5.
The table below shows the formula and colour of three different types of cobalt(II) chloride.
| Formula | Colour |
|---|---|
| CoCl2\text{CoCl}_2CoCl2 | blue |
| CoCl2⋅2H2O\text{CoCl}_2\cdot2\text{H}_2\text{O}CoCl2⋅2H2O | violet |
| CoCl2⋅6H2O\text{CoCl}_2\cdot6\text{H}_2\text{O}CoCl2⋅6H2O | pink |
When water is added very slowly to solid CoCl2\text{CoCl}_2CoCl2, the colour changes from blue to violet and then to pink.
Write a chemical equation for the change from the violet solid to the pink solid.
Which of these words describes the change taking place when the pink solid is heated to form the blue solid?
