C5.2 What reactions happen in the hydrosphere?
HYDROSPHERE – all the water on the Earth including oceans, seas, lakes and rivers) – the water contains dissolved compounds called salts.
Ions in crystals of a solid ionic compound are arranged in a regular way forming a lattice
When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.
For example:
Sodium + Chlorine → Sodium Chloride
Magnesium + Oxygen → Magnesium Oxide
In each of these reactions, the metal atoms give electrons to the non-metal atoms, so that the metal atoms become positive ions and the non-metal atoms become negative ions.
There is a strong electrostatic force of attraction between these oppositely charged ions, called an ionic bond.
Seawater in the hydrosphere is ‘salty’ because it contains dissolved compounds – examples of dissolved ionic compounds are:
- Sodium chloride, NaCl
- Magnesium chloride, MgCl2
- Magnesium sulfate, MgSO4
- Sodium sulfate, Na2SO4
- Potassium chloride, KCl
- Potassium bromide, KBr
Ions in ionic compounds can be detected and identified because they have distinct properties. For example, an insoluble compound may precipitate on mixing two solutions of ionic compounds – this technique is used to identify metal ions.
If a solution of any soluble transition metal compound is mixed with sodium hydroxide solution then there is displacement reaction – the sodium is the more reactive metal, and displaces the transition metal from its compound. The transition metal hydroxide is produced as a result. As this is insoluble in water it appears as a solid in the liquid – solid produced in a liquid in this way is called a precipitate.
The colour of the precipitate indicates which transition metal ion was in the initial solution
In order to identify a negative ion, a range of different tests can be carried out – these involve adding a reagent to the unknown sample, which reacts with the ions to form an insoluble solid.