Group 1: Alkali Metals
These are very soft metals (low density) and can be cut with a knife - and when you do this you will see a very shiny surface (in common with all metals), which soon disappears as the metal reacts with air. This is why alkali metals are stored in oil.
They also have low melting points compared to other metals The melting points reduce as you go down the group.
Reaction in Water
They are often everyones favourite metal because of their violent reactions with water which are very exothermic.
When an alkali metal is placed in water it will float and produce a gas (hydrogen) which it may well burn - producing a flame above it. The reactions get more and more vigourous as you move down the group. To the extent where it would be too dangerous to perform the reactions past potassium, in a classroom.
The reactions are as follows:
Alkali metal + water ® Alkali metal hydroxide + hydrogen
So the reaction when we add potassium to water will be:
Potassium + water ® Potassium hydroxide + hydrogen
2K (s) + 2H2O(l) ® 2KOH(aq) + H2 (g)
The KOH that is produced makes the solution alkaline.
The image below shows a sample of lithium as it is usually prepared: in cubes, suspended in oil.