Potassium and sodium react violently with:

Sodium (Na) and potassium (K) are alkali metals, found in Group 1 of the periodic table. They are among the most reactive metals. Each has only one electron in its outermost shell, which it loses very easily to form a positive ion.

When sodium or potassium is put into cold water, it reacts vigorously. It forms a metal hydroxide (an alkali) and hydrogen gas, and a large amount of heat is released (the reaction is highly exothermic):

2Na + 2H2O → 2NaOH + H2 + heat

2K + 2H2O → 2KOH + H2 + heat

The heat produced is so great that the hydrogen gas catches fire. Sodium melts into a silvery ball that darts around on the surface of the water, and the hydrogen may burn with a yellow flame. Potassium reacts even more violently, and the hydrogen catches fire immediately with a lilac (violet) flame.

Because they react so readily with water and with oxygen in the air, sodium and potassium are stored under kerosene (paraffin oil) to prevent contact with air and moisture. They are soft enough to be cut with a knife.

The reactivity of metals with water decreases down the reactivity series:

• Potassium, sodium and calcium react with cold water (calcium less violently; it floats because of hydrogen bubbles sticking to it).
• Magnesium reacts very slowly with cold water but quickly with hot water.
• Aluminium, zinc and iron do not react with cold or hot water but react with steam to form metal oxides and hydrogen.
• Lead, copper, silver and gold do not react with water at all.
• Reactivity of alkali metals increases down the group: lithium < sodium < potassium < rubidium < caesium, because the outer electron is farther from the nucleus and more easily lost.

Why not the other options?

• ✗Hot water only describes magnesium.
• ✗Steam only describes metals such as aluminium, zinc and iron.
• ✗Neither describes metals such as copper, silver and gold.