Why does lava float first?

Answer

Evi, your question is not quite clear to me. What do you mean float? on water?
Lava is molten rock. It is very hot (over 1000°C). But it doesn’t float on water.
Once it comes into contact with water, it cools very quickly and becomes as hard as stone.
What does float on water is pumice. That is also the result of a volcanic eruption, but that is volcanic material that is blown into the air by the volcano. When that material hardens, it’s full of large gaps (because of the gas). This makes pumice extremely light and floats on water.