Question 5
A piece of sodium dropped into water floats on the surface. Why?
The right answer is the one marked above.
Because sodium's density is less than water's — 0.927 g cm−3.
It certainly does react with the water: sodium reacts rapidly with cold water to form sodium hydroxide and hydrogen gas. But the floating is about density alone. Magnesium, at 1740 kg m−3, is denser than water and sinks.