Because the lead is heavier than an equal bulk of water.
When cork is put in water, why does it rise to the surface?
Because the cork is lighter than an equal bulk of water.
Does heat increase the weight of any substance?
Heat does not increase the weight of any substance.
How do we know this?
Because a piece of iron when cold will weigh as much as when heated red-hot.
When a liquid is heated, does it become lighter or heavier?
It becomes lighter, bulk for bulk, when heated.
Why does a liquid become lighter when heated?