What is bigger 3.5 L or3 kg

That's like asking whether inches or ounces are bigger.

'Liter' is a unit of volume (space), and 'kilogram' is a unit of mass.

They have different physical dimensions, they're used to measure

different kinds of quantities, and neither one can be converted into

the other one.

The amount of mass contained in a liter of space depends on

what substance is there. If your liter is full of water, then there's

1 kilogram of mass in it, but if the liter is full of gold, then there's

about 19 kilograms of mass in it. And if your liter is empty, then

there's no mass in it at all ... zero kilograms.