Question

How to find out sp gravity of a material lighter than water?

Answer 1

If it is a liquid, just poor some in a jar so you know the volume. Now weigh it and subtract the weight of the jar. Now you can divide the kilos by the cubic meters if you want to stick to SI.

If it is a solid, try to float it on water. Then use Archimedes observation: the force acting on the solid and pushing it upwards (so it will not sink) is the same as the weight (in Newton) of the displaced volume of water.

For instance, water ice floats on water. Now assume we see an iceberg that has 1/10 is sticking above the water while 9/10 is under water.

Then the total weight (in Newton) of the entire iceberg must equal the weight of the displaced volume of water, which is the same as the weight of the volume of the ice that is under water.

So 10/10 parts of iceberg weigh the same as 9/10 parts of water. We can conclude that the density of the iceberg is 9/10 of that of water.