Question

If the relative density of a substance is 0.8, is it heavier or lighter than water?
A. lighter
B. heavier
C. of equal weight
D. none of these

Answer 1

Hint: Relative density of a substance is given as the ratio of the density of the substance to the density of water.
We can write it in the equation form as
$\rho {\text{ = }}\dfrac{{{\rho _s}}}{{{\rho _w}}}\,$ ……………..(1)
Where ${\rho _s}$ is the density of substance and ${\rho _w}$ is the density of water.
Using this, find the density of the substance. If the density is greater than the density of the water it means the substance is heavier and if the density is less than the density of water the substance is lighter.

Complete step by step answer:
The relative density of a substance is given as the ratio of the density of a substance to the density of water.
$\Rightarrow \rho {\text{ = }}\dfrac{{{\rho _s}}}{{{\rho _w}}}\,$
$\Rightarrow {\rho _w} = 1000\,kg/{m^3}$
Now substitute this value in equation 1, we get,
$\rho {\text{ = }}\dfrac{{{\rho _s}}}{{{\rho _w}}}$
$\Rightarrow {\rho _s} = \rho \times {\rho _w} $
$\Rightarrow {\rho _s} = 0.8 \times 1000 = 800\,kg/{m^3} $
Therefore, the density of the substance is,
$\Rightarrow {\rho _s} = 800\,kg/{m^3}$

Since this value of density is less than the density of water. we can say that it is lighter than water. So, the correct answer is option (A).

Note:
The relative density of water is 1. So, we can directly compare the relative density of the substance which is 0.8 with value 1. Since 0.8 is less than 1 we can say the substance is lighter than water.