Relative density of a substance is expressed by comparing the density of that substance with the density of:

1. air
2. mercury
3. water
4. iron

The unit of relative density is:

1. g cm^-3
2. kg m^-3
3. m³ kg^-1
4. no unit

The correct expression for relative density when the weight of a body in air is W₁, and in water is W₂ is given by :

1. R.D. = $\dfrac{\text{W}$2}{\text{W}1 - \text{W}_2}

2. R.D. = $\dfrac{\text{W}$1}{\text{W}1 + \text{W}_2}

3. R.D. = $\dfrac{\text{W}$1 + \text{W}2}{\text{W}_1}

4. R.D. = $\dfrac{\text{W}$1}{\text{W}1 - \text{W}_2}

If the weight of a body in air is W₁, in a liquid is W₂ and in water is W₃, then :

1. R.D. of liquid = $\dfrac{\text{W}$2 - \text{W}3}{\text{W}1 - \text{W}3}

2. R.D. of liquid = $\dfrac{\text{W}$1 - \text{W}2}{\text{W}1 - \text{W}3}

3. R.D. of liquid = $\dfrac{\text{W}$2 - \text{W}3}{\text{W}1 + \text{W}3}

4. R.D. of liquid = $\dfrac{\text{W}$1 - \text{W}2}{\text{W}1 + \text{W}3}