Density- Types of Density, Definitions, and Formulas

Have you ever wondered why a ship floats on water but a nail sinks? Seems a bit weird, does it not? The heavier object (in this case ship) floats but the lighter object (in this case nail) sinks.

Lets us try a similar routine for chemical compounds. What will happen if we try to add oil to the water?

Most of you know already know the answer that such a mixture will not mix and remain as separate layers. What causes such behaviour?

One of the parameters which control miscibility is density. Let us take a closer look at it now.

Figure of Oil in Water

Table of Contents

Types of Density
Absolute Density
Relative Density
Vapour Density
Practice Problems on Density
FAQs on Density

Types of Density

Generally, we study two types of density

Absolute Density
Relative Density

Absolute Density

Absolute density (d) is defined as the mass of any material per unit of volume. Generally, expressed in g cm^-3.

For gas, Absolute density (d) is defined as the molar mass of any gas per unit of molar volume of gas.

Units of Density

Density expressed in g cm^-3 or g mL^-1

S.I unit of density is Kg m^-3

Relative Density

Relative density is defined as the density of a substance with respect to any other substance.

Specific gravity: Specific gravity is defined as the density of any substance divided by the density of water at 4^oC (generally used for solids and liquids).

(Specific gravity is unitless quantity)

Density of water at 4^oC is 1 g cm^-3

For gas:

Vapour Density

Vapour density is defined as the density of the gas with respect to hydrogen gas at same temperature and pressure.

Vapour density(at the same Temperature & Pressure)

Vapour density (at same Temperature & Pressure)

We know, according to Avogadro’s hypothesis, All gases having equal volume have the same number of gaseous molecules (not atoms) at same temperature and pressure.

[So, at the same Temperature and Pressure, the molar volume of gas = molar volume of H₂ gas ]

Vapour density(at same Temperature & Pressure)

Vapour density

Practice Problems on Density

Question1. The specific gravity of a solution is 2.1, then calculate the mass of 180 ML of the same solution.

A. 210 g
B. 389 g
C. 378 g
D. 350 g

Answer: (C)

Solution: Specific gravity is unitless quantity,

Question2. What is the relative density of NO₂ gas with respect to CO₂ gas at the same temperature and pressure?

A. 1.50
B. 1.045
C. 2.045
D. 1.90

Answer: (B)

according to Avogadro’s hypothesis, All gases having equal volume have the same number of gaseous molecules (not atoms) at same temperature and pressure.

[So, at the same T and P molar volume of CO₂ = molar volume of NO₂ gas ]

Question3. Find vapour density of nitrogen gas is

A. 7
B. 14
C. 28
D. 32

Answer: (B)

Solution: Nitrogen gas in free state present in the form of N₂.

Question4. The vapour density of nitrogen gas is

A. 7
B. 14
C. 28
D. 32

Answer: (B)

Solution: Nitrogen gas in free state present in the form of N₂.

FAQs on Density

Question1. Why density of water is higher than ice?

Answer: As we start heating ice, hydrogen bonds break and water molecules start occupying the empty space present in ice (ice has a cage-like structure).

Question2. Does the density of any material depend on the quantity of matter?

Answer: No, Density does not depend on the quantity of matter. On reduction of the quantity of material, its mass and volume will change but there will not.

Question3. Density is additive in nature or not like mass and volume.

Answer: No, the Density of two substances can’t be added algebraically like mass and volume, because density is an intensive property.

Question4. Density is temperature-dependent or not?

Answer: Yes, density depends on temperature. At higher temperatures, the kinetic energy of constituent particles increases and density is reduced.