Short Answer

Density of a solution is the mass of the solution per unit volume. It tells us how heavy a solution is compared to its size. The formula used is:
Density = mass ÷ volume.

Density helps compare different solutions and understand how concentrated they are. A solution with more dissolved solute usually has a higher density. Density is expressed in units like grams per millilitre (g/mL) or grams per cubic centimetre (g/cm³).

Detailed Explanation

Density of a Solution

The density of a solution is an important physical property that describes how much mass is present in a specific volume of the solution. It is a measure of how tightly packed the particles (solute + solvent) are inside the solution. Solutions with more solute generally become heavier for the same volume, so their density increases. Density plays a major role in chemistry, physics, engineering, environmental science, and many industries such as food, medicine, and chemicals.

Density gives a clear idea of whether a solution is light or heavy compared to another solution or pure water. For example, sugar water is denser than plain water because dissolved sugar adds more mass without greatly increasing the volume.

Definition of density of a solution

The density of a solution is defined as:

Density = mass of solution ÷ volume of solution

This formula indicates how much mass (in grams) exists in a certain volume (in millilitres or cubic centimetres) of the solution.
Common density units are:

• g/mL
• g/cm³
• kg/L

Although different units are used, the physical meaning remains the same.

Factors affecting the density of a solution

Several factors determine the density of a solution:

1. Amount of solute

When more solute is dissolved in a solvent, the mass increases while the volume does not increase much.
Example: Salt water is denser than pure water because the salt adds weight.

Thus, higher solute concentration = higher density.

2. Nature of solute and solvent

Different solutes add different amounts of mass.
Similarly, solvents have different densities.
Example: Alcohol has lower density than water, so alcohol-water mixtures have densities lower than pure water.

3. Temperature

Temperature has a major effect on density because volume changes with temperature.

• When temperature increases → particles move apart → volume increases → density decreases
• When temperature decreases → particles move closer → volume decreases → density increases

This is why warm water is less dense than cold water.

4. Pressure (mainly for gases)

For liquids and solids, pressure does not significantly affect density.
However, gas densities change greatly with pressure because gases are compressible.

How density of a solution is measured

Density can be measured using:

• A hydrometer (for liquids like milk or sugar syrup)
• A pycnometer (a precise measuring bottle)
• A densitometer (electronic instrument)

In labs, density is often determined by measuring mass using a weighing balance and volume using a measuring cylinder or pipette.

Importance of solution density

Density plays a key role in many scientific and daily activities.

1. Helps identify substances

Each substance has a unique density.
For example, pure water has a density of 1 g/mL at 25°C.
If measured density is different, the sample may be impure.

2. Used in concentration calculations

Density allows chemists to convert between:

• mass and volume
• molarity and mass percent
• volume percent and molality

Accurate solutions require density data.

3. Plays a role in separation techniques

In processes like centrifugation, flotation, and decantation, density differences help separate mixtures.

Example: Oil floats on water because it has lower density.

4. Important in industries

Density is used to control quality in:

• Petroleum industry
• Food products (milk, juices, syrups)
• Pharmaceuticals
• Chemical manufacturing

Even slight changes in density may indicate spoilage or impurity.

5. Helps understand natural phenomena

Many natural processes depend on density differences:

• Warm air rising and cool air sinking
• Ocean water movement
• Layering of liquids like oil and water

Density helps explain why some substances float while others sink.

Examples of density in real life

Example 1: Sugar solution

Adding sugar to water increases its density because sugar particles occupy space between water molecules, adding mass.

Example 2: Saline solution

Medical saline (salt solution) has higher density than pure water because of dissolved salt.

Example 3: Petrol and water

Petrol floats on water because its density is lower than that of water.

Relationship between density and concentration

A more concentrated solution usually contains more solute particles. These extra particles increase mass, making the solution denser.
Thus:

• Low concentration → low density
• High concentration → high density

This relationship helps chemists determine concentration from density tables.

Conclusion

Density of a solution is the mass per unit volume of the entire solution. It depends on solute amount, type of solute and solvent, temperature, and pressure (for gases). Density helps identify substances, prepare solutions, control industrial processes, and understand natural phenomena. Since density changes with concentration, it is a useful tool in laboratory calculations and everyday chemical applications.