How does the density of a liquid change with temperature?

The density of a liquid generally decreases as its temperature increases. This occurs because the liquid expands when heated.

Explanation of Density and Temperature Relationship

When a liquid is heated, the molecules within it gain kinetic energy. This increased energy causes the molecules to move more vigorously and push further apart. As the average distance between molecules increases, the overall volume of the liquid expands. Since density is defined as mass per unit volume (density = mass/volume), and the mass remains the same while the volume increases, the density of the liquid decreases.

Here's a breakdown of the relationship:

• Heating a liquid:

  • Molecules gain kinetic energy.
  • Molecules move more and further apart.
  • The liquid expands.
  • Volume increases.
  • Density decreases.

• Cooling a liquid:

  • Molecules lose kinetic energy.
  • Molecules move less and closer together.
  • The liquid contracts.
  • Volume decreases.
  • Density increases.

Why Liquids Expand With Temperature

According to the reference provided, "With the rise in temperature of a liquid, its density decreases because liquids tends to expand with rise in temperature as the internal energy of molecules increases." This confirms the mechanism described above. The increase in temperature leads to an increase in the internal energy of molecules, which in turn causes expansion and a decrease in density.

Practical Examples

• Water: Water is a notable exception, as its density increases from 0°C to 4°C before decreasing. This anomaly affects aquatic life. However, above 4°C water density behaves normally.
• Cooking Oil: Cooking oil expands when heated, which means that if you filled a container with it at room temperature, then heated it, you might notice that the oil level has increased slightly and the oil has become less dense.

Summary Table

Change in Temperature	Molecular Behavior	Liquid Volume	Density
Increase	Increased molecular movement	Increases	Decreases
Decrease	Decreased molecular movement	Decreases	Increases