Thermal Expansion of Liquids

We all know that when different substances get exposed to increased heat, they react in different ways. When solids, liquids, or gases undergo some temperature change, their dimensions and volumes also tend to change. 

Thermal expansion refers to the expansion of and growth in the dimensions of solids, liquids, or gases when their temperature undergoes an increase. It is a phenomenon that we encounter in our daily lives. So, let us understand thermal expansion in detail.

Thermal Expansion

The ability of matter to change its form, area, volume, and density in reaction to a change in temperature is known as thermal expansion. It does not include the transitions between phases.

The temperature has a monotonic effect on a material’s average molecular kinetic energy. When a solid is heated, the atoms begin to vibrate and move more, increasing the space between them. Materials that contract as the temperature rises are rare and only occur in a few temperature ranges. 

The material’s coefficient of linear thermal expansion, which varies with temperature, is the relative expansion (also known as strain) divided by the change in temperature. Particles travel faster as their energy rises, eliminating intermolecular interactions and expanding the material.

Thermal Expansion of Liquids

Because the intermolecular interactions in liquids are relatively weak, and the component molecules are more mobile, their thermal expansion is generally greater than that of solids. We can theoretically calculate the coefficient of linear expansion from the coefficient of volumetric expansion (αV ≈ 3αL).  

Unlike solids, liquids have no distinct form and take on the shape of the container. As a result, because liquids have no fixed length or area, linear and areal expansions of liquids are meaningless. However, the experimental value of αL is occasionally used to determine αV.

Heating expands liquids in general. On the other hand, water is an exception to this rule: below 4 °C, it contracts when heated, resulting in a negative thermal expansion coefficient. Water exhibits more usual behaviour at higher temperatures, with a positive thermal expansion coefficient.

Solids’ thermal expansion is ordinarily unaffected by temperature, particularly at very low temperatures. In contrast, liquids’ thermal expansion is at various temperatures; they can grow at varying speeds.

Coefficient of Thermal Expansion

The volumetric coefficient of thermal expansion of a gas, liquid, or solid in general is given by:

α = αv =  1V (dVdT)p

The derivative’s subscript “p” denotes that the pressure is constant during the expansion. Subscript V emphasizes that this generic formulation uses the volumetric (rather than linear) expansion. 

The fact that the pressure is kept constant is critical in the case of gas because the volume of a gas varies significantly with pressure and temperature. We may observe this aspect in the ideal gas at low density.

Linear Expansion

When we expose a substance to heat, it reacts and increases in length. Linear expansion refers to the change in length with respect to its original length due to increased temperature change. The formula is expressed as:

αLL1 = dL / dT

Where:

• αL is the coefficient of linear expansion
• L1 is the initial length of the material
• dL is the unit change in length
• dT is the unit change in temperature

Area Expansion

The thermal area expansion coefficient links a change in temperature to a change in the area dimensions of a substance. The fractional change in area per degree of temperature change is what this term refers to without regard for the pressure. The formula is: 

ΔA = 2αAAΔT

Where:

• A is the area,
• ΔA is the change in the area
• αA is the area expansion coefficient
• ΔT is the temperature difference

Volume Expansion

We may express the volumetric (or cubical) thermal volume expansion coefficient as follows, ignoring the effects of pressure on a solid. We can state the formula as: 

ΔV = βV1 ΔT

Where:

• V1 is the initial volume
• ΔT is the temperature change
• ΔV is the increase in volume
• β is the coefficient of volumetric expansion

Conclusion

When we expose different substances to heat, they react in different ways. When solids, liquids, and gases undergo an increased temperature change, their dimensions and volumes also tend to change. 

Thermal expansion refers to the expansion and growth of the dimensions of solids, liquids, or gases when their temperature increases. Linear expansion is the fractional change in length with respect to its original length. The formula for linear expansion is: 

ΔA = 2αAAΔT

Volume/ volumetric/ cubical expansion occurs when the increased temperature acting on gas, solid, or liquid increases the volume of the material.