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When an object is heated, the kinetic energy of its molecules increases, causing them to move faster and maintain a greater average distance from each other. This increased molecular spacing results in the object expanding. Thermal expansion occurs in all three phases of matter: solids, liquids, and gases.

Linear expansion is the change in length when a substance is heated. Areal expansion is the change in area of a body when heated. Volume expansion is the change in volume when heat is applied. Solids undergo all three types, while liquids and gases exhibit only volume expansion.

Railroad tracks and bridges have expansion joints that allow them to freely expand and contract with temperature changes. These joints prevent structural damage and stress caused by thermal expansion. Without them, materials would buckle or crack as temperatures fluctuate throughout the day and seasons.

Water is the most important exception to thermal expansion. While water expands when heated above 4°C, it also expands when cooled between 4°C and 0°C. Water reaches its highest density at 4°C, making it less dense both above and below this temperature, which is unusual for most substances.

When surface water cools to 4°C, it becomes denser and sinks, leaving warmer water near the top. If the surface temperature drops below 4°C, that water becomes less dense and stays near the top, forming ice. This ice layer insulates the liquid water below, allowing fish and aquatic life to survive in the 4°C water beneath.

The coefficient of linear expansion is the proportionality constant that relates the change in length of a substance to its initial length and temperature change. Different materials have different coefficients, determining how much they expand per unit temperature increase. This constant is essential for calculating thermal expansion in solids.

The expansion of alcohol in a thermometer is a common example of thermal expansion. As temperature increases, the alcohol expands and rises up the thermometer tube, indicating the temperature. This visible expansion of liquids with heat change makes thermometers practical instruments for measuring temperature.