Thermal expansion occurs to some degree in all substances. The density of liquids typically decreases as temperature increases, with most liquids having the highest density near their freezing points and the lowest density near their boiling points. In other words, substances tend to expand when heated and contract when cooled. This is because the particles in a liquid move more rapidly as they absorb thermal energy and when they move faster they tend to move...
Thermal expansion occurs to some degree in all substances. The density of liquids typically decreases as temperature increases, with most liquids having the highest density near their freezing points and the lowest density near their boiling points. In other words, substances tend to expand when heated and contract when cooled. This is because the particles in a liquid move more rapidly as they absorb thermal energy and when they move faster they tend to move farther apart. Collisions between particles are more energetic and cause more separation.
Water, however, is an exception to this trend. Liquid water decreases in density or contracts as the temperature is lowered until it reaches a temperature of about 4 degrees C. At this temperature it begins to expand again. This is called the density anomaly, and it’s due to hydrogen bonding in water. The attractions between oppositely charged ends of different molecules result in the molecules arranging in a very open formation when they get close together. We’re aware that ice floats in water while most solids are less dense than their liquid phase. Water continues to expand until it solidifies into a structure based on hexagonal crystals that contain a lot of empty space. The image below shows this open structure.
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