above the temperature of 4 degrees Celsius. Is this just some fact we're supposed to accept? I'm just curious.
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Accept??? Sure. As long as you accept the basic concepts of density and molecules and the kinetic molecular theory. As the temperature increases, the molecules have more kinetic energy, and as they move around more they occupy greater volumes, which decrease the density. So, the density of water decreases as temperature increases. The converse is also true. As the molecules slow down, the density increases until it reaches a maximum near 4 degrees C. Below that, the density begins to decrease as hydrogen bonding becomes more prevalent.
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water has its MAXIMUM density at 4 degrees. when you cool water that is above 4 degrees, it gets less dense because at 4 degrees, hydrogen bonds begin to form which actually locks the molecules into a more regular, more open 3-dimensional structure which has a greater volume per unit mass (hence lower density) than water above four degrees. below four degrees ice begins to form and once the crystals are fully formed, further cooling results in lower density and resumes the normal trend of cooling resulting in higher density