Why is the density of water highest at 4 degrees Celsius, and when the temperature is lower than 4 d

There are hydrogen bonds between the water molecules, which makes the water molecules tightly aligned. At 4 degrees Celsius, water molecules are most closely arranged, so the density is the largest.

The density of water is the largest at 1 103kg m3, when the temperature is higher, the density of water decreases with the increase of temperature, and at 0, the water does not obey the law of thermal expansion and contraction, and the density increases with the increase of temperature.

Note: Most substances have a higher density in the solid state than in the liquid state; As a result, a solid piece of pure "matter" sinks into a pure "matter" in its liquid state. But a piece of ordinary ice floats on the water, because solid water is "lower" in density than liquid water

This is a very important property of water. At room temperature, liquid water increases in density as the temperature decreases, just like a normal substance. But at 4°C, close to freezing, the water reaches its maximum density, and as the temperature of the water continues to drop towards freezing, the liquid water expands in its standard state, and thus becomes "low" in density

The physical cause of this phenomenon is related to the crystal structure of ordinary ice, which is also known as hexagonal ice ihWater, gallium, bismuth, antimony, and silicon all expand as they solidify; Most of the other materials shrink. However, it is important to note that not all species of ice are less dense than liquid water.

For example, both high-density amorphous ice and ultra-high-density amorphous ice have a higher density than liquid pure water. Therefore, the reason why ordinary ice is denser than water is not easily intuitive, and it has a lot to do with the inherent unusual properties of hydrogen bonds.

In general, the expansion of water when it solidifies is due to its tandem molecular structure arranged with the unusual elasticity of hydrogen bonds, and the hexagonal crystal form (i.e., the form adopted in the standard state) with particularly low energyThat is, when the water cools, it tries to pile up in a lattice form, and the lattice will elongate the rotational and vibrational components of the bonds, so that a water molecule will be pushed by a few neighboring molecules, which actually reduces the density of water when the water is frozen in the standard state

The statement that water is densest at 4 degrees Celsius is true.

Analysis: The density of general objects decreases when the temperature increases, and increases when the temperature decreases, because the general objects obey the law of thermal expansion and cold contraction. That is, when the temperature increases, the volume of the object increases (the mass does not change), so the density decreases, and when the temperature drops, the volume of the object decreases (the mass does not change), so the density increases.

Density: Density is a measure of the mass within a specific volume, density is equal to the mass of an object divided by volume, which can be expressed by symbols, in the International System of Units and Chinese legal units of measurement, the unit of density is kilograms per cubic meter, and the symbol is kg m3.

Generally speaking, no matter what substance it is in, no matter what state it is in, with the change of temperature and pressure, the volume or density will also change accordingly. The relationship between the three physical quantities of temperature t, pressure f, and density is called the equation of state.

Your question itself is wrong, the density of water at 4 is the maximum.

It is a fact that the density of water is at its highest at 4:

Tests in glacial lakes have shown that in frozen lakes, the density of the water below the ice surface increases from top to bottom (of course, the heavier is lower and the lighter is higher), and the temperature increases from top to bottom, from 0 in the surface water to 4 in the bottom waterBecause of this property, the fish in the lake are able to hide in the bottom water in the harsh winter without being frozen into ice.

— Why is this happening? ——

The maximum density of water at 4 is determined by the special structure of hydrogen bond association between water molecules. According to recent X-ray studies, it has been proved that ice has a tetrahedral crystal structure. This tetrahedron is formed by hydrogen bonding, which is an open relaxation structure, because five water molecules cannot take up the volume of all tetrahedra, and hydrogen bonds in ice connect these tetrahedra into a whole.

This directional and orderly arrangement formed by hydrogen bonding has a small space utilization rate, accounting for about 34, so the density of ice is small.

A large number of hydrogen bonds are dismantled when the water solution is dissolved, so that the whole is divided into tetrahedral groups and sporadic smaller "water molecule groups" (i.e., some association molecules formed by the association of hydrogen bonds), so liquid water is no longer completely arranged in order like ice, but has a certain degree of disordered arrangement, that is, the distance between water molecules is not fixed as in ice, and H2O molecules can enter another microcrystalline from one tetrahedral crystallite. In this way, the voids between the molecules are reduced, and the density increases.

When the temperature increases, the tetrahedral group of water molecules is continuously destroyed, and the disordered arrangement of the molecules increases, so that the density increases. But at the same time, the thermal motion between molecules also increases the distance between molecules, which reduces the density. These two contradictory factors reach equilibrium at 4 o'clock, therefore, the density of water is highest at 4 o'clock.

After 4, the thermal motion of the molecules increases the distance between the molecules, and the density of the water begins to decrease again.