Let's put a mass of lard in the pot first, and then add it when it melts. This solidified lard will surely sink into the melted lard.
Below, it will never float on the water.
Melting tin in a pot is the same. Tin blocks always sink under molten tin. Almost all substances are the same. When they are solid, they are dense.
The temperature is higher than that in liquid state.
Ice is less dense than water, so it can float on the water. In nature, this situation is rare.
Why is the density of ice less than that of water? There is only one reason, that is, the volume of water increases when it freezes, which violates the principle of "expansion with heat and contraction with cold"
Contraction "routine.
It is common sense that water expands in volume when it freezes. Before winter comes, we put the water tank in the open air together.
Buried underground, covered with a straw cover, in order to prevent water from freezing, the water tank will burst.
Someone has done such an experiment: in winter, he filled the steel shells of two bombs with water, stuffed them with cork and left them in the open air for one night.
. The next morning, I went to see the cork of a bomb shell washed six or seven meters away by ice, and an icicle grew in the hole; Another bomb shell cracked.
There was a crack, and the ice squeezed out.
In winter, the tanks placed in the open air should be turned upside down to avoid freezing and cracking when water accumulates. Open water pipes should be wrapped in straw.
Forget it, it's still a trivial matter that the water is frozen and can't be released. If it is frozen, the loss will be great. Straw must also be used for flowers, fruit trees and seedlings that are not hardy.
Bandage. If the water contained in tree cells freezes, it will rupture the cell membrane and destroy the plant tissue.
In winter, the cement coated on the outer layer of brick wall often falls off in blocks; Cracks and even arching often appear on the pavement of cement paving site.
Come; It's all ice As long as there is a little crack in cement, rain will seep in; If it freezes and expands, put the cement
It's ruined.
The same is true of rock cracking. Water accumulates in cracks in rocks, and cracks are propped up little by little when the frozen volume expands in winter;
The next year, there was more water in the cracks, and when the ice formed, the cracks became bigger. In this way, year after year, big stones fell from the mountain.
Here comes the mountain.
If water does not expand when it freezes, wouldn't it save us a lot of trouble and loss? At first, I thought about it, as if it were like this.
That makes sense, but on second thought, if water shrinks when it freezes, isn't the density of ice greater than that of water? Don't you have to sink underwater?
Did you go? There is no ice on the water surface, which does not hinder navigation, but once the water freezes, it will sink to the bottom. Isn't it necessary to freeze the upper water when it's cold?
? The ice at the bottom of the water is accumulating more and more, and finally all rivers, lakes and seas will be frozen at the bottom of the water and will never melt. The heat of the sun is as high as
Of course, animals and plants in the ocean can't survive without reaching the bottom of the sea. As we know, plants and animals on land originally came from the seaside.
Climb ashore. The sea is frozen into ice, and whether there will be life on land has become a problem. Such a comparison, you will definitely make the right one.
Here comes the verdict.
Ice is not only less dense than water, but also difficult to transfer heat. The ice on the water surface can protect the lower water and make the lower water freeze slowly.
Order. Even small ponds and rivers rarely freeze at the bottom in winter. Even under the thick ice of the Arctic Ocean, the sea water is still shining.
Often flowing, the fish are still swimming freely.
Water expands when it freezes. If it is not allowed to expand, even if the temperature drops below 0℃, water will not freeze. in other words
It is said that when water is under pressure, its freezing point will drop. Someone put water in a strong steel cylinder and did an experiment to prove the idea.
We don't have a steel cylinder, which also proves that water will lower the freezing point under pressure. In winter, we found a big piece of ice and divided it in half.
Between two tables. Bend a wire into a circle and wrap it around the ice. There is a heavy scale hanging under the wire ring. We'll see the wires
Will slowly buckle the ice. However, the ice on the K surface of the wire is still very regular, and there is no trace of being buckled by the wire. What's going on here?
It turned out that there was a weighing hammer hanging under the wire ring, which exerted great pressure on the ice, and part of the pressed ice turned into water. The wire is broken again.
Buckle it, and the water on it loses pressure and immediately becomes ice again.
Skating makes use of this principle. There is a pair of skates on the ice sole, and the weight of our whole body is on these thin skates. Ice knife plus
The pressure on the ice is so great that the ice becomes water. There is a thin layer of water between the skates and the ice, just like adding lubricant, so it can be smooth.
Slide forward. Glass is more slippery than ice, but no one can skate on it because it won't melt under pressure.
Change.
The density of water is the highest at 4℃, but the density of ice is lower than that of water (attached figure).
Because water molecules have strong polarity, they can form associated molecules through hydrogen bonding, as shown in the following structural formula (dotted lines indicate hydrogen bonds).
Liquid water contains not only simple water molecules (H2O), but also associated molecules (H2O)2 and (H2O)3. When water is not frozen at 0℃, most water molecules exist as (H2O)3 associated molecules. When the temperature rises to 3.98 ℃( 10 1 kPa), most of the water molecules are (H2O). If the temperature continues to rise above 3.98℃, the law of general substances expanding with heat and contracting with cold will prevail. When the water temperature drops to 0℃, water turns into ice. When water freezes, almost all molecules associate to form a huge associated molecule. The arrangement of water molecules in ice is that each oxygen atom has four hydrogen atoms as its nearest neighbors (two valence bonds and two hydrogen bonds), as shown in the figure below. This arrangement leads to an open structure, that is, there are large gaps in the structure of ice, so the density of ice is inversely proportional to the density of water at the same temperature.
Water molecules are connected by hydrogen bonds.
In addition, it takes a certain amount of energy to decompose the associated molecules, which is enough to explain why the specific heat capacity of water is large.