In the winter, temperatures in a lake get close enough to freezing that the colder water is less dense and floats to the top. 

Ice is less dense than water because of the way it forms a hexagonal crystalline structure. Each water molecule consists of two hydrogen atoms bonded to the bottom of an oxygen atom. When ice forms, the hydrogen atoms of one water molecule form weak hydrogen bonds with the top of the oxygen atoms of two other water molecules. Lining up the water molecules in this pattern takes up more space than having them jumbled randomly together (as is the case in liquid water). And because the same mass of molecules takes up more space when frozen, ice is less dense than liquid water. For this same reason, water below 4° Celsius becomes increasingly less dense as it gets colder. Close to freezing temperatures, the molecules in the liquid water begin to line up into the space-filling hexagonal structure.

Water is the name of the liquid phase of H₂O molecules, and ice is the name of the solid phase.

The second thing we need to think about is density, which tells us how tightly matter particles are packed together. Tightly packed matter is more dense than loosely packed matter. Also, less dense matter floats on top of more dense matter. So when you place ice cubes in your soft drink(which is mostly water) and your ice cubes float, you know that ice is less dense than water. This is a very special property of water. Almost every other type of matter gets more dense when it changes phase from a solid to a liquid, but water gets less dense. Why? Because ice is a crystal, which means it has a regular pattern with spaces in between molecules. The spaces in the crystal are larger than the spaces between molecules in the liquid. More space = less dense, so ice is less dense than water.

So now we understand why ice floats on top of water, but how does this work on a lake? Imagine that it is the beginning of winter, and the temperature has just gotten below freezing. Air changes temperature faster than water -- this is why a swimming pool seems much warmer in the evening. The air cools down at night, but the pool water remains almost as hot as it was during the day. So although the air is freezing cold, the water isn't freezing yet. The water at the very top of the lake is in direct contact with the cold air, so it freezes first. And because ice is less dense than water, the sheet of ice doesn't sink. If the temperature stays very cold, the sheet of ice gets thicker, but the ice also acts as a barrier, or insulator, between the cold air and the warmer water underneath. The sheet of ice helps the rest of the water from becoming ice! 

The amazing fact that water freezes on the surface of a lake but not below it allows plants and fish to live through the winter in northern lakes and ponds. Imagine a lake without fish! So it is a good thing that H₂0 has the very strange property that its solid phase is less dense than its liquid phase.

Firstly, the surface water is going to cool; As the water cools the density increases and the surface water drowns to the bottom and the bottom water rises up. This will continue till the whole water comes at 4 degree Celsius.

Now, further cooling will occur but as the surface water is going to get affected first, it will cool first. But now the density won’t increase, instead the density is going to decrease, which means that the surface water, instead of drowning (like before), would keep on floating on the top, disallowing the bottom water to come up and further cool. The warmer water under the surface will rise gradually because of its smaller density, convection thus takes place and the whole lake will be cooled down. So, the surface water keeps on cooling till it gets converted to ice continuing to float on the surface of water keeping the inside of water at a temperature of 4 degree Celsius. This is because the density is in decreasing order of- Solid> Liquid> Gas. Hence, the density of water in solid form (ice) is smaller than the density of water in liquid form. Therefore, we can say that water with a lower temperature on the lake surface will freeze first and then the freezing will spread to the lake bottom. Because heat is transmitted by conduction, the cooling rate is very slow. The water temperature under the ice layer usually maintains. 

This anomaly allows the aquatic organism to survive in freezing cold, as the temperature below is warmer than above. If water does not have this special property, we can imagine that water will start to freeze from the bottom and convection will increase the rate of freezing.

When water freeze, it gets less dense. That is why ice cubes float in a glass of water. It also takes a lot of energy removal to freeze water, and the air is usually much colder than the earth at the bottom of the lake. So the cold comes from the air above the lake, and starts freezing the water from the surface.

Simply cooling the molecules in a liquid does not make them solid. The molecules must begin to arrange themselves in orderly crystal lattice, and this takes a little more energy (called the latent heat of fusion). Forming a solid from individual liquid molecules is similar, the first few molecules must move into proper position and alignment to start building crystal lattice. Once this lattice begins to form, it becomes much easier for other liquid molecules to attach and continue growing the crystal lattice. The colder a liquid becomes, the more likely it is that some of the molecules will form that first crystal, but if they are not moving around much it may not happen.

Besides, the temperature also influence the density of the ice forming. When there is the changing of the temperature the water forming also can be effected. At the low temperature the water will change their  transitional stage from liquid to solid so that the density of the water also change. So the density will reduce in ice stage. So it will float at the upper of the water. If the temperature at the lower point, the sheet of ice will becomes thicker, the ice also acts as a barrier, or insulator, between the cold air and the warmer water underneath. The sheet of ice can helps the rest of the water from becoming ice.

1) The air is the source of the cold, and the earth below retains warmth better. Thus, the surface is frozen, while underneath it is heated (or kept from freezing) from below.

2) Ice is less dense than liquid water, and when a solid crust of ice is on a lake, the water underneath is under pressure. The freezing point drops with increasing pressure for a substance the liquid of which is denser than the solid

3) Water is an unusual substance. When water freezes, it gets less dense. That is why ice cubes float in a glass of water. It also takes a lot of energy removal to freeze water, and the air is usually much colder than the earth at the bottom of the lake. So the cold comes from the air above the lake, and starts freezing the water from the surface. As the ice is formed, it floats and stays at the surface of the lake until a solid sheet of ice is formed. This ice prevents the cold air from reaching the water beneath it - the ice acts like a blanket for the water in the lake and helps keep it warmer than it otherwise would be. So a lot of things cause the lake to freeze only on top. Of course, if it stayed cold enough for long enough, the lake would freeze solid

The meaning of lake from Thesaurus.com are a body of fresh or salt water on considerable size, surrounded by land or any similar body or pool of other liquid; an expanse of water entirely surrounded by land and unconnected to the sea except by rivers or streams related to adjective lacustrine. Lakes can be formed by glacier, tectonic plate movements, river from billabong and wind currents, and volcanic or meteorite activity. Some are only seasonal with drying up during parts of year.

The reason why only the upper part of layers of lakes is freezing are due to the density. When water freezes, it gets less dense. That is why ice cubes float in a glass of water. It also takes a lot of energy removal to freeze water, and the air is usually much colder than the earth at the bottom of the lake. So the cold comes from the air above the lake, and starts freezing the water from the surface

Lakes or fresh water are salty because there is in a region the water are evaporates more quickly due to high concentration. The study of lakes and other freshwater basins is known as limnology. Lakes are divided into two which are natural and artificial which is man-made.  The natural lakes in Malaysia are Tasik Bera and Tasik Chini while artificial is Tasik Kenyir.

When the winter cold wind blow on the surface of water, the surface water freezed and a thin layer of ice is form. When this process keeps going, the layer of ice is getting thinker and finally cover up the whole surface of the lake.

When this happened, the thicker layer of ice functions as a barrier or an insulator between the outer cold wind and the inner less cold lake water. It effectively reduce the heat from the water beneath the ice layer from escaping to the atmosphere. Hence, the bottom of a lake usually does not get freeze during winter.

Next is density, which tells us how tightly matter particles are packed together. Tightly packed matter is more dense than loosely packed matter. Also, less dense matter floats on top of more dense matter. So when you place ice cubes in your soft drink(which is mostly water) and your ice cubes float, you know that ice is less dense than water. This is a very special property of water. Almost every other type of matter gets more dense when it changes phase from a solid to a liquid, but water gets less dense. Why? Because ice is a crystal, which means it has a regular pattern with spaces in between molecules. The spaces in the crystal are larger than the spaces between molecules in the liquid. More space = less dense, so ice is less dense than water.

Now in lakes, in beginning of winter, and the temperature has just gotten below freezing. Air changes temperature faster than water -- this is why a swimming pool seems much warmer in the evening. The air cools down at night, but the pool water remains almost as hot as it was during the day. So although the air is freezing cold, the water isn't freezing yet. The water at the very top of the lake is in direct contact with the cold air, so it freezes first. And because ice is less dense than water, the sheet of ice doesn't sink. If the temperature stays very cold, the sheet of ice gets thicker, but the ice also acts as a barrier, or insulator, between the cold air and the warmer water underneath. The sheet of ice helps the rest of the water from becoming ice.

The fact that water freezes on the surface of a lake but not below it allows plants and fish to live through the winter in northern lakes and ponds. Imagine a lake without fish! So it is a good thing that H₂0 has the very strange property that its solid phase is less dense than its liquid phase.