Ice is the solid formed by water when it freezes. It is colorless, and has hexagonal crystals. The temperature of its freezing point is usually 00c. There are at least 6 different modification of ice and most of them exist under high pressure. The forms are represented by roman numerals thus, the ordinary ice formed at atmospheric condition is known as ice I. It is less dense than water due to the regular arrangement of water molecule through hydrogen bonds.
Ice floats in water. Water reaches its highest density at 40c. As it cools further into ice it becomes less dense. Since ice is less dense than water, it displaces the weight of water equal to its weight, thereby floating on water. On the other hand, most liquids are denser in their solid state unlike water which is different because of hydrogen bonding. Ice is 9% less than water. This means that it takes about 9% space than water. Therefore a liter of ice is less heavy than a liter of water.
For a substance to float, the liquid must be heavier than the substance so that the heavier liquid displaces the lighter substance making it to float. This is why ice can float in water, and also the principle behind the survival of aquatic living organisms. Fishes can survive in water because lakes and rivers freeze from the top to the bottom. If ice sinks in water, the water will be displaced to the top. This exposes the water to a cooler temperature which causes rivers and lakes to freeze solid and fill with ice.
When ice and water are placed in contact, melting and freezing occur at the same time. The melting is due to molecules of water that escape to the ice, and the freezing is due to the molecules of ice that escape to the water. At the point when freezing and melting is happening at the same rate, the amount of ice and the amount of water will not change. The water and ice is said to be in dynamic equilibrium. This is maintained at 00c; the temperature of the freezing point of water, unless condition changes in a way that it favors one condition over another.
The balance between the melting point and the freezing point can easily be upset. If the water and ice mixture is cooled, the molecules move more slowly and are attracted by ice. So freezing occurs at a greater rate than melting. On the other hand, if the mixture is heated, the molecules move faster and are attracted by water. So the melting occurs at greater rate than the freezing.
The addition of salt to the mixture of water and ice will also upset the equilibrium of the mixture. When salt is added to the mixture, some of the water is replaced by the salt thereby lowering the total number of water captured by ice. So the rate of freezing goes down and melting is favoured. This is why salt melts ice. Salt is not the only substance that melts ice. In fact any substance placed into the mixture of ice and water will melt the ice increasing the melting process. These substances are called foreign substances. The effect of the substance does depend on the size of solute, but it occurs because the concentration of water in a solution is less than the concentration in pure water.
To re-establish equilibrium or balance between the solution of water and ice, the temperature of the solution will be lowered below the freezing point of water.