Freezing takes place on lowering the temperature of a liquid substance. Substances have different freezing points because of different molecular structures.
As we know very well, Matter is made up of small particles. These all small particles are also so minute that we cannot see with naked eyes.
Whenever heat is supplied to them they start moving from their original position. This heat energy is also represented as providing kinetic energy to the particles. On receiving heat this starts colliding with each other, if we keep on raising the temperature, at the end of this stage, they will turn themselves into another form.
The reverse is also true. On expelling the heat of a liquid, particles slow down. Taking them to freezing point, this further becomes the reason for again the start of intermolecular forces.
You might have observed that when a cube of ice is taken out of a refrigerator, it melts into water. If this water is kept back in the refrigerator, it re-freezes into ice. This indicates that the properties of water and ice are the same, i.e. their chemical composition is the same. On melting of ice or on freezing of water no new substance is formed. Exothermic and endothermic process changes the physical state of the substance.
Is Freezing Endothermic Or Exothermic? Freezing is an exothermic process in which heat is expelled out. When the freezing process takes place the particles of a liquid substance slowly lose energy thus turns into a solid-state. Another name of freezing is solidification.
In another way, we can also explain it as liquid and gaseous particles have a weak force of attraction. To change them into a freezing solid state, a greater force of attraction is required. This can be achieved by lowering the heat. Extracting the heat from the substance allows liquid or gas particles to turn themselves into closed packed.
Endothermic and Exothermic Process
Depending upon the energy absorbed or given out, reactions are of two types:
1. Exothermic reaction: Such reactions involves the given out of the heat.
2. Endothermic reaction: In these reactions, heat is absorbed to start the reaction and to change the state.
No matter whether physical or chemical, a reaction involves the breaking up of force of attraction or bonds between particles (atoms). During the physical process, with the release or absorption of energy, there is a change in state. Whereas in the chemical process, a new product is formed with the release or gain of energy.
These two types of energy are different from each other i.e. there is either a surplus or a deficit of energy during the reaction. Therefore, in chemical reaction energy is either absorbed or released.
The freezing process remains reversible through the endo and exothermic process. You have noted that when a cube of ice is taken out of a refrigerator, it melts and changes into water. It turns again into ice when put back in the refrigerator. This shows that on removing the cause of change the substance returns to its original state.
How Freezing Is An Exothermic Process?
As we said earlier, The heat we are giving to a matter is the kinetic energy and this energy certainly increases the particle collisions of the matter. What does this mean? Explaining in simpler words, this means that a gaseous state has more heat energy than a liquid and a liquid has more energy than a solid state.
When heat is supplied to a block of ice, it will turn into a liquid and when more heat is given to it, it will turn into a gaseous state.
During the process of freezing, heat is taken out from this substance. For example- When water is cooled it changes into ice. We can easily lower the temperature of the water by placing it into the refrigerator compartment. Water turns into ice because energy has been extracted from it which was a reason for being liquid. So that is why Freezing is an exothermic process.
You must remember that freezing is the reverse of melting. When water freezes to become ice when there is a change from a liquid state to a solid state.
Melting and freezing point of Water
If a teacher asks you about the Melting and freezing point of water then your answer should be zero degrees Celsius.
You also so bear in mind, the freezing point of a liquid is the same as the melting point of its solid form. When water is at zero degrees Celsius, it is the initial state when water starts to turn into ice. As the temperature becomes lower it will achieve a solid-state.
Similarly, the melting point of water is also zero degrees celsius. It is the temperature at which ice starts its changing state. The temperature keeps rising more, it will move towards a gaseous state as it is achieving more heat.
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Similarly, Instead of giving them the heat energy, if we take out heat from the substance, it also changes its state vice-versa.
Changes in State Occur at Fixed Temperatures
For a pure substance, a change in state occurs at a fixed temperature (provided we do not change the pressure). In all cases, 1 atmosphere is taken as the standard pressure.
Solid melts at a fixed temperature, called its melting point, and a liquid boils at a fixed temperature, called its boiling point. A liquid freezes at its freezing point and a gas (or vapour) liquefy at its temperature of liquefaction. Interestingly, the melting point of a solid is the same as the freezing point of the corresponding liquid. And the boiling point of a liquid is the same as the temperature of liquefaction of the corresponding gas.
The melting point and the boiling point of a pure substance are fixed at a particular pressure.
For example, at a pressure of 1 atmosphere, ice melts at 0°C and water freezes at 0°C, and water boils at 100°C and steam liquefies at 100°C. However, the melting point of solid decreases and the boiling point of a liquid increases with pressure.
Energy Change in Change in State
Energy change is a general feature of all changes–physical or chemical. So, a change in state is also accompanied by an energy change. This will be evident from the following observations.
1. If you put some ice in a glass, water droplets will collect on the outer surface of the glass. Ice needs heat to melt, which it takes from the surroundings. On giving heat to the ice, the water vapour in the air condenses on the outer surface of the glass. Reactions always involve an energy change. Let us look at a couple of illustrative examples.
In a reaction, suppose you need 500 kJ of energy to break the bonds of the reactants, but you get only 400 kJ of energy from the formation of the new bonds. So, you will have to spend 100 kJ of energy for the reaction to occur.
2. In another reaction, suppose you need 500 kJ of energy to break the bonds of the reactants, but you get 600 kJ of energy from the formation of new bonds. So, 100 kJ of energy will be released during this reaction.
In most cases, energy is absorbed or given out in the form of heat. However, in many cases, light is also absorbed or given out.
The effect of impurities on the melting point and the boiling point of a substance
The melting point and the boiling point of a substance change when soluble impurities are present in it.
When there are soluble impurities in a substance, its melting point is lowered. For example, sodium chloride is soluble in water, and so the melting point of water, in this case, ice) containing sodium chloride is lower than 0°C at 1 atmosphere. Mix some common salt with ice and record the temperature it will go to the minus side. This is the principle on which a freezing mixture works.
A freezing mixture is a mixture of ice and common salt or calcium chloride, and it helps us attain as low a temperature as -5°, -10° or -15°C, which may be required for an experiment. Ice-cream vendors store ice cream in a freezing mixture.
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