A change is referred to as a difference that occurs in the properties of the substances such as shape, size, colour, state and internal structure etc. For example, a burning candle melts its wax, and this melting wax evaporates to produce a black substance called soot and carbon dioxide.
The most noticeable thing is that changes can occur either instantly or take a longer time, and, they can be temporary or permanent. Natural changes: The changes occurred by nature are called natural changes. Some common examples are the rotation of the earth on its axis causing day and night, the revolution of the earth around the sun causing different seasons and growing babies into adults(Figure 1). Man-made changes: The changes that are influenced by human efforts are called man-made changes. Some common examples are the manufacturing of vehicles, making food products and building houses(Figure 1).
The changes can be broadly differentiated based on the properties of the substances: physical, chemical, reversible and irreversible changes.
REVERSIBLE AND IRREVERSIBLE CHANGES
The substances undergo the changes in which they get back to their original or initial state. Such changes are reversible changes. The permanent changes are not reversible, but temporary changes are reversible. For example, the heating of water leads to the formation of water vapours, a process called vapourisation. If we cooled these vapours, they again back to their liquid state and the process is called condensation
Conversely, the substances undergo changes that cannot be reversed back to their original state. Such changes are called irreversible changes. Some examples are the growth of living things, ripening of fruit and cutting papers into small pieces In irreversible changes, the substances may have internal structural changes or they may form into new products.
The changes in the physical properties of the substances occur, but no change occurs in their chemical composition. Such changes are called physical changes. The following characteristics of physical changes are :-
These changes only affects the physical properties of the substances such as temperature, shape, size, odour, position and texture.
The result cannot lead to the formation of the new substance.
No change in the internal structure of the substances occurred.
These changes are temporary.
Some examples of physical changes have physical processes such as:-
i. Evaporation: The water changes into water vapours.
ii. Melting: Ice changes into water.
iii. Freezing: The water changes into solid.
All these processes have state conversions that occurred due to the changes in the temperature, but no chemical composition is changed(Figure 3).
Physical changes are mostly reversible but not all physical changes are reversible such as breaking glass, tearing of paper and bursting of balloons. Some methods that we apply in our daily life like heating, cooling, cutting and pushing or pulling, etc.
The changes that occur in the chemical composition of the substances that lead to the formation of new substances are called chemical changes. A chemical change can take place when two or more substances chemically react together. In a chemical change, the substances that are combined are chemically called reactants, whereas the newly formed substances are called products. Some examples are:-
The burning of paper turns into ash. This newly formed product ash has different properties than the paper.
The combination of the atoms or molecules of different elements together formed a new compound. For example, water is formed, when two atoms of hydrogen are linked with one atom of oxygen in the presence of heat.
Fermentation is also a chemical change. For example, yeast and bacteria convert sugar into alcohol and carbon dioxide. This process is called fermentation.
The following characteristics of chemical changes are:-
i. The newly formed substances have different properties and compositions than the reactants.
ii. These changes are considered as irreversible.
iii. The absorption or emission of heat are both involved in chemical changes.
Common Causes of Physical and Chemical Changes
1. Mixing of two or more substances: The mixing of two or more substances leads to physical change. For example, the occurrence of evaporation recovered the dissolved salt in the water. The mixing of two or more substances also leads to the chemical change. For example, the reaction between lemon juice and baking soda causes hissing sounds and bubbles, forming a substance with completely different properties.
2. Chemical reaction: It is a cause of chemical change. For example, rusting of iron takes place due to corrosion(a rough black coating on the iron).
3. Heating and cooling: The heating and cooling effects cause changes in the physical states of matter. For example, the physical processes depend on heating and cooling factors.
4. Application of force: The force of air changes the shape and size. For example, a balloon inflated due to the force of air.
Effects of Heating and Cooling
The effects of heating and cooling can lead to various changes. For example, the heating of iron expands it while it contracts when gets cooled.
Applications of contraction and expansion
The expansion and contraction are caused due to the effects of various parameters such as temperature and force, etc. The following applications of contraction and extraction are:-
The wooden wheel of a cart has a metal rim because it helps the cartwheel to move smoothly. On heating the metal rim, it expands. Afterward, the heated metal rim expels out of the cartwheel that is later placed over the cartwheel and poured with the cold water which makes it contracts and fitted in the cartwheel(Figure 4).
Figure 4: Cooling and heating of metal rim.
The small gaps between two adjacent sections of rails on the railway track are also an example of the heating and cooling effect(Figure 5).
Figure 5: Small gaps between rails on the railway track.
One of the excellent examples of heating and the cooling effect is a clinical thermometer that contains liquid mercury for measuring the temperature of the human body. On cooling the bulb of the thermometer, the temperature drops down while putting it under the mouth of the human can raise the temperature significantly