Physical and Chemical Changes
Science Class Seven
What is change?
We see many changes around us, such as growing of plants, shredding of leaves by trees, burning of fuel, cutting of papers, cutting of vegetables, rising of the sun, setting of the sun, changing of the place of the moon in the sky, etc. This means our surrounding is changing continuously. This says that changes are very important in our life.
Properties of Matters
To know the change properly, it is necessary to know about the properties of matter. Because before knowing about change, it is necessary to know about the properties of matter.
Change means a change in the properties of a thing.
Properties of matters are two types, Physical Properties and Chemical Properties.
Physical Properties
The shape, size, colour, state of matters, etc. are called the Physical Properties of matters.
Chemical Properties
The internal structure, capacity to react with other substances, etc. are called the Chemical Properties of matters.
Types of Changes
For ease of study, changes are divided into two types, Physical Change and Chemical Change.
Physical Change
Change in physical properties of a substance is called Physical Change.
Changing in the shape, size, colour, temperature, state, etc. of a matter is called the Physical Change.
In a physical change, no new substance is formed.
Physical changes are reversible. This means we can get the initial substance back after a physical change using simple methods.
Example :
Folding of a paper, bending of a rod, tearing off a piece of paper, breaking of brick into two or more pieces, painting of a car, movement of a car or anything, change of water into ice, change of water into vapour, change of vapour into water, change of water into ice, heating of an iron rod, melting of wax, etc. are some examples of physical change.
After folding a paper only the shape of the paper is changed. We can unfold the paper and can get the paper of the previous size back, hence it is a physical change.
After melting ice into the water we can get the ice back by freezing of water, hence melting of ice into water is a physical change. In the melting of ice into water, no new substance is formed. Ice is another form of water.
After vaporisation of water, we can get water back by cooling of vapour, hence vaporisation of water is a physical change. Vapour is another form of water. In the change of vapour into water, no new substance is formed, in this only state of water goes under change.
Melting of wax. We can get wax back after cooling of melted wax. In this no new substance is formed, only the state of wax goes under change. Thus melting of wax or cooling of liquid wax into solid wax is an example of physical change.
Chemical Change
Change in chemical properties of a substance is called Chemical Change.
Change in the internal structure of a substance and formation of new substance is called the Chemical Change.
In a chemical change, one or more new substances are formed.
A chemical change is generally irreversible. This means after the chemical change, we cannot get the initial substance back using simple physical methods.
Since in a chemical change, one or more than one new substance are formed, so a chemical change is known as Chemical Reaction also.
Example
Growth of trees, growth of a person, burning of paper, burning of fuel, burning of wax, etc. The burning of anything is an example of chemical change.
The burning of paper is a chemical change. By burning paper, we get heat, light, smoke, and ash from paper. In the burning of paper, three or more substances are formed. We cannot get the paper back after burning it. By burning, the chemical properties of paper change, and new substances are formed. Thus, the burning of paper is a chemical change.
Bursting of crackers. In the bursting of crackers heat, light, sound, smoke, ash, etc. are formed. We cannot get crackers back after bursting. In the bursting of crackers, the chemical nature of crackers changes. Thus, the bursting of crackers is a chemical change.
Burning of anything, such as coal, wood, paper, cloth, cotton, magnesium ribbon, etc. is a chemical change. Because by burning of anything one or more substances are formed and chemical nature of substance burnt is changed. We cannot get the substance back after burning.
The growth of living things is a chemical change, for example after the growth of a plant its internal structure changes and we cannot get the previous plant back using any method from the grown plant.
Burning of Magnesium Ribbon
As the burning of anything is a chemical change, hence burning of magnesium ribbon is also a chemical change.
A magnesium ribbon burns with a dazzling light. In the burning of magnesium ribbon, light, heat, smoke, and ash are formed.
The ash of the magnesium ribbon is called magnesium oxide. The chemical formula of magnesium oxide is MgO.
The reaction involved in the burning of a magnesium ribbon can be written as follows:
Magnesium (Mg) + Oxygen (O2) ⇒ Magnesium oxide (MgO) + Heat + Light
Since, without the presence of oxygen burning is not possible, hence reaction involved in the burning of anything is shown by the addition sign of oxygen, as it is shown in the above reaction of the burning of magnesium.
Here, we can see that in the burning of the magnesium ribbon, three new substances are formed. These substances are magnesium oxide, heat, and light.
Since, in the burning of magnesium new substances are formed, thus burning of magnesium is a chemical reaction.
Reaction between Magnesium Oxide and Water
Dissolving of Magnesium oxide formed in the burning of magnesium ribbon into water.
By dissolving magnesium oxide into water, it changes into magnesium hydroxide. The chemical formula of magnesium oxide is MgO, and that of magnesium hydroxide is Mg(OH)2.
The reaction involves in the dissolving of magnesium oxide (MgO) into water can be represented as follows:
Magnesium oxide (MgO) + Water (H2O) ⇒ Mg(OH)2
When a red litmus paper is dipped into the magnesium hydroxide solution, it turns blue. This shows that magnesium hydroxide [Mg(OH)2] is a base.
Since, in the reaction between magnesium oxide and water, a new substance magnesium hydroxide is formed, so, it is a chemical change.
Reference: