Physical and Chemical Changes
Science Class Seven
Summary: A quick Reading
We see different types of changes every day around us. Such as rising and setting of the sun, changing shape of the moon, running of cars, growing plants, etc. This means our surroundings are changing continuously.
Before understand change, it is necessary to understand the two important properties of substances, these properties are Physical Properties and Chemical Properties.
Physical Properties
The colour, shape, size, states, temperature, mass, etc. are called the physical properties of a substance.
Chemical Properties
The internal structure, capacity to react with other substance, etc. are called the Chemical Properties of matters.
Changes can be divided into two types : Physical Change and Chemical Change.
Physical change
The change in physical properties of a substance or matter is called physical change.
Physical changes are reversible, so we can get the matter back after physical change using simple methods.
In physical change no new substance is formed, only the texture, temperature, colour, state, etc. of a substance is changed.
Example : Melting of ice, vaporisation of water, freezing of water, folding of a paper sheet, cutting of a paper sheet, melting of wax, moving of a car, heating of an iron rod, etc. are some examples of physical changes.
The melting of ice into water, and vaporisation of water into vapour only changes in the physical state of water. This is one of the examples of physical change.
Chemical Change
If a new substance is formed after the change, the change is known as chemical change.
The chemical changes are irreversible. This means we cannot get the matter back using simple methods after a chemical change.
The chemical change is also known as a chemical reaction.
Example
The change of milk into curd, change of milk into paneer, burning of wax, burning of a sheet of paper, etc.
The burning of anything is a chemical change. Because in the burning of a substance new substances are formed.
Burning of Magnesium Ribbon
The burning of magnesium ribbon is a chemical change similar to all other burnings.
In the burning of magnesium ribbon, magnesium oxide is formed along with heat, dazzling light, and smoke.
The reaction involved in the burning of magnesium ribbon can be written as follows:
Magnesium (Mg) + Oxygen (O2) ⇒ Magnesium oxide (MgO) + Heat
The Reaction of Iron with Copper Sulphate
In the reaction of Iron with copper sulphate, iron sulphate and copper is formed. In this reaction, copper gets deposited over an iron nail or iron blade which is used as Iron.
The colour of copper sulphate is blue and that of iron sulphate is green.
This reaction can be written as follows:
Copper sulphate solution (CuSO4) + Iron (Fe) ⇒ Iron sulphate (FeSO4) + Copper (Cu)
As with other reactions, the change of copper sulphate into iron sulphate and the deposit of copper over the iron is a chemical change.
The Reaction between Vinegar and Baking Soda
In the reaction between vinegar and baking soda, carbon dioxide, water, and sodium acetate are formed.
This reaction can be shown by a given chemical equation.
Vinegar (Acetic acid) + Baking soda (sodium bicarbonate) ⇒ Carbon dioxide (CO2) + Sodium acetate (CH3COONa) + Water (H2O)
The Reaction of Carbon dioxide with Lime Water
When carbon dioxide is passed in the lime water (calcium hydroxide), lime water turns milky because of the formation of calcium carbonate.
The reaction between carbon dioxide and lime water can be shown as follows:
Lime water (Ca(OH)2) + Carbon dioxide (CO2) ⇒ Calcium carbonate (CaCO3) + Water (H2O)
Rusting of Iron
The reddish brown colour substance that gets deposited over articles made of iron is called rust, and the phenomenon of getting deposited reddish brown colour substance over the article made of iron is called rusting of iron.
When articles made of iron come in contact with moisture and oxygen for a long time, iron turns into iron oxide, which gets deposited over the articles made of iron as a layer. This iron oxide is known as rust.
The reaction involved in the rusting of iron can be shown as follows:
Iron (Fe) + Water (H2O) + Oxygen (O2) ⇒ Iron oxide (Fe2O3)
If not prevented on time, this process continues and gradually whole article made of iron is converted into rust.
For rusting, water and oxygen must be in contact with iron for a long time. Hence, rusting can be prevented by preventing the water or oxygen or both to come in contact with iron.
To prevent rusting, a layer of paint, a layer of grease is applied over the article made of iron. This layer prevents the iron to come in contact with water and oxygen.
Galvanisation
The depositing of a layer of chromium or zinc over articles made of iron is called GALVANISATION.
The galvanisation is done over the water pipes which are used to supply drinking water in our homes in order to prevent them from rusting.
Crystallisation
The formation of big beads of salt after solidification is called crystallisation. The common salt, sugar, etc. are examples of such crystals.
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