- Books Name
- CBSE Class 7 Science Book
- Param Publication
- CBSE Class 7
A change which alters the specific properties of a material by bringing about a change in its molecular composition, is called a chemical change.On the other hand we can say that when two or more substances reacts in such a way that there is formation of one or more new substances, the change is called a chemical change or a chemical reaction.
During a chemical change, reactants undergo changes to form products. Chemical changes, generally, are irreversible changes.
• Chemical equations :
All chemical changes are accompanied by chemical reactions. These reactions can be described in sentence form, but the description would be quite long. Chemical equations have been framed to describe the chemical reactions.
A chemical equation links together the substances which react with the new substances that are formed.
• Reactants :
The substances which take part in bringing about chemical change are called reactants.
• Products :
The substances which are produced as a result of chemical change are called products.
• These reactions involve breaking and making of chemical bonds.
• Irreversible change:
A change in which we cannot get back the initial substance by reversing the action is an irreversible change.
To observe the formation of a new substance on heating a magnesium ribbon and then adding water to it.
• Take a small piece of a magnesium ribbon.
• Clean its tip by rubbing it with a sandpaper.
• Hold it with a pair of tongs over the flame of a burner.
Now, we will observe that it burns with a brilliant white light. It leaves behind a powdery ash after burning. The ash obtained is not the same as the magnesium ribbon. Magnesium has lost its properties and a new substance, magnesium oxide (MgO) is formed.
• Collect the ash in a beaker and mix it with a small amount of water.
• Stir it properly.
• With the help of a dropper, put a drop of this solution on red and blue litmus papers to test its chemical nature.
Now, we will observe that red litmus paper turns blue and blue litmus paper remains as it is. This shows that the solution obtained is basic in nature.
Inference : Magnesium oxide, on dissolving in water, forms magnesium hydroxide which is a new substance. Thus, it is a chemical change.
This change is represented as:
To observe the formation of a new substance on putting an iron nail in copper sulphate solution.
• Take 100 mL water in a 250 mL beaker.
• Dissolve a spoonful of copper sulphate (blue vitriol or neela thotha) in it.
• Add a few drops of dilute sulphuric acid to it.
Now, we will get a blue-coloured solution.
• Divide this solution into two equal parts.
• To the second part, drop an iron nail and leave it for 30 minutes.
• Compare the colour of this solution with the second part of the solution .
Now, we will observe that the blue colour of the solution changes to green colour. Also a brown-coloured layer get deposited on the iron nail.
Copper sulphate solution is blue in colour. It changes to green colour due to the formation of a new substance, i.e., iron sulphate. The brown deposit on the iron nail is of copper, another new substance. Hence, this is a chemical change.
This change can be represented as :
• Displacement reaction
Iron is a more reactive metal than copper. Iron replaces copper from copper sulphate solution to form iron sulphate. Such reactions, in which a more reactive metal replaces a less reactive metal from its salt solution is called displacement reaction.
• When we leave cut slices of apple, brinjal, potato, etc. they acquire a brown-coloured layer. This change of colour is due to the formation of a new substance on reaction with atmospheric air.