Maharashtra Board Class 8 Science Chapter 13 Chemical Change and Chemical Bond PDF Download

Chemical Change and Chemical Bond

1. What are the methods of classification of changes?

2. What is the difference between physical and chemical change?

3. Classify the following changes into physical and chemical change.

13.1 Some Observations of Chemical Change

Ripening of mango, melting of ice, boiling of water, dissolution of salt in water, ripening of banana, fragrance on ripening fruit, darkening of a cut potato, bursting of an inflated balloon, sound of bursting fire cracker, foul smell from a spoiled food.

During any chemical change, composition of original substance changes to form new substance with a different composition and properties. How to identify a chemical change?

Take the lemon juice in a clean glass. Take two drops of the lemon juice in a spoon and taste. Add a pinch of baking soda in the glass of lemon juice. Did you notice bubbling around the particles of soda? Did you hear a sound on taking your ear near the glass? Now again taste it. Did it taste as sour as it was in the beginning? (Above activity is to be done using clean apparatus and edible material. Then only it is possible to test the 'taste', otherwise keep in mind that the testing of 'taste' cannot be done.)

Many perceivable observations are noticed during the above activity. A gas is seen to be liberating in the form of bubbles. A low sound is heard. The white solid particles of the baking soda disappear. The original sour taste becomes mild or diminishes. From this, it is understood that a new substance having a different taste is formed. At the end of the above change, the taste of the substance was different means its composition was different. Thus, during the above change, the composition of the original substance changed to form a new substance with different properties. Thus, the change that takes place on adding baking soda to lemon juice is a chemical change. Sometimes some characteristic observations are perceived during a chemical change. These enable us to know that a chemical change has taken place. Some of these observations are enlisted in the table.

Teacher's Note

When lemon and baking soda mix, they make bubbles and the taste changes. This is like when you make lemonade at home and add sugar - the liquid changes completely.

Exam Trick

Remember: If taste, colour, or smell changes, it is a chemical change. If only the shape changes, it is a physical change. Gas bubbles also show chemical change.

Points to Remember

Chemical changes make new substances with different properties.
You can see bubbles, smell, colour change, or taste change during chemical changes.
In a chemical change, the composition of matter changes completely.
Physical changes only change the shape or size, not what the thing is made of.

First step of writing a chemical equation is to write a word equation by using the names of the concerned substances. When the chemical formula is written in place of each of the names, it becomes a chemical equation. While writing a chemical equation, original substances are written on the left side and newly formed substances are written on right side and an arrow is drawn in between. Arrow head points towards the substances formed. Arrow indicates the direction of the reaction. Substances written on the left side of the arrow are original substances that take part in the reaction. They are called reactants. New substances formed as a result of the reaction are called products. Place for the products of a reaction is on the right side of the arrow.

Chemical Change and Word Equation

During a chemical change the chemical composition of the original matter changes and new substances having different properties and different chemical composition are formed. A chemical equation can be written for a chemical change, if the exact change in chemical composition is known. Names and chemical formulae of the original substance and newly formed substance are used while writing a chemical equation. For example, when baking soda is added to lemon juice a chemical change takes place in the citric acid present in the lemon juice and the gas formed is carbon dioxide. The word equation can be written for this chemical reaction as follows:

Citric acid + Sodium bicarbonate → Carbon dioxide + Sodium citrate

Acid + Alkali → CO₂ + Salt

This is neutralization reaction.

Teacher's Note

When you add baking soda to lemon juice, bubbles come out - this is carbon dioxide gas. Your mother uses the same gas to make carbonated water at home.

Exam Trick

Remember: Reactants go on the left, products go on the right, and the arrow points to the products. Think of it like: things that go in → arrow → things that come out.

Points to Remember

Reactants are substances that go into a chemical change.
Products are new substances formed after the change.
An arrow in an equation shows the direction of the reaction.
Word equations use names of substances.
Chemical equations use the formulas of substances.

Natural Chemical Changes

a. Respiration: Respiration is a continuously occurring biological process. In this process, we inhale the air and exhale carbon dioxide and water vapour. After an in depth study it is learnt that glucose in the cells reacts with oxygen in the inhaled air to form carbon dioxide and water. The word equation and the chemical equation of this chemical reaction are as follows. (Here, the chemical equation is not balanced.)

Word equation:

Glucose + Oxygen \(\xrightarrow{\text{respiration}}\) Carbon dioxide + Water

Chemical equation:

\[\text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2 \xrightarrow{\text{respiration}} \text{CO}_2 + \text{H}_2\text{O}\]

Chemical Changes in Everyday Life

We find many examples of chemical changes in our surrounding, body, home and laboratory. Let us see some chemical changes for which word and chemical equation can be written easily.

Take some freshly prepared lime water (solution of calcium hydroxide) in a test tube. Keep on blowing in it with a blow tube. What is seen after some time? Did the colourless lime water turn milky? After some more time you will find that a white insoluble solid settles at the bottom of the test tube. This is a precipitate of calcium carbonate. The turning milky of lime water means that the blown gas mixed in it was carbon dioxide.

Carbon dioxide + Calcium hydroxide → Calcium carbonate + Water

Write a chemical equation for the above word equation.

Teacher's Note

When you blow through a straw into lime water, it turns white like milk. This white powder is calcium carbonate, the same thing that is in chalk.

Exam Trick

Remember: Lime water + carbon dioxide = white milk-like colour. This test shows there is carbon dioxide in your breath!

Points to Remember

Respiration is how our body gets energy from food using oxygen.
During respiration, glucose and oxygen make carbon dioxide and water.
Lime water turns white when carbon dioxide bubbles through it.
The white colour shows calcium carbonate has formed.
This is how we test if carbon dioxide is present in a gas.

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