physical and chemical changes class 7

PHYSICAL AND CHEMICAL CHANGES - CHAPTER 6- IMPORTANT TOPICS

physical and chemical changes class 7

We are observing different types of changes around us in our daily life. Some changes are taking place very slowly like the growth of your nail or hair. But there are some changes which occur very fast. Certain changes are reversible like melting and freezing of water. Setting curd from milk is a desirable change, but milk getting spoiled is an undesirable change. In this session, we can see another type of classification. They are PHYSICAL CHANGE and CHEMICAL CHANGE.

PHYSICAL CHANGES

A change in which the physical properties of a substance undergoes change is called a physical change.

Normally physical properties like shape, size, state, and color are changed. Most of the physical changes are reversible. During a physical change, new products are not formed.

paper boat

If you are making a paper boat, it can be unfolded to get back the paper in the same shape, size, and color. So it is a physical change.

If you are taking some ice and melting it. water is obtained. To get back ice, we can keep that water in a freezer. Similarly  the steam obtained by boiling water can be condensed back to water. Hence change in state of matter is a physical change

Iron on heating will become red hot and it will get its color back on cooling. This is also an example of physical change. Think about such changes happening around you and make a list of five physical changes.

CHEMICAL CHANGES

In a chemical change, one or more new products are formed. Most of the chemical changes are not reversible. It is also called a chemical reaction, as it leads to the formation of new things.

 

On burning a piece of paper, we can observe the formation of ash. But on tearing a piece of paper, we are not getting any new product. Rusting iron is another example of a chemical change. Iron articles will get rusted when exposed to air. The rust obtained is the new product formed and we cannot get back iron from it.

Chemical changes are accompanied by the following changes:

• color may change
• the sound may be produced
• gas may be evolved
• heat, light or any radiation might be evolved or absorbed
• smell may be changed

More examples of chemical changes/chemical reactions

1. Burning of Magnesium ribbon

Burn a small strip of magnesium ribbon on flame. It burns with Oxygen in the air with a dazzling white light leaving behind white powdery ash.

The reaction can be represented as:

Magnesium + Oxygen → Magnesium oxide

Mg + O2 → MgO

Dissolve the white powder obtained (MgO) with water and test it with red and blue litmus paper.

The red litmus paper will turn blue and there is no color change with blue litmus paper. This indicates the formation of a basic compound.

Magnesium oxide + water → Magnesium hydroxide

MgO + H2O → Mg(OH)2

2. Reaction of Iron and Copper sulphate

Place an iron nail in a Copper sulphate solution that contains a few drops of dilute sulphuric acid. Observe the color change after some time.

We can observe the color change from blue to green and some brown powdery deposits on the iron nails. This is due to the chemical reaction between iron and copper sulphate.

Blue-colored copper sulphate reacts with iron to form green-colored iron sulphate and brown-colored copper.

The reaction is as follows:

Copper sulphate + Iron → Iron sulphate + Copper

copper sulphate

3. Reaction of Acetic acid and Baking soda

Add a pinch of baking soda to acetic acid (vinegar). We can observe the formation of gas bubbles with a hissing sound. The bubbles are due to the formation of carbon dioxide.

Acetic acid + Baking soda → Carbon dioxide + other substances

On passing the gas obtained in the above reaction through lime water (Calcium hydroxide), the colorless solution turns milky.

The reaction is as follows:

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Ca(OH)2 + CO2 → CaCO3 + H2O

When carbon dioxide is passed through colorless calcium hydroxide, calcium carbonate is formed which turns milky. This is a standard test of carbon dioxide.

4. Rusting of Iron

When an iron article is exposed to air, it reacts with oxygen and moisture to form hydrated iron oxide (rust). It is represented by the equation:

Iron + Oxygen + Water → Iron oxide

Fe + O2 + H2O → Fe2O3.x H2O

The presence of both oxygen and water is essential for rusting.

If the air is more humid, rusting becomes faster.

Prevention of rusting

Rusting of iron can be prevented:

•   By painting
• By oiling 
•    By greasing
•           By galvanization
•   By alloying

Galvanization- It is the process of depositing a layer of zinc on iron. The iron pipes to carry water and storage tanks are galvanized to prevent rusting.

Alloying-  is the process of mixing different types of metals or metals with non-metals. Stainless steel is made by mixing iron with carbon, chromium, nickel, and manganese.