Class 7 Physical and Chemical Changes Important Questions for exams

Class 7 Physical and Chemical Changes Important Questions for exams. There are many more useful educational material which the students can download in pdf format and use them for studies. Study material like concept maps, important and sure shot question banks, quick to learn flash cards, flow charts, mind maps, teacher notes, important formulas, past examinations question bank, important concepts taught by teachers. Students can download these useful educational material free and use them to get better marks in examinations.  Also refer to other worksheets for the same chapter and other subjects too. Use them for better understanding of the subjects.

Chapter: Physical and Chemical Changes

Smart Notes

Difference between physical and chemical changes

The properties such as __________, __________, __________ and __________ of a substance are called its physical properties.

Answer: colour, shape, size and texture (other physical properties are smell, hardness, density, melting point and boiling point).

During a chemical change / reaction, when new products are formed, it is usually accompanied by

1. release or absorption of energy in the form of heat, light, etc.
2. production of sound
3. change in smell
4. change in colour
5. formation of gas

Classify the following changes into physical and chemical changes:

Rusting: Rusting is the process in which iron reacts with oxygen and moisture in the air to form a reddish brown coating, called rust, on its surface. The presence of common salt speeds up rusting. If either oxygen or moisture is absent, rusting does not occur.

1. Presence of oxygen (air)
2. Presence of moisture (water)

Rusting is faster when the content of salt is high in air.

Rusting can be prevented by

1. applying a coat of paint or grease / oil
2. depositing a layer of a metal like zinc or chromium on iron.

Why do ships suffer a lot of damage from rusting?

Answer: In spite of ships being painted, they suffer a lot of damage because a part of the ship is always in water and water droplets cling on the part which is above water. Ships sail in the sea and sea water contains salts. Salt makes rusting faster.

Practice exercise: Physical and chemical changes

Fill in the blanks:

1. Chemical reactions are accompanied by absorption or evolution of energy.
2. Matter can be classified as solid, liquid and gas.
3. New substances are formed in a chemical reaction.
4. A change in physical state of a substance is called physical change.

Complete the following table giving reasons, whether the classification in column B is correct or not:

Write word equations for the following chemical changes:

1. The oxide of magnesium is dissolved in water to form magnesium hydroxide.
2. Baking soda is added to an aerated drink to form bubbles of carbon dioxide and other substances.
3. Carbon dioxide is exhaled in lime water to form carbonic acid.
4. Zinc granules are added to hydrochloric acid to form zinc chloride and liberate hydrogen gas.

Answer:

1. Magnesium oxide + water → magnesium hydroxide
2. Sodium hydrogencarbonate (baking soda) + acid in the drink (e.g. citric acid) → a salt (sodium citrate) + water + carbon dioxide
3. Carbon dioxide + calcium hydroxide (lime water) → calcium carbonate + water
4. Zinc + hydrochloric acid → zinc chloride + hydrogen

(The source said part (c) forms “carbonic acid”; carbon dioxide and lime water actually form calcium carbonate, which makes the lime water milky, so the question and answer are written that way.)

Which of the following changes is useful?

1. Burning of paper
2. Rusting
3. Formation of curd
4. Cutting of trees

Answer: (c) Formation of curd. Curd is a useful food made from milk, whereas rusting and cutting of trees cause loss, and burning of paper wastes the paper.

Define the following terms:

1. Crystallisation
2. Galvanisation
3. Rusting

Answer:

1. Crystallisation: the process of getting pure crystals of a solid from its saturated solution on cooling or slow evaporation.
2. Galvanisation: the process of depositing a layer of zinc on iron to protect it from rusting.
3. Rusting: the process in which iron reacts with oxygen and moisture in the air to form a reddish brown coating called rust.

Study the diagram and answer the questions:

In which case would the iron nails get rusted fastest and why?

Answer: In tube 4 (salt solution). The nails get both water and air, and salt speeds up rusting.

In which case would the iron nails not get rusted and why?

Answer: In tube 1, because anhydrous calcium chloride absorbs moisture, so there is only dry air and no water; and in tube 2, because boiled water has no dissolved air and the oil layer stops air from dissolving in it, so there is no oxygen.

Food items like apples, if cut and exposed to air, become brownish. Explain why.

Answer: The cut surface of the apple comes in contact with oxygen of the air. Substances in the apple react with oxygen (helped by enzymes) to form new brown-coloured substances. This is a chemical change.

H.O.T.S.

When you buy a new water bottle, you find a little pouch of silica gel inside. Why is it kept in a water bottle? Can you use it at other places also?

Answer: Silica gel absorbs moisture. It keeps the inside of the bottle dry so that the bottle does not get a damp smell, fungus or (for a metal bottle) rust before it is sold. Yes, it is also used in packets of shoes, bags, medicines, electronic goods and food items to keep them dry.

A change may be both physical and chemical. Do you agree? Justify.

Answer: Yes. Some changes have both parts. For example, when a candle burns, the wax first melts (a physical change, as the wax can solidify again) and then the wax vapour burns to form carbon dioxide and water vapour (a chemical change).

Let us do: Magic with Chemistry

Secret message: On a blank paper, write a secret message to your friend with lemon juice. How does your friend read the message? Hold the paper above a lighted candle. The message will appear by itself.

Magical drink: Take some soap solution in a container. Dip a clean handkerchief (dipped in turmeric and dried beforehand) into the container. Lo and behold, your handkerchief will turn red.

Greeting card: You can design your own T-shirt or make a personalised greeting card with a difference for your friend. Apply a paste of turmeric on a piece of paper and let it dry completely. Now dip a brush in soap solution and paint a freehand drawing on the paper.