Get the simplified Class 7 Science Extra Questions and Class 7th Curiosity Chapter 5 Changes Around Us Physical and Chemical Important Extra Question Answer with complete explanation.
Class 7 Science Chapter 5 Changes Around Us Physical and Chemical Extra Questions
Class 7 Science Chapter 5 Extra Questions on Changes Around Us Physical and Chemical
Changes Around Us Physical and Chemical Class 7 Very Short Question Answer
Question 1.
What kind of change is shown by tearing of paper?
Answer:
Tearing of paper is a physical change as no new substance is formed.
Question 2.
Melting of wax is a change, where a solid changes to liquid state. Give one more such change, which you observe in your surroundings.
Answer:
Melting of ice is also a change, where solid changes into liquid state.
Question 3.
Give example of a physical change, which occurs by the action of heat.
Answer:
Melting of ice to form water is a physical change, which occurs by the action of heat.
Question 4.
How can you say that ripening of a fruit is a chemical change?
Answer:
Ripening of a fruit is a chemical change because after ripening, a new product with different properties is formed.
Question 5.
We should eat freshly cut apple. Why?
Answer:
We should eat freshly cut apple because if we leave the apple after cutting, it starts turn to brownish by the action of air on it and its food value decreases.
Question 6.
Complete the following reaction.
Calcium hydroxide + Carbon dioxide → ______ + ______
Answer:
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Question 7.
Name the gas, which turns lime water milky.
Answer:
Carbon dioxide gas (C02) turns lime water milky.
Question 8.
Write word equations for two chemical reactions with the help of materials given in the box.
Answer:
(i) Iron + Air + Water → Iron oxide
(ii) Vinegar + Baking soda → Carbon dioxide + Other substances
Question 9.
What colour of flame is observed when magnesium ribbon burnt in air?
Answer:
When magnesium is burnt in air then a brilliant white flame is observed.
Question 10.
What is the nature of magnesium oxide solution?
Answer:
Magnesium oxide solution is basic in nature because it turns red litmus solution to blue.
Question 11.
What else is required for combustion to occur besides fuel and heat?
Answer:
Oxygen is the main component of air which is needed besides fuel and heat as it supports the burning process.
Question 12.
Increased consumption of fuels in cars, trains, aeroplanes, etc, is desirable or undesirable change?
Answer:
Increased consumption of fuels in cars, trains, aeroplanes, etc. is undesirable change due to the increasing the amount of carbon dioxide in the atmosphere.
Question 13.
Basalt has chemically changed to produce which coloured layer?
Answer:
The black-coloured rock called basalt has chemically changed to produce a red-coloured layer.
Changes Around Us Physical and Chemical Class 7 Short Question Answer
Question 1.
Classify the following processes into physical or chemical changes.
(a) Beating of aluminium metal to make aluminium foil
(b) Cooking
(c) Breaking of glass
(d) Burning of papers
Answer:
Physical changes are beating of aluminium metal to make aluminium foil and breaking of glass. Chemical changes are cooking of food and burning of paper.
Question 2.
Is rusting of iron a physical or chemical change? Explain.
Answer:
Rusting of iron forms a new substance called iron oxide when iron reacts with air and moisture. Therefore, rusting of iron is a chemical change.
Question 3.
The process of digestion is a chemical change. Explain.
Answer:
In the process of digestion, the various food materials breakdown to form new substances, which can be absorbed by the body. So, the process of digestion is a chemical change.
Question 4.
Explosion of a cracker is a chemical change. Explain.
Answer:
When we burn a cracker, it explods. Explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere. Many new products are formed. So, it is a chemical change.
Question 5.
In addition to the formation of new products, what changes do the chemical changes accompany?
Answer:
A chemical change occurs, when two substances react chemically to form a new substance with different chemical properties. All new substances, which we use in various fields of our life, are produced as a result of chemical changes. These are also called chemical reactions, e.g. rusting of iron, cooking of food, etc.
Question 6.
Which type of change takes place in the following and state whether the energy is evolved or absorbed during the change?
Burning of a candle, lightning of a bulb, preparation of food by green plants, volcanic eruption, evaporation of water, cooking of food.
Answer:
- Burning of a candle Chemical change as well as physical change and energy is evolved.
- Lightning of a bulb Physical change, energy is evolved.
- Preparation of food by green plants Chemical change, energy is absorbed.
- Volcanic eruption Chemical change, energy is evolved.
- Evaporation of water Physical change as no new chemical substance is formed, energy is absorbed.
- Cooking of food Chemical change because new substance with different taste, smell and appearance are formed which cannot be reversed, energy is absorbed.
Question 7.
Explain the following.
(i) Lime water turns milky on passing carbon dioxide gas through it.
(ii) Bubbles are produced when vinegar is added to a solution of baking soda.
Answer:
(i) Aim To observe the difference between physical and chemical changes by blowing air into tap water and lime water, and understand the formation of new substances.
Materials Required Two glass tumblers or small transparent bottles, water (tap water), lime water (calcium hydroxide solution), two straws, and a marker (to label the tumblers).
| Activity | Observation(s) | Type of Change |
| Blowing air into tap water. (Tumbler A) | Air bubbles form, but no change in appearance of water. | No change |
| Blowing air into lime water. (Tumbler B) | Lime water turns milky due to formation of calcium carbonate. | Chemical change |
Conclusion:
We conclude that when carbon dioxide from exhaled air reacts with lime water, a new substance (calcium carbonate) is formed, which turns the lime water milky. This is a chemical change. In contrast, blowing air into tap water creates only bubbles and no new substance is formed.
(ii) Aim To observe the reaction between vinegar (or lemon juice) and baking soda, and identify whether it is a chemical or physical change.
Materials Required A test tube or two small used bottles, teaspoon of vinegar or lemon juice, a pinch of baking soda (sodium hydrogen carbonate), freshly prepared lime water, and a flexible straw (optional, for the bottle setup).
| Activity | Observation(s) | Type of Change |
| Adding vinegar or lemon juice to baking soda. | A fizzing or bubbling sound is observed and gas bubbles are formed in the test tube. | Chemical change |
| Passing the gas into lime water. | Lime water turns milky, indicating the formation of a new substance. | Chemical change |
| Mixing baking soda with water (without vinegar) | No fizzing or bubbling is observed. | Physical change |
Conclusion:
We conclude that the reaction between vinegar and baking soda produces carbon dioxide gas, which turns lime water milky, proving that a new substance has formed.
Vinegar + Baking soda → Carbon dioxide + Other substances.
This is a chemical change. In contrast, when baking soda is mixed with plain water, no new substance is formed, indicating a physical change.
Question 8.
Write the differences between physical and chemical changes.
Answer:
Differences between physical and chemical changes are as follows
| Physical change | Chemical change |
| No new substance is formed. | New substance is formed. |
| It is a temporary change | It is a permanent change. |
| Physical change is easily reversible. | Chemical change is irreversible. |
| Very little energy (heat, etc.) is absorbed or given out in a physical change. | A lot of energy (in the form of heat, light, sound etc.) is absorbed or given out in a chemical change. |
Question 9.
Magnesium ribbon burns in air and changes to white substance, i.e. magnesium oxide. When magnesium oxide dissolves in water, what type of change take place? Give reason in support of your answer. Express the change in the form of equation.
Answer:
Mixing of ash obtained by the burning of magnesium with water is a chemical change. When magnesium is burnt in air, it forms magnesium oxide in the form of white powder (ash).
Magnesium + Oxygen → Magnesium oxide + Heat + Light
When magnesium oxide dissolves in water, it forms a new substance, magnesium hydroxide.
Magnesium oxide + Water → Magnesium hydroxide
So, it is a chemical change.
Question 10.
State three conditions that are necessary for combustion?
Answer:
The three necessary condition for the combustion process to occur are following:
- A combustible substance, also called as fuel.
- Oxygen to support burning.
- Heat that allows the fuel to reach its ignition temperature.
Question 11.
How does the burning of a candle involve both physical and chemical changes?
Answer:
When a candle burns, the wax melts it becomes liquid, and sometimes solidifies again which is a physical change because the wax only changes state. The vapour of the melted wax burns in the flame, producing new substances like carbon dioxide which is a chemical change. Thus, burning a candle includes both physical (melting and solidifying) and chemical (burning) changes.
Question 12.
Why cannot a chemical change be normally reversed?
Answer:
In a chemical change, the new substances formed are quite different from the original ones. Therefore, a chemical change cannot be normally reversed.
Question 13.
Give two examples for each of the following cases
(i) Physical changes, which are reversible.
(ii) Physical changes, which are not reversible.
(iii) Chemical changes NCERT Exemplar
Answer:
(i) (a) Folding of paper (b) Melting of ice
(ii) (a) Tearing of paper (b) Breaking of glass
(iii) (a) Reaction between vinegar and baking soda. (b) Burning of a matchstick.
Question 14.
Explain with an example how chemical changes take place in rocks.
Answer:
Chemical changes in rocks happen when substances like water or air react with the minerals in the rocks, e.g., black basalt rock that contains iron can react with water vapour or air over time. This reaction forms iron oxide, which gives the rock a red-coloured surface. This change in colour and composition is a sign of a chemical change.
Question 15.
What is weathering, and how does it help in soil formation?
Answer:
Weathering is the process by which rocks undergo both physical and chemical changes due to natural factors like temperature, water, air, and plant growth. These changes break down rocks into smaller particles. Over a long time, this breakdown leads to the formation of soil, which is essential for supporting plant life.
Question 16.
Why are most erosion-related changes irreversible? Give an example.
Answer:
Most changes caused by erosion are irreversible because they occur slowly over thousands of years and involve permanent breaking down and relocation of rocks and soil, e.g., the smoothness of river rocks caused by continuous water flow cannot be reversed to their original rough form. Once these sediments settle and form new rocks, the original structure is lost forever.
Changes Around Us Physical and Chemical Class 7 Long Question Answer
Question 1.
Give an example of a chemical reaction for each of the following situations NCERT Exemplar
(i) Heat is evolved.
(ii) A gas is evolved.
(iii) Sound is produced.
Answer:
(i) When the magnesium ribbon is burnt, a new
substance, magnesium oxide is formed along with heat and light. So, the burning of magnesium is a chemical change.
Magnesium + Oxygen → Ribbon Air
Magnesium oxide + Heat + Light White powder
(ii) When baking soda and vinegar are mixed together then a chemical change takes place and bubbles of carbon dioxide gas are formed along with some other substances.
Baking soda + Vinegar → Carbon dioxide + Other substances
(iii) Explosion of a firework produces heat, light, sound, and unpleasant gases. Explosion of a firework is a chemical change.
Question 2.
If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.
(i) Do you think rust is different from iron?
(ii) Can you change rust back into iron by some simple method?
(iii) Do you think formation of rust on iron is a chemical change?
(iv) Give two other examples of a similar type of change.
Answer:
(i) Yes, rust is iron oxide (Fe2 03). Thus, rust and iron are not the same substance.
(ii) No, in the process of rusting, a new substance, rust (iron oxide) is formed. It cannot be reversed by any method.
(iii) Yes, rusting of iron is a chemical change. During the rusting of iron, it combines with the oxygen in the presence of water (moisture) to form a new compound ‘iron oxide’.
This iron oxide is a rust.
Iron + Oxygen + Water → Iron oxide (From damp air) (Rust)
It is a permanent change which cannot be reversed back.
So, rusting of iron is a chemical change.
(iv) Two other examples of chemical change are
(a) Setting of curd from milk.
(b) Burning of magnesium ribbon to form magnesium oxide.
Changes Around Us Physical and Chemical Class 7 Case Based Questions
Question 1.
Sania adds a piece of magnesium ribbon to a test tube containing dilute hydrochloric acid.
She observes the following events
I. The test tube gets hot.
II. Gas bubbles are formed.
III. The magnesium ribbon dissolves in the acid.
(i) Which observation(s) shows that a chemical change may have taken place in the test tube?
(ii) After observing the given events. Sania adds a few more magnesium ribbons to the same test tube. This time she observes tiny pieces of magnesium ribbon floating in the test tube. Sania thinks that the reaction was not completed.
What does sania need to do to complete the reaction? Add more magnesium ribbons to the test tube/Add more acid to the test tube.
(iii) Sania later repeats her activity using magnesium powder instead of magnesium ribbon. Will there be a change in her first three observations? Explain your answer.
Answer:
(i) The observations I, II and III all shows that a chemical change may have taken in the test tube.
(ii) Sania need to add more acid to the test tube to complete the reaction.
(iii) No, because magnesium powder and magnesium ribbon are made of the same substance, i.e. magnesium. Hence, both will react in the same way and give the same observations.
Question 2.
Anu observed a candle burning during a power cut. She noticed the wax near the wick melted into a small pool.
The wax slowly disappeared as the candle burned. The next day, she touched the remaining wax, and it was solid again.
(i) What type of change is the melting of wax – physical or chemical?
Answer:
Melting of wax is a physical change because it only changes state and no new substance is formed.
(ii) What kind of change happens when the wax burns and produces heat and light?
Answer:
Burning of wax is a chemical change as new substances like carbon dioxide and heat are produced.
(iii) Why can we say burning a candle involves both physical and chemical changes?
Answer:
Because melting of wax is a physical change and burning of wick is a chemical, therefore burning of candle shows both types of changes.
Changes Around Us Physical and Chemical Class 7 Skill Based Questions
Question 1.
Amit dissolved sugar in water and observed the change. His friend Raj took a little sugar in a spatula and directly heated it on a bunsen burner on a high flame. Answer the following questions.
(i) Which type of changes is observed by the Amit? Is it reversible or irreversible?
Answer:
The change observed by Amit is reversible.
(ii) Who of the two will observe chemical change?
Answer:
(ii) Raj will observe a chemical change.
Question 2.
Rahul was a student of class VII. His father purchased a new bicycle for him on his birthday. After few months, he found that the cycle chain and the handle gets rusted. His father advised him to apply a coating of paint to the cycle and not to keep it in the open in future.
Now, answer the following questions
(i) Why his cycle gets rusted?
Answer:
His cycle is rusted because iron reacts with air and moisture.
(ii) What do you mean by rusting of iron?
Answer:
Rusting is the formation of rust when iron reacts with water (moisture) and air (oxygen).
Question 3.
Abhi burned a substance X over the candle flame and white dazzling light is observed. Also white powdery ash left behind. He mixed the white ash formed with small amount of water. Then he dipped red and blue litmus papers into the solution one by one.
(i) Identify the substance X.
Answer:
The substance X is magnesium.
(ii) Which substance is formed after substance X .. gets burnt?
Answer:
When magnesium (X) is burnt in the presence of oxygen then magnesium oxide is formed.
(iii) What is the nature of solution formed after mixing the white ash into the water, acidic or basic?
Answer:
The nature of the solution formed after mixing the white ash into the water is basic.
Question 4.
At the base of a mountain, Anaya noticed heaps of small stones, soil, and sand. Her teacher explained that these are called sediments and are formed from weathering of rocks. She learned that physical forces like wind, rain, freezing water, and plant roots break large rocks into small pieces. Over a long time, chemicals in water or air can also change the colour and composition of some rocks, e.g. Black basalt rock changes to red when iron reacts with oxygen to form iron oxide.
Answer the following questions.
(i) What is weathering, and how does it contribute to the formation of soil?
Answer:
Weathering is the breaking down of rocks by physical and chemical processes. It contributes to soil formation by turning large rocks into smaller particles over time.
(ii) Give one example each of a physical and a chemical change involved in weathering.
Answer:
Physical change Breaking of rocks due to freezing water or plant roots. Chemical change Formation of iron oxide (rust) when iron in rocks reacts with air or water.
(iii) Why are weathering-related changes considered irreversible?
Answer:
Weathering changes are irreversible because the original rock cannot be restored and these changes take thousands of years.
Changes Around Us Physical and Chemical Extra Questions for Practice
Question 1.
What is the term used for the minimum temperature at which substance catches fire?
Question 2.
Which type of change is photosynthesis?
Question 3.
Which gas is released when vinegar is added to baking soda solution?
Question 4.
Differentiate between desirable and undesirable changes with examples.
Question 5.
Write the word equation when calcium hydroxide (lime water) is reacted with carbon dioxide, and state how can you test the carbon dioxide gas.
Question 6.
Write two points which tell us that formation of curd from milk is a chemical change.
Question 7.
What happens when magnesium is burned in air? Write a word equation for this process. Name the type of change which takes place.
Question 8.
Explain the process of rusting and how rusting damages iron objects?