Changes - Physical and Chemical | Class 7 | Science | Chapter 13 | Maharashtra State Board
Explore the dynamic world of changes in this lesson! Delve into the distinction between physical and chemical changes, understanding the natural processes that shape our environment. Uncover the significance of useful changes that enhance our lives and the consequences of harmful changes on the planet. Investigate the rhythmic patterns of periodic changes and the unpredictable nature of non-periodic changes. Gain insights into the fundamental concepts of physical and chemical transformations. Join us on a journey of discovery, where science comes alive through engaging explanations and real-world examples.
Questions & Answers
1. Distinguish between the following.
(a) Physical change and Chemical change.
Ans.
| Physical change | Chemical change |
1. | In a physical change, the material involved in the change is structurally the same before and after the change. | A chemical change occurs when the substance’s composition is changed. |
2. | Physical changes are limited to changes that result in a difference in display without changing the composition. | This requires the breaking and forming of chemical bonds during a chemical reaction. |
3. | Some common physical changes include changes in texture, colour, temperature, shape, and state of matter. | This results in the rearranging of atoms in substances to form the products of a chemical reaction, which are brand new molecules that cannot be easily reverted back to their original state. |
4. | Examples of physical changes are boiling, melting, freezing, and shredding. | Examples of chemical changes are burning, cooking, rusting, and rotting. |
(b) Periodic change and Non-periodic change.
Ans.
Periodic change | Non-periodic change | |
1. | These are events that repeat themselves after a fixed period of time and their occurrence can be predicted. | These are events that do not repeat themselves after regular intervals of time. |
2. | For example, the occurrence of day and night is a periodic change. Other examples include the heartbeat of a person, the rotation and revolution of the earth, and seasonal changes. | For instance, the burning of a matchstick is a non-periodic change. Other examples include the eruption of a volcano, the running of a batsman between the wickets in a cricket match, and dancing. |
(c) Natural change and Man-made change.
Ans.
Natural change | Man-made change | |
1. | These are changes that occur naturally without the interference of human beings. They happen even if no human beings existed on the planet. | These are changes that occur due to the activities of humans. They are created by humans and do not occur naturally. |
2. | For example, the changing of seasons, day to night changes, flowering of a plant, and aging of plants. | For example, burning of fuels, making of chapattis, formation of curd, cutting hair, switching on lights, and cutting a lemon to extract its juice. |
2. Under which different types can the following changes be classified?
(a) Conversion of milk into yoghurt.
Ans. Man-made (useful).
(b) Bursting of cracker.
Ans. Man-made (fast change).
(c) Occurrence of an earthquake.
Ans. Non-periodic.
(d) Revolution of earth around the sun.
Ans. Periodic. (Natural change)
(e) Stretching of a spring.
Ans. Physical, reversible change.
3. Give reasons.
(a) While purchasing canned food, its expiry date should be checked.
Ans. Checking the expiry date on canned food is important to ensure food safety.
Expiry dates indicate the period during which the product is expected to remain safe for consumption. Consuming canned food beyond its expiry date may pose health risks due to potential spoilage, loss of nutritional value, or the development of harmful bacteria and toxins.
(b) An iron article should be given a coat of paint.
Ans. Providing an iron article with a coat of paint helps prevent corrosion. The paint acts as a protective barrier, shielding the iron from exposure to moisture and oxygen, which are key factors leading to rust formation. This protective layer enhances the durability and aesthetic appearance of the iron object, extending its lifespan.
(c) A Wooden article should be polished.
Ans. Polishing a wooden article serves to protect and enhance its appearance. The polish provides a protective layer that helps prevent damage from moisture, scratches, and other environmental factors. Additionally, it enriches the natural colour and grain of the wood, giving the article a more attractive and finished look. Polishing also contributes to the longevity of the wooden item by maintaining its structural integrity.
(d) Copper and brass utensils should be tinned.
Ans. Copper and brass utensils should be tinned to prevent corrosion. Tinning involves coating the utensils with a layer of tin, which acts as a protective barrier, inhibiting the reaction between the metal and acidic or reactive substances. This helps maintain the integrity of the utensils, prevents the development of harmful compounds, and enhances their durability.
(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.
Ans. A dry handkerchief quickly gets wet when dipped in water because the fabric absorbs water readily due to capillary action and the porous nature of the fibers. On the other hand, a wet handkerchief takes longer to dry because the water absorbed into the fabric must undergo evaporation, a process that requires heat energy and time to remove the moisture from the material.
4. What will you take into account while identifying the following:
(a) A physical change in a substance.
Ans. When identifying a physical change in a substance, consider the following:
1. Reversibility: Physical changes are usually reversible, and the substance can return to its original state.
2. No New Substance Formed: The chemical composition remains the same; no new substance is produced.
3. Energy Change: There is typically no significant change in energy during a physical change.
4. Change in State or Appearance: Physical changes involve alterations in the state (solid to liquid, etc.) or appearance of the substance.
5. No Gas or Precipitate Formation: Unlike chemical changes, physical changes do not result in the production of gas or the formation of a precipitate.
Examples include melting, freezing, dissolving, changes in shape, and changes in state.
(b) A chemical change in a substance.
Ans. When identifying a chemical change in a substance, consider the following:
1. Irreversibility: Chemical changes are often irreversible, and the substance transforms into one or more new substances.
2. New Substance Formed: A chemical change results in the formation of a new substance with a different chemical composition.
3. Energy Change: Chemical changes often involve the absorption or release of energy, manifesting as heat, light, or sound.
4. Colour Change: A noticeable change in colour may indicate a chemical reaction.
5. Gas or Precipitate Formation: Production of gas bubbles or the formation of a solid precipitate are common indicators of a chemical change.
Examples include burning wood, rusting of iron, cooking an egg, or the reaction between acids and bases. In these cases, the identity of the substance changes, and the process is not easily reversible.
5. Read the paragraph and note down the various types of changes it mentions.
It was nearing six o’clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lighting too. Sahil was enjoying his dinner in the dim light.
Ans.
1. Clock showing six o’clock time – It is a periodic change.
2. Sun was setting – A periodic change.
3. Breeze was blowing – A natural change.
4. Leaves shaking – A natural change.
5. Making toys from wet soil – Physical change.
6. Making dough from wheat flour – Physical change
7. Frying purees – Chemical change.
8. Raining – Natural change.
9. Lightning – Natural change.
E X T R A
1. Fill in the blanks and rewrite the sentences:
1. Burning of candle is a chemical change.
2. Cooking of vegetable is useful change.
3. A man painted his main gate made up of iron to prevent it from rusting.
4. Galvanization is a process used to prevent the rusting of iron.
5. Making sugar solution is a physical change.
6. A physical change is generally temporary.
7. Grinding of wheat grain changes its size, it is a physical change.
8. Iron benches kept in lawns and gardens get rusted. It is a change because a new chemical product is formed.
2. State whether the following statements are True or False. Correct and rewrite the false statement:
1. Iron and rust are the same substance.
Ans. False, Rust is oxide of iron.
2. Iron pipes coated with zinc do not get rusted easily.
Ans. True.
3. Formation of manure from leaves is a physical change.
Ans. False, it is a chemical change.
4. Cutting a log of wood into pieces is a chemical change.
Ans. False, it is a physical change.
5. Digestion of food is a chemical change.
Ans. True.
6. Melting of wax is a chemical change.
Ans. False, it is a physical change.
7. Dissolving sugar in water is physical change.
Ans. True.
8. Changes in which new substances are formed are called physical
changes.
Ans. False, they are called chemical change.
9. Breaking down of larger food particles into smaller particles is a chemical change.
Ans. False, it is a chemical change.
10. Setting of curd is a physical change.
Ans. False, it is a chemical change.
11. Heating of iron is a physical change.
Ans. True.
12. Cooking a vegetable is chemical change.
Ans. True.
13. Freezing water into ice is a chemical change.
Ans. False, it is a physical change.
3. Define the following.
1. Physical change:
Ans. A physical change is a transformation in the state or appearance of a substance without altering its chemical composition. These changes are typically reversible, involve no new substance formation, and often do not result in a significant energy change. Examples include changes in state (solid to liquid), changes in shape, or dissolving in a solvent.
2.Chemical change
Ans. A chemical change is a process in which one or more substances undergo a transformation, resulting in the formation of new substances with different chemical compositions. Chemical changes are typically irreversible, involve an energy change, and often exhibit signs such as colour change, gas production, or the formation of a precipitate. Examples include combustion, rusting of iron, and chemical reactions in living organisms.
3. Periodic change
Ans. A periodic change is a variation or fluctuation in a system that follows a regular and repeating pattern over time. These changes occur at consistent intervals and are characterized by predictability. Examples of periodic changes include the Earth's rotation leading to day and night, the oscillation of a pendulum, or the seasonal cycle.
4.Non-periodic change
Ans. A non-periodic change, also known as aperiodic or irregular change, does not follow a regular or repeating pattern over time. These changes lack consistent intervals and are often unpredictable. Examples include random weather events, economic fluctuations, or the behaviour of chaotic systems that exhibit irregular and complex patterns without a clear repetition.
5. Reversible change
Ans. A reversible change is a transformation in a substance that can be undone, restoring the original state or form. These changes do not alter the chemical composition of the substance. Examples include changes in state (solid to liquid), dissolving in a solvent, or changes in shape. Reversible changes are typically associated with physical transformations.
6. Irreversible change.
Ans. An irreversible change is a transformation in a substance that cannot be undone, resulting in the formation of new substances with different chemical compositions. These changes are often associated with chemical reactions and may involve significant energy changes. Examples include combustion, rusting of iron, or cooking an egg, where the original substances cannot be easily restored.
7. Condensation
Ans. Condensation is the phase transition process in which a substance changes from a gaseous state to a liquid state. This occurs when the temperature of a gas decreases, causing its molecules to lose energy and come together, forming liquid droplets. Common examples include the formation of dew on grass or water droplets on a cold surface.
8. Corrosion of metals
Ans. Corrosion of metals is the gradual deterioration of metal surfaces due to chemical reactions with the environment. It typically involves the formation of oxides or other compounds, compromising the metal's structural integrity. Common factors contributing to corrosion include exposure to moisture, oxygen, and various environmental pollutants. Rusting is a common example of corrosion in iron and steel.
9. Galvanization
Ans. Galvanization is the process of coating iron or steel with a layer of zinc to protect the metal from corrosion. This protective layer acts as a barrier, preventing direct contact between the iron or steel and the surrounding environment, particularly moisture and oxygen. Galvanization is a widely used method to enhance the durability and longevity of metal structures, such as fences, pipelines, and roofing materials.
10. Tinning
Ans. Tinning is the process of coating a metal, often copper or iron, with a thin layer of tin. This protective layer helps prevent corrosion, enhances the metal's resistance to oxidation, and improves its appearance. Tinning is commonly used for utensils, electrical wires, and other metal items to extend their lifespan and maintain their functionality.
11. Natural change
Ans. A natural change refers to alterations or processes that occur in the environment without direct human influence. These changes are typically driven by natural forces, such as geological, climatic, or biological processes. Examples include weathering of rocks, formation of mountains, and evolution of species. Natural changes are inherent to the Earth's systems and occur over various timescales.
12. Man-made change
Ans. A man-made change, also known as anthropogenic change, refers to alterations or transformations in the environment that result from human activities or interventions. These changes are directly influenced by human actions, such as urbanization, deforestation, industrialization, and pollution. Unlike natural changes, man-made changes are driven by human choices and behaviours, often impacting ecosystems and the Earth's overall environment.
13. Useful change
Ans. A useful change refers to a transformation or alteration in a system that brings about benefits or positive outcomes. This change is purposeful and often results in improvements, increased efficiency, or enhanced functionality. Examples include technological advancements, process optimizations, or innovations that contribute positively to society, industry, or individual well-being.
14. Harmful change
Ans. A harmful change refers to a transformation or alteration in a system that brings about negative consequences or adverse effects. This change is detrimental and often results in harm, damage, or negative impacts on the environment, society, or individual well-being. Examples include pollution, deforestation, or the release of harmful substances that can have detrimental effects on ecosystems and human health.
4. Name or Give two example of:
1. Two physical changes.
Ans. 1. Melting of wax
2. Melting of ice
2. Two chemical changes.
Ans. 1. Burning of wax candle
2. Burning of wood or paper.
3. Two periodic changes.
Ans. 1. Occurrence of day and night
2. change of seasons
4. Two non-periodic changes.
Ans. 1. A meteor moving across the sky.
2. A bird sitting on a tree flies away.
5. Two reversible changes.
Ans. 1. Melting of wax.
2. Freezing water into ice.
6. Two irreversible changes.
Ans. 1. Ripening of a mango.
2. Burning of wood
7. Two natural changes.
Ans. 1. Lighting in the sky.
2. Earthquake.
8. Two manmade changes.
Ans. 1. Baking bread
2. Cooking food.
9. Two useful changes.
Ans. 1. Ripening of mango
2. Curdling of milk.
10. Two harmful changes.
Ans. 1. Spoiling of milk.
2. Air pollution.
5. Distinguish between the following.
1. Reversible change and Irreversible change.
Ans.
Reversible change | Irreversible change | |
1. | The changes that can occur in a forward and reverse direction again and again is called reversible. | The changes which we cannot reverse back to something are called irreversible change. |
2. | E.g.: (a) Melting a wax candle and obtaining the original solid wax again, (b) Stretching a rubber or spring. | E.g.: Burning of paper or wood cannot be undone. Wood cannot be obtained from the ash. |
6. Under which different types can be following changes be classified:
1. Formation of curd from milk.
Ans. Man-made change (useful), chemical
2. Burning of wood or paper.
Ans. Man-made, chemical change.
3. Glowing of bulb.
Ans. Man-made (fast).
4. Rusting of iron.
Ans. Chemical change (harmful).
5. Burning of Camphor.
Ans. Chemical change.
6. Souring of milk.
Ans. chemical change (harmful).
7. Digestion of food.
Ans. Chemical change (useful), irreversible.
8. Change of season.
Ans. Periodic change, natural change.
9. Day and night formation.
Ans. Periodic change.
10. Heartbeat.
Ans. Periodic change.
11. Water pollution.
Ans. Man-made, harmful change.
12. Air pollution.
Ans. Man-made, harmful change.
13. Burning of fossil fuel.
Ans. Man-made (irreversible change).
14. Ripening of fruit.
Ans. Natural, chemical.
15. Change-in climate.
Ans. Natural, periodic.
16. Melting of ice into water.
Ans. Physical change.
17. Germination of seed.
Ans. Natural, slow, desirable.
18. Bursting of balloon.
Ans. Irreversible change (fast).
7. Give reasons:
The burning of wax is a chemical change.
Ans. 1. Because when the cotton wick embedded in the candle is burnt, ash and smoke is formed.
2. It is not possible to make wick again from the smoke and ash.
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