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Class 7 Science NCERT Notes – Chapter 5: Physical and Chemical Changes Around Us (PDF, MindMap, Q&A, Quizzes)
ByAKS
Chapter 5 (Science): Physical and Chemical Changes Around Us – CBSE Class 7 NCERT Science Detailed Study Notes.
1. Observing and Categorizing Changes
Changes are constantly occurring in our surroundings. We observe these changes through our senses of sight, smell, touch, hearing, and taste. These alterations can affect a substance’s size, shape, smell, or other properties. To understand them better, we can categorize changes into different types, primarily as physical or chemical changes.
Examples of common changes include:
- Melting ice cubes
- Boiling water
- Burning wood
- Drying wet clothes
- Cutting a piece of paper
- Making popcorn from corn
2. Physical Changes
A physical change is a change in which only the physical properties of a substance, such as its shape, size, and state (solid, liquid, gas), are altered. Crucially, in a physical change, no new substance is formed. The original material remains the same, even if its appearance is different.
Key Characteristics:
- Alters physical properties (shape, size, state).
- No new substances are created.
- Many physical changes are reversible.
Examples from the Text:
- Folding Paper: Creating objects like airplanes or boats by folding paper changes its shape, but it is still paper. Unfolding it returns it to its original form.
- Inflating a Balloon: Inflating a balloon changes its size and shape. If the air is let out, the balloon returns to its original state. However, if the balloon is pricked, its shape changes permanently, but the material (rubber) is still the same.
- Crushing Chalk: Grinding a piece of chalk into powder changes its size and form, but the substance is still chalk.
- Changes in the State of Water: Water changing between its solid (ice), liquid (water), and gaseous (steam) states is a classic example of a physical change.
- Melting and Boiling: Melting ice cubes and boiling water are physical changes related to a change in state.
3. Chemical Changes
A chemical change is a process in which one or more new substances are formed. These new substances have different properties from the original materials. Chemical changes occur through a process called a chemical reaction, which can often be represented by a chemical equation.
Key Characteristics:
- Results in the formation of one or more new substances.
- Involves a chemical reaction.
- Often accompanied by observable signs like the production of heat, light, sound (fizzing), or a change in color.
- Chemical changes are generally irreversible.
Examples from the Text:
- Blowing Exhaled Air into Lime Water: When carbon dioxide (from exhaled air) is blown into lime water (calcium hydroxide), the lime water turns milky or cloudy. This indicates the formation of a new, insoluble white substance called calcium carbonate. The chemical equation is:
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water - Vinegar and Baking Soda Reaction: Mixing vinegar (or lemon juice) with baking soda (sodium hydrogen carbonate) produces a fizzing sound and gas bubbles. This gas is carbon dioxide, which can be confirmed by passing it through lime water and observing it turn milky.
- Burning a Magnesium Ribbon: When a magnesium ribbon is burnt, it reacts with oxygen from the air to form a new substance, magnesium oxide (a white powder). This reaction also produces heat and light. The chemical equation is:
Magnesium + Oxygen → Magnesium oxide + Heat + Light
4. Combustion: A Type of Chemical Change
Combustion is a specific type of chemical reaction where a substance reacts with oxygen to produce heat and/or light. Substances that can undergo combustion are called combustible substances (e.g., wood, paper, cotton, kerosene).
The Three Requirements for Combustion (The Fire Triangle):
- Combustible Substance (Fuel): The material that will burn.
- Oxygen: A component of air that supports combustion. An experiment with two candles, one covered by a glass tumbler, demonstrates that a continuous supply of air (oxygen) is necessary for burning. The covered candle extinguishes once the oxygen is used up, producing carbon dioxide.
- Heat: The fuel must be heated to its ignition temperature, which is the minimum temperature at which a substance catches fire. A substance will not burn, even in the presence of oxygen, if it is not heated to this temperature.
5. Simultaneous Physical and Chemical Changes
Some processes involve both physical and chemical changes occurring at the same time. The burning of a candle is a prime example.
- Physical Changes: The heat from the flame melts the solid wax into liquid wax. This liquid wax is drawn up the wick and evaporates into wax vapor. The melting, solidification (if it drips and cools), and evaporation of wax are all physical changes.
- Chemical Change: The wax vapor reacts with oxygen in the air (combustion) to produce a flame, heat, light, and new substances like carbon dioxide. This burning of the vapor is a chemical change.
6. Classifying Changes by Reversibility and Desirability
Changes can also be grouped based on whether they can be undone or whether they are useful.
Reversible and Irreversible Changes
- Reversible Changes: These are changes where the original substance or object can be recovered. Many physical changes, like melting ice (it can be refrozen) or boiling water (it can be condensed), are reversible.
- Irreversible Changes: These are changes where the original substance cannot be brought back. Most chemical changes are irreversible. Examples include making popcorn from corn, chopping vegetables, and ripening of fruits.
Desirable and Undesirable Changes
- Desirable Changes: These are changes that are useful or beneficial to us. Examples include the ripening of fruits, the changing of milk into curd, and the cooking of food.
- Undesirable Changes: These are changes that are harmful or not wanted. Examples include the rusting of iron and the decay of food.
- Context-Dependent Changes: A change can be undesirable in one context but desirable in another. For example, the decomposition of food is undesirable when it’s meant for consumption, but it is desirable for converting food waste into useful compost.
7. Slow Natural Changes
Nature is host to large-scale, slow changes that involve both physical and chemical processes.
Weathering of Rocks
Weathering is the collective term for the physical and chemical processes that break down rocks.
- Physical Weathering: Large rocks are broken into smaller pieces without changing their chemical composition. This can be caused by temperature fluctuations, the freezing of water in cracks, and the growth of tree roots.
- Chemical Weathering: The chemical composition of the rock is altered. For example, water or chemicals in water can react with minerals in the rock. A black basalt rock containing iron can turn red due to the formation of iron oxide when exposed to air and water over time.
- Result of Weathering: Over long periods, weathering leads to the formation of soil.
Erosion
Erosion is a physical process where rock fragments, soil, and sediments are broken down and transported from one place to another by natural forces like flowing water and wind. The smoothing of river pebbles is an example of erosion. When these forces slow down, the transported material (sediments) settles and can harden over thousands of years to form new rocks.
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II. Quiz and Q&A Section
Short-Answer Questions
Answer each question in 2-3 sentences.
- What is the definition of a physical change?
- Provide two examples of physical changes from the source text.
- What is the key characteristic that distinguishes a chemical change from a physical change?
- Describe what happens when you blow exhaled air into lime water.
- What new substance is formed when carbon dioxide reacts with lime water?
- What is the process that forms new substances during a chemical change called?
- What is combustion?
- List the three essential requirements for combustion to occur.
- Define “ignition temperature.”
- What are combustible substances? Give one example.
- Explain why a candle covered with a glass tumbler extinguishes after a short time.
- How can you test for the presence of carbon dioxide produced by a burning candle?
- Describe the physical changes that occur when a candle burns.
- Describe the chemical change that occurs when a candle burns.
- What is a reversible change? Provide an example.
- What is an irreversible change? Provide an example.
- Are all changes desirable? Explain with an example.
- Give an example of a change that can be desirable in one situation but undesirable in another.
- What is weathering of rocks?
- Differentiate between physical weathering and chemical weathering.
- What is the eventual result of the weathering of rocks?
- What is erosion, and is it a physical or chemical change?
- What happens when the magnesium ribbon is burnt in the air?
- How can a blanket be used to extinguish a fire on a person’s clothes?
- What is bioluminescence, as mentioned in the text?
Multiple-Choice Questions (MCQs)
Choose the best answer for each question.
- Which of the following is an example of a physical change?
- a) Burning wood b) Rusting of iron c) Melting ice cubes d) Curdling of milk
- A chemical change is a process in which:
- a) Only the state of a substance changes. b) One or more new substances are formed. c) The shape of an object is altered. d) The change is always easily reversible.
- The turning of lime water milky is used as a test for which gas?
- a) Oxygen b) Nitrogen c) Carbon dioxide d) Hydrogen
- What are the products of the reaction between calcium hydroxide (lime water) and carbon dioxide?
- a) Calcium oxide and water b) Calcium carbonate and oxygen c) Calcium carbonate and water d) Calcium and carbon hydroxide
- The reaction of a substance with oxygen that produces heat and/or light is called:
- a) Erosion b) Weathering c) Evaporation d) Combustion
- Which of the following is NOT required for combustion?
- a) Fuel (combustible substance) b) Carbon dioxide c) Oxygen d) Heat to reach ignition temperature
- The minimum temperature at which a substance catches fire is its:
- a) Boiling point b) Melting point c) Ignition temperature d) Freezing point
- The burning of a candle involves:
- a) Only a physical change b) Only a chemical change c) Both physical and chemical changes d) Neither a physical nor a chemical change
- Which of these is an example of an irreversible change?
- a) Boiling water b) Folding paper c) Making popcorn from corn d) Inflating a balloon
- The decomposition of food waste into compost is an example of a:
- a) Desirable chemical change b) Undesirable physical change c) Reversible physical change d) Desirable physical change
- The process that breaks down rocks and transports the pieces via wind and water is:
- a) Weathering b) Sedimentation c) Combustion d) Erosion
- Chemical weathering of a basalt rock containing iron can result in the formation of:
- a) Magnesium oxide b) Calcium carbonate c) Iron oxide d) Sodium hydrogen carbonate
- Burning a magnesium ribbon is a chemical change because:
- a) The ribbon changes shape. b) A new substance, magnesium oxide, is formed. c) It produces light. d) It can be reversed by cooling.
- Which of the following is a combustible substance?
- a) Water b) Sand c) Paper d) Iron nail
- What are the two main types of changes discussed in the text?
- a) Fast and Slow b) Reversible and Irreversible c) Physical and Chemical d) Natural and Artificial
- What physical process is responsible for the smoothing of river pebbles?
- a) Combustion b) Weathering c) Erosion d) Condensation
- Michael Faraday used which object to discuss various physical and chemical processes?
- a) A balloon b) A piece of paper c) A candle d) A magnesium ribbon
- Mixing vinegar and baking soda produces which gas?
- a) Oxygen b) Nitrogen c) Carbon dioxide d) Water vapor
- Freezing of water within cracks in rocks causing them to break is an example of:
- a) Chemical erosion b) Physical weathering c) Chemical weathering d) Combustion
- Which of the following changes is reversible?
- a) Burning of wood b) Ripening of fruits c) Dissolving sugar in water d) Grinding wheat grains to flour
III. Essay Questions and Answers
- Explain in detail what a physical change is, providing at least three distinct examples from the source text and explaining why each is a physical change.
- Answer: A physical change is a change where a substance’s physical properties, such as its shape, size, or state, are altered, but no new substance is formed. The chemical identity of the substance remains the same. The source text provides several examples. First, folding a sheet of paper into an object like a boat is a physical change because only its shape is altered; it is still paper and can be unfolded. Second, crushing a piece of chalk into powder is a physical change because while the size of the particles has been reduced, the substance is still chalk. Third, the change of water from a solid (ice) to a liquid (water) is a physical change; the state has changed, but the H₂O molecule itself has not, and the process is reversible through freezing.
- Describe the experiment involving lime water and exhaled breath. Explain what happens, why it happens, and what scientific principle it demonstrates.
- Answer: The experiment involves taking two glass tumblers, one with tap water and one with lime water, and blowing exhaled air into each through a straw. In the tumbler with tap water, only bubbles are formed. In the tumbler with lime water (calcium hydroxide), the liquid turns milky or cloudy. This happens because the exhaled air contains carbon dioxide, which undergoes a chemical reaction with the lime water. This reaction forms a new substance, calcium carbonate, which is a white solid that is insoluble in water, causing the milky appearance. This experiment demonstrates a chemical change, where a new substance is formed, and it also serves as the standard chemical test for the presence of carbon dioxide.
- Define combustion and detail the three necessary components for it to occur, often referred to as the “fire triangle.” Use an example from the text to illustrate these components.
- Answer: Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. For combustion to occur, three components are required, forming what is known as the “fire triangle.” The first is a combustible substance, also called a fuel. The second is a supply of oxygen, which is a component of air. The third is heat, which is required to raise the temperature of the fuel to its ignition temperature—the minimum temperature at which it will catch fire. An example is burning a piece of paper with a lighted matchstick. The paper is the combustible substance (fuel), the air provides the oxygen, and the flame from the matchstick provides the heat to raise the paper to its ignition temperature, causing it to burn.
- Using the burning of a candle as an example, explain how a single process can involve both physical and chemical changes simultaneously.
- Answer: The burning of a candle is a perfect example of a process that involves both physical and chemical changes. The physical changes begin when the heat from the flame melts the solid candle wax into a liquid. This liquid wax is then drawn up the wick through capillary action and, due to the intense heat, evaporates into a gas (wax vapor). The melting of the solid to a liquid and the evaporation of the liquid to a gas are both physical changes in state. The chemical change occurs when this wax vapor reacts with oxygen from the surrounding air. This reaction is combustion, which produces the flame, light, heat, and new substances like carbon dioxide and water vapor. Therefore, the melting and vaporizing of wax are physical changes, while the burning of the vapor is a chemical change.
- Differentiate between reversible and irreversible changes. Provide two examples of each from the text and explain your reasoning.
- Answer: Reversible changes are transformations where the original object or substance can be returned to its initial state. An example is melting ice cubes; the resulting water can be frozen back into ice. Another reversible change is boiling water; the steam can be condensed back into liquid water. Irreversible changes, on the other hand, are transformations that cannot be undone to recover the original substance. For instance, making popcorn from corn is irreversible because the popped corn cannot be turned back into a kernel. Similarly, chopping vegetables is irreversible because the smaller pieces cannot be reassembled into the original whole vegetable.
- Explain the concepts of desirable and undesirable changes. Provide an example of a change that could be considered both, depending on the context.
- Answer: Desirable changes are those that are useful or beneficial, such as the ripening of fruits to make them edible or cooking food to make it digestible. Undesirable changes are harmful or unwanted, such as the rusting of an iron gate, which weakens it, or the decay of stored food, making it inedible. Some changes can be both desirable and undesirable depending on the situation. The decomposition of food is a key example. It is an undesirable change when food meant for eating spoils, but it is a highly desirable change when food waste is intentionally decomposed to create compost, a valuable fertilizer.
- What is weathering? Describe the two types of weathering mentioned in the text and how they contribute to the formation of soil.
- Answer: Weathering is the collective process of physical and chemical changes that break down rocks. The first type, physical weathering, involves breaking large rocks into smaller pieces without altering their chemical makeup. This can be caused by climatic temperature changes, the expansion of freezing water in rock cracks, or the pressure from growing tree roots. The second type, chemical weathering, occurs when water or other chemicals react with the minerals in the rock, changing its composition. An example is a black basalt rock containing iron turning red after long exposure to water and air, forming iron oxide (rust). Both of these processes work over very long periods to break down rocks into smaller and smaller particles, which eventually leads to the formation of soil.
- Describe the experiment with vinegar and baking soda. What observations indicate a chemical change, and how is the product of this reaction identified?
- Answer: In this experiment, a teaspoon of vinegar or lemon juice is placed in a test tube, and a pinch of baking soda is added. Upon mixing, one would observe a fizzing or bubbling sound and see gas bubbles forming. These observations indicate that a chemical reaction is occurring and a new substance, a gas, is being produced. To identify this gas, it is passed through freshly prepared lime water in another test tube. The lime water turns milky, which is the definitive test for carbon dioxide. This confirms that the reaction between vinegar and baking soda is a chemical change that produces carbon dioxide gas.
- Why is oxygen essential for combustion? Describe the candle experiment that demonstrates this principle.
- Answer: Oxygen is a key reactant in the chemical process of combustion; it is the component of air that supports burning. The text describes an experiment to demonstrate this: two identical candles are lit. One is left uncovered, while the other is covered with an inverted glass tumbler. The uncovered candle continues to burn because it has a continuous supply of air, and therefore oxygen. The covered candle, however, stops burning after a short time because it uses up the limited supply of oxygen trapped inside the tumbler. This experiment clearly shows that oxygen is essential for combustion to continue.
- What is erosion? Explain how this natural process works and why it is classified as a physical change.
- Answer: Erosion is a natural process where rock pebbles, soil, and other sediments are broken down and moved from one location to another by forces like wind and flowing water. For example, fine sand found on riverbeds is formed as erosion breaks down rocks and transports the particles. This process is classified as a physical change because it only involves the breaking down and movement of existing materials, not the creation of new chemical substances. The composition of the rock fragments and soil remains the same; only their size and location are changed.
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IV. Answer Keys
Short-Answer Question Answer Key
- A physical change is one in which only physical properties like shape, size, and state of a substance change, and no new substance is formed. The original material remains chemically the same.
- Two examples are folding a piece of paper, which only changes its shape, and melting an ice cube, which only changes its state from solid to liquid.
- The key distinction is the formation of a new substance. In a chemical change, one or more new substances are formed, whereas in a physical change, no new substances are created.
- When exhaled air is blown into lime water, the lime water turns milky or cloudy. This is because the carbon dioxide in the breath reacts with the lime water.
- The new substance formed is calcium carbonate, a white, insoluble solid that causes the milky appearance. A small amount of water is also formed.
- The process that forms new substances during a chemical change is called a chemical reaction.
- Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
- The three requirements for combustion are a combustible substance (fuel), oxygen, and heat that allows the fuel to reach its ignition temperature.
- Ignition temperature is the minimum or lowest temperature at which a substance can catch fire and begin to burn.
- Combustible substances are materials that undergo combustion. Examples from the text include wood, paper, cotton, and kerosene.
- A covered candle extinguishes because it is cut off from a continuous supply of air. It uses up the limited amount of oxygen inside the tumbler and then the flame goes out.
- You can test for the carbon dioxide by adding a small amount of lime water to the petri dish under the tumbler. The lime water will turn milky, confirming the presence of CO2.
- The physical changes are the melting of the solid wax into a liquid and the evaporation of that liquid wax into a vapor due to the heat of the flame.
- The chemical change is the combustion of the wax vapor. The vapor reacts with oxygen in the air to produce a flame, heat, light, and new substances.
- A reversible change is a change that can be undone to get the original substance back. An example is melting ice, which can be refrozen back into ice.
- An irreversible change is one that cannot be undone to get the original substance back. An example is making popcorn, as you cannot turn it back into a corn kernel.
- No, not all changes are desirable. For example, the rusting of iron or the decay of food are undesirable changes because they are harmful or cause damage.
- The decomposition of food is undesirable when it spoils food for consumption. However, it is desirable when it is used to convert food waste into compost.
- Weathering is the collective term for the physical and chemical changes that cause rocks to break down over time.
- Physical weathering breaks rocks into smaller pieces without changing their chemical makeup (e.g., from freezing water). Chemical weathering alters the rock’s chemical composition (e.g., iron in rock reacting with water to form iron oxide).
- The eventual result of the weathering of rocks over a very long time is the formation of soil.
- Erosion is a physical process where natural forces like wind and flowing water break down and move rock, soil, and sediments from one place to another.
- When a magnesium ribbon is burnt, it undergoes a chemical change by reacting with oxygen to form a new substance, magnesium oxide. This reaction also produces heat and light.
- Wrapping a person in a blanket cuts off the supply of air (and thus oxygen) to the fire. Without oxygen, the combustion process stops, and the fire is extinguished.
- Bioluminescence is the production of light (without heat) in living organisms, caused by a chemical change. An example given is the light produced by fireflies.
MCQ Answer Key
- c) Melting ice cubes
- b) One or more new substances are formed.
- c) Carbon dioxide
- c) Calcium carbonate and water
- d) Combustion
- b) Carbon dioxide
- c) Ignition temperature
- c) Both physical and chemical changes
- c) Making popcorn from corn
- a) Desirable chemical change
- d) Erosion
- c) Iron oxide
- b) A new substance, magnesium oxide, is formed.
- c) Paper
- c) Physical and Chemical
- c) Erosion
- c) A candle
- c) Carbon dioxide
- b) Physical weathering
- c) Dissolving sugar in water
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V. Glossary of Key Terms
| Term | Definition |
| Bioluminescence | The production of light (without heat) in living organisms, caused by a chemical change. |
| Chemical Change | A change in which one or more new substances are formed. |
| Chemical Equation | A short-form representation of a chemical reaction. |
| Chemical Reaction | A process through which new substances are formed. |
| Combustible Substance | A substance that can undergo a combustion reaction (e.g., wood, paper). |
| Combustion | A chemical reaction in which a substance reacts with oxygen and produces heat and/or light. |
| Desirable Change | A change that is useful or beneficial. |
| Erosion | The process where rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water. |
| Ignition Temperature | The minimum temperature at which a substance catches fire. |
| Irreversible Change | A change that cannot be reversed to get the original object or substance back. |
| Physical Change | A change in which only physical properties like shape, size, and state of a substance are altered, with no new substance being formed. |
| Reversible Change | A change that can be undone, allowing the original object or substance to be brought back. |
| Sediments | Heaps of sand, soil, and stones, often formed by the weathering of rocks or deposited by erosion. |
| Undesirable Change | A change that is harmful or not wanted. |
| Weathering | The collective physical and chemical changes in rocks that lead to their breakdown and eventually the formation of soil. |