GSEB Class 7 Science Solutions Chapter 6 Physical and Chemical Changes

Question 1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f) Digestion of food
Answer:
(a) Chemical change
(b) Physical change
(c) Chemical change
(d) Physical change
(e) Physical change
(f) Chemical change
In simple words: This question asks you to decide if a change creates something new (chemical) or just alters its appearance (physical). Think about whether the material itself is fundamentally changed.

Exam Tip: For each process, ask yourself: "Is a new substance formed?" If yes, it's a chemical change. If no, it's a physical change.

Question 2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
Answer:
(a) False
Correct statement: Cutting a log of wood into pieces is an irreversible physical change.
(b) Formation of manure from leaves is a physical change. (True/False)
Answer:
False
Correct statement: Formation of manure from leaves is a chemical change.
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
Answer:
True
(d) Iron and rust are the same substances. (True/False)
Answer:
False
Correct statement: Iron and rust are two different chemical substances.
(e) Condensation of steam is not a chemical change. (True/False)
Answer:
True
In simple words: For each statement, you need to decide if it is correct or incorrect. If it's incorrect, you also need to write a sentence that makes it true. Focus on the core definition of physical versus chemical changes.

Exam Tip: Understand the definitions of physical and chemical changes thoroughly. A physical change alters appearance but not chemical composition, while a chemical change forms new substances.

Question 3. Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of __________.
(b) The chemical name of baking soda is __________.
(c) Two methods by which rusting of iron can be prevented are __________ and __________.
(d) Changes in which only __________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called __________ changes.
Answer:
(a) calcium carbonate
(b) sodium hydrogen carbonate
(c) painting or greasing, galvanisation
(d) physical
(e) chemical
In simple words: This question tests your knowledge of common chemical terms and processes. You need to remember the specific substances formed or names of chemical prevention methods.

Exam Tip: Memorize key terms and reactions, especially for common substances like baking soda and concepts like rusting and different types of changes.

Question 4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer: The reaction between baking soda and lemon juice can be given as below:
Lemon juice + Baking soda \( \longrightarrow \)
(Citric acid) (Sodium hydrogen carbonate)
\( \implies \) \( \text{CO}_2 \) (bubbles) + Other substances
(Carbon dioxide)
It is a chemical change. A new substance, carbon dioxide gas, is produced, which causes the bubbling. This type of reaction also produces new substances like water and sodium citrate.
In simple words: When baking soda and lemon juice combine, they make bubbles because a new gas, carbon dioxide, is created. This means it's a chemical change, not just a physical one.

Exam Tip: The formation of a new gas (bubbles), a change in temperature, or a color change often indicates a chemical change, not merely a physical one.

Question 5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Answer: When a candle burns, both physical and chemical changes occur:
• Physical change: The wax melts and turns into vapour. This is just a change in state, not a new substance.
• Chemical change: The wax vapours burn in the air to produce carbon dioxide, heat, and light. This forms new substances.
LPG (Liquefied Petroleum Gas) is another example where both types of changes happen. When LPG leaves the cylinder and turns from liquid to gas, it's a physical change. When the gas burns in the air, it's a chemical change.
In simple words: A burning candle shows two kinds of changes: the wax melts (physical change), and the melted wax burns to create new things like smoke and light (chemical change). Cooking food is another example: cutting vegetables is physical, but cooking them is chemical.

Exam Tip: Always identify the state change (melting, vaporization) for the physical aspect and the formation of new products (ash, gases, light, heat) for the chemical aspect.

Question 6. How would you show that setting of curd is a chemical change?
Answer: We can say that setting of curd is a chemical change because we cannot get the original substance, which is milk, back from the curd. Also, a new substance is formed that has a different taste, smell, and other chemical properties compared to milk. This irreversibility and formation of new characteristics are key indicators of a chemical transformation.
In simple words: Making curd from milk is a chemical change because you can't turn the curd back into milk, and the curd has a completely different taste, smell, and properties.

Exam Tip: Irreversibility and the formation of new substances with different properties are definitive signs of a chemical change. Focus on these points.

Question 7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer: Burning of wood is a chemical change because when wood burns, new substances like charcoal, carbon dioxide, heat, and light are formed. The original wood is transformed into entirely different materials. Conversely, cutting wood into small pieces is a physical change because no new substance is formed. The wood's chemical composition remains the same; only its size and shape are altered. We can only reduce the size of the wood, not change its fundamental nature.
In simple words: Burning wood is a chemical change because it makes new things like ash and smoke. Cutting wood into smaller pieces is a physical change because it's still wood, just smaller.

Exam Tip: Emphasize the formation of new substances (or lack thereof) to distinguish between chemical and physical changes. Chemical changes involve altering the molecular structure, while physical changes only affect the appearance.

Question 8. Describe how crystals of copper sulphate are prepared.
Answer: To prepare crystals of copper sulphate, take a cup full of water in a beaker and add a few drops of dilute sulphuric acid to it. Heat the water slowly. When it begins boiling, gradually add copper sulphate powder. Continue adding copper sulphate powder until no more powder can dissolve in the water. Throughout this process, continuously stir the solution to ensure proper mixing. Filter the hot solution to remove any impurities, then leave it for cooling in a undisturbed place. After some time, you will observe the formation of copper sulphate crystals. These crystals grow around any small seed crystals or imperfections as the solution cools.
In simple words: Heat water with a little acid, then slowly mix in copper sulphate powder until no more dissolves. Stir it well, filter it, and then let it cool slowly. Over time, you'll see copper sulphate crystals form.

Exam Tip: Remember the key steps: heating, dissolving to saturation, filtering, and slow cooling. Slow cooling is crucial for forming large, well-defined crystals.

Question 9. Explain how painting of an iron gate prevents it from rusting?
Answer: Painting an iron gate helps prevent rusting because it creates a protective barrier over the iron surface. It is well-known that the presence of oxygen and moisture is essential for rusting to occur. The paint layer physically blocks the iron from coming into direct contact with both oxygen and moisture present in the air. By isolating the iron from these two crucial components, painting effectively stops the chemical reaction that causes rust.
In simple words: Painting an iron gate stops it from rusting because the paint covers the iron. This cover stops air and water from touching the iron, and iron needs both air and water to rust.

Exam Tip: Focus on the concept of a protective barrier that prevents contact with oxygen and moisture – the two primary agents of rusting.

Question 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: Rusting of iron objects occurs faster in coastal areas compared to deserts because the air in coastal regions has a much higher moisture content. Rusting is a chemical process that requires both oxygen and water (moisture). Deserts, by contrast, have very low humidity and therefore less moisture in the air. Since coastal areas provide more of the necessary moisture, the chemical reaction that leads to rusting proceeds at a quicker rate there.
In simple words: Iron rusts faster near the sea because coastal air has more moisture, which iron needs to rust. Deserts have very dry air, so rusting is slower.

Exam Tip: Link the rate of rusting directly to the availability of moisture. Higher moisture content (like in coastal areas) accelerates the rusting process.

Question 11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change-A) then it burns (Change-B). The following statements pertain to these changes. Choose the correct one.
(i) Process-A is a chemical change.
(ii) Process-B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer: (ii) Process-B is a chemical change.
In simple words: When LPG changes from liquid to gas (Process A), it's a physical change. When it burns (Process B), new substances are made, so that's a chemical change.

Exam Tip: Remember that a change of state (liquid to gas) is a physical change, whereas combustion (burning) is always a chemical change because new substances are formed.

Question 12. Anaerobic bacteria digest animal waste and produce biogas (Change-A). The biogas is then burnt as fuel (Change-B). The following statements pertain to these changes. Choose the correct one.
(i) Process-A is a chemical change.
(ii) Process-B is a chemical change.
(iii) Both processes A and B are chemical change.
(iv) None of these processes is a chemical change.
Answer: (iii) Both processes A and B are chemical change.
In simple words: When bacteria turn waste into biogas, it's a chemical change because new things are made. When that biogas burns, it's also a chemical change because it produces heat, light, and other substances.

Exam Tip: Digestion and decomposition (Process-A) are biological processes involving chemical changes. Burning (Process-B) is also a chemical change. Both fundamentally alter the substances involved.

Extended Learning – Activities And Projects

Question 1. Describe two changes that are harmful. Explain why you consider them harmful. How can you prevent them?
Answer: Harmful changes:
• Rusting of iron
• Decaying of fruits.
Rusting of iron is harmful because it slowly damages and destroys iron articles. Since iron is commonly used in constructing bridges, ships, cars, and other structures, rusting can lead to enormous financial losses and safety hazards. Similarly, the decaying of fruits causes health hazards as it makes them unfit for consumption and can spread spoilage to other food items.
Prevention: Rusting can be prevented by applying oil, polishing, and painting iron surfaces. Fruits can be preserved by keeping them at a low temperature or by using specific preservatives to slow down decay.
In simple words: Rusting iron is bad because it breaks down important things like bridges and cars, costing a lot of money. Decaying fruit is bad because it can make us sick. You can stop rust with paint or oil, and keep fruit fresh by chilling it or using special preservers.

Exam Tip: When explaining harmful changes, clearly state the change, why it is harmful, and specific, practical prevention methods.

Question 2. Take three glass bottles with wide mouths. Label them A, B, and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water that has been boiled for several minutes, to the same level as in A. In bottle C, take the same boiled water and of the same amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so that it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.
Answer: In test tube A, which contains ordinary water, rusting occurs because the iron nail is in direct contact with both air and water. The air provides oxygen, and the water provides moisture, both of which are crucial for rusting. In test tube B, the nail becomes dull but does not rust as much. This is because the water was boiled, reducing the amount of dissolved oxygen, though some might still be present. In test tube C, there is no rusting because the cooking oil forms a layer on the surface of the boiled water, preventing the air (oxygen) from coming into contact with the iron nail. The boiled water itself also has less dissolved oxygen. From these observations, we can conclude that both air (oxygen) and water (moisture) are essential for rusting to take place.
In simple words: In bottle A, the nail rusts because it has both air and normal water. In bottle C, with boiled water and oil on top, the nail doesn't rust because the oil stops air from reaching it. This shows that both air and water are needed for iron to rust.

Exam Tip: This experiment demonstrates the necessary conditions for rusting. Clearly explain how each setup (A, B, C) manipulates the presence of air and water, and relate this to the observed rusting or lack thereof.

Question 3. Prepare crystals of alum.
Answer: Preparation of crystals of alum: Start by placing a beaker half-filled with water on a stand and heating it until it begins boiling. Mix some impure alum powder into the boiled water. Continue mixing the powder until no more alum powder can dissolve in the water, creating a saturated solution. Filter the hot solution into another clean beaker to remove any insoluble impurities. To obtain a large crystal, suspend a small, pure alum crystal in this solution using a thread. Allow the entire solution to cool very slowly, ideally for 24 hours, without disturbance. As the solution cools, the dissolved alum will gradually crystallize, and you will see a larger crystal of alum forming around the small seed crystal. The crystals can then be carefully removed by filtration.
In simple words: Heat water, then mix in alum powder until no more dissolves. Filter this hot liquid. Let it cool slowly for a day, and you'll see alum crystals grow.

Exam Tip: The key steps for crystal preparation are saturation (dissolving maximum solute), filtration (removing impurities), and slow cooling (allowing large crystals to form).

Question 4. Collect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/ others which fuels are less polluting and why.
Answer: The following types of fuels are typically used in our area:
• Solid fuels: These include coal, dung cake, and wood.
• Liquid fuel: Kerosene oil is a common liquid fuel.
• Gaseous fuel: LPG (Liquefied Petroleum Gas) is widely used.
We observe that solid fuels produce more smoke and unburnt particles when burned. Because of this, they are considered more polluting fuels and contribute significantly to air pollution. Liquid fuels, such as kerosene, are generally less polluting than solid fuels, as they produce less smoke and fewer harmful particles. Gaseous fuels, like LPG, are considered the cleanest fuels because they burn completely and produce very little smoke or harmful emissions, resulting in minimal pollution.
In simple words: In our area, people use solid fuels (like wood or coal), liquid fuels (like kerosene), and gas (LPG). Solid fuels make a lot of smoke and dirt, so they pollute a lot. Liquid fuels pollute less. Gaseous fuels like LPG are the cleanest because they burn cleanly and make very little pollution.

Exam Tip: When comparing fuels, link their polluting impact to the amount of smoke, unburnt particles, and harmful gases they release during combustion.