Chapter 5

                                    Physical and Chemical Changes

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

Ans: (a) Photosynthesis - Chemical change

(b) Dissolving sugar in water - Physical change (c) Burning of coal - Chemical change (d) Melting of wax - Physical change (e) Beating aluminium to make aluminium foil - Physical change (f) Digestion of food - Chemical change


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)

Ans: False.


(b) Formation of manure from leaves is a physical change. (True/False)

Ans: False.


(c) Iron pipes coated with zinc do not get rusted easily. (True/False)

Ans: True.


(d) Iron and rust are the same substances. (True/False)

Ans: False.


(e) Condensation of steam is not a chemical change. (True/False)

Ans: True.


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 _________.

Ans: calcium carbonate.

(b) The chemical name of baking soda is _________.

Ans:  sodium bicarbonate.

(c) Two methods by which rusting of iron can be prevented are

_________ and _________.

Ans: Painting, Galvanization.


(d) Changes in which only _________ properties of a substance change

are called physical changes.

Ans:  physical.


(e) Changes in which new substances are formed are called _________

changes.

Ans: chemical changes.


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.

Ans: When baking soda (sodium bicarbonate) is mixed with lemon juice (containing citric acid), it's a chemical change. Here's why:

  • New substance formation: The reaction between baking soda and lemon juice produces a new gas - carbon dioxide (CO2). This is the gas that forms the bubbles you observe.
  • Chemical composition change:
    • Baking soda (NaHCO3) is a base.
    • Lemon juice contains citric acid (C6H8O7).
    • The reaction creates new substances: carbon dioxide (CO2), water (H2O), and sodium citrate (Na3C6H5O7).

The starting materials (baking soda and lemon juice) have different chemical compositions than the products formed (carbon dioxide, water, and sodium citrate). This change in composition signifies a chemical change.


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.

Ans: Physical change: Melting of wax is a physical change since it again turns into solid wax on cooling. Chemical change: The wax near to flame burns and gives new substances like carbon dioxide. 2) Lighting torch bulb using the dry cell is another example where both physical and chemical changes takes place.


6. How would you show that setting of curd is a chemical change?

Ans: The curd is formed from milk. Both curd and milk have different properties. So, there is a formation of a new substance with different properties and once the curd is formed, it cannot be reversed back to milk. Therefore, the setting of curd is a chemical change.


7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes

Ans: When we burn wood, it turns into ashes which is a new substance and the process is irreversible one, hence it is a chemical change. While cutting the wood into small pieces no new substance is formed. It is a physical change.


8. Describe how crystals of copper sulphate are prepared.

Ans: Crystal of pure copper sulphate can be obtained from an impure sample by re-crystallization. In re-crystallisation of copper sulphate the impure sample is dissolved in water, heated, and then cooled which then later forms pure crystals. These copper sulphate crystals are then separated by the filtration method.


9. Explain how painting of an iron gate prevents it from rusting.

Ans: Painting an iron gate prevents rusting by acting as a barrier between the iron and the elements that cause rust: oxygen and moisture (water vapor) in the air. Here's a breakdown of the process:

  1. Iron and Rust: Iron is susceptible to rusting, which occurs when it reacts with oxygen and moisture in the air. This reaction produces iron oxide, commonly known as rust, a reddish-brown flaky substance that weakens the iron.

  2. Paint as a Barrier: When you paint an iron gate, the paint forms a coating that acts as a physical barrier between the iron surface and the surrounding environment.

  3. Limiting Contact: This paint layer makes it difficult for oxygen and moisture to come into direct contact with the iron. Without these elements, the rusting reaction cannot readily occur, significantly slowing down or preventing rust formation.

  4. Types of Paint: Different paints offer varying levels of protection against rust. Here are some commonly used options:

    • Oil-based paints: These paints provide excellent water resistance and create a strong barrier against rust.
    • Alkyd paints: These are oil-modified paints that offer good water resistance and rust protection.
    • Acrylic paints: While generally less water-resistant than oil-based paints, some acrylic paints with rust-inhibiting additives can be used for iron surfaces.

Additional factors to consider:

  • Quality of Paint: Using high-quality paint specifically formulated for metal surfaces provides better protection.
  • Proper Application: Ensure thorough and even application of the paint to create a complete and effective barrier.
  • Maintenance: Over time, paint can chip or crack, exposing the iron underneath. Regularly inspect the paint and reapply as needed to maintain optimal protection.

By painting an iron gate and maintaining the paint job, you can significantly increase its lifespan and prevent rust formation.


10. Explain why rusting of iron objects is faster in coastal areas than in

deserts.

Ans: Rusting of iron occurs only if iron comes in contact with moist air. In coastal areas, air contains high percentage of moisture because of sea or ocean, while in deserts air is dry and hot. As the percentage of moisture in air is more in coastal area compared to desert, rusting becomes faster.


11.  The gas we use in the kitchen is called liquified petroleum gas (LPG). In

the cylinder it exist 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.

Ans: (iv) None of these processes is a chemical change.


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 changes. (iv) None of these processes is a chemical change

Ans: (iii) Both processes A and B are chemical changes.


1. Describe two changes that are harmful. Explain why you consider them

harmful. How can you prevent them?

Ans: 
  1. Rusting of Iron:
  • Harmful effects: Rusting weakens the structure of iron objects, reducing their lifespan and functionality. It can lead to failures in bridges, buildings, and machinery, potentially causing accidents and property damage. Additionally, rusting can contaminate surrounding materials and stain surfaces.
  • Prevention: Here are some ways to prevent rusting:
    • Painting: Apply a coat of paint to create a physical barrier between the iron and oxygen and moisture in the air.
    • Galvanization: Coat the iron with a layer of zinc. Zinc is more reactive than iron and will corrode first, protecting the iron underneath.
    • Oiling: For smaller objects, applying a thin coat of oil can provide temporary protection against rust.
    • Using stainless steel: Stainless steel is an alloy of iron that contains chromium, which helps form a protective oxide layer that resists rust.
  1. Food Spoilage:
  • Harmful effects: Spoiled food can harbor harmful bacteria that can cause foodborne illnesses. These illnesses can lead to vomiting, diarrhea, cramps, and other unpleasant symptoms. Additionally, spoiled food is often unappealing in taste and appearance, leading to food waste.
  • Prevention: Here are some ways to prevent food spoilage:
    • Proper storage: Store food at the appropriate temperature (refrigeration or freezing for perishables).
    • Maintain cleanliness: Keep your refrigerator and kitchen surfaces clean to minimize bacterial growth.
    • First-in, first-out (FIFO) principle: Use older food items first to avoid them sitting and spoiling.
    • Proper packaging: Store food in airtight containers to prevent exposure to air and contamination.
    • Freezing: Freezing food is an effective way to extend its shelf life by significantly slowing down bacterial growth.

By implementing these preventive measures, you can minimize rusting of iron objects and food spoilage, promoting safety, saving money, and reducing waste.


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 which 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 com pletely 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 few days. Take out nails from each bottle and observe them. Explain your observations.

Ans: 

Experiment: Observing Rust on Iron Nails

Materials:

  • 3 glass bottles with wide mouths (labeled A, B, and C)
  • Tap water
  • Pot for boiling water
  • Stove
  • Iron nails (all similar size and quality)
  • Cooking oil
  • Teaspoon
  • Paper towels

Procedure:

  1. Fill bottle A with tap water to about half its volume.
  2. Boil tap water for several minutes.
  3. Fill bottle B with the boiled water to the same level as bottle A.
  4. Fill bottle C with the same amount of boiled water as the other bottles.
  5. Add a few iron nails to each bottle, ensuring they are completely submerged in the water.
  6. Add a teaspoon of cooking oil to bottle C only. Swirl gently to create a thin film of oil on the water surface.
  7. Label the bottles clearly (A, B, and C) and place them somewhere undisturbed for a few days.

Observations:

After a few days, take out the nails from each bottle and observe the following:

  • Bottle A (tap water): You will likely see rust on the nails. Tap water contains dissolved oxygen that can react with iron to form rust (iron oxide).
  • Bottle B (boiled water): The nails might show some minor rusting, but it will likely be less severe compared to bottle A. Boiling water removes some of the dissolved oxygen, slowing down the rusting process.
  • Bottle C (boiled water with oil): The nails should show minimal to no rust. The oil layer creates a barrier between the water and the air, preventing oxygen from reaching the nails and significantly inhibiting rust formation.

Explanation:

Rusting of iron occurs when iron reacts with oxygen and moisture (water vapor) in the air. This reaction is accelerated by the presence of dissolved oxygen in water.

  • Bottle A: The tap water contains dissolved oxygen, which readily reacts with the iron nails to form rust.
  • Bottle B: Boiling water removes some of the dissolved oxygen, reducing the available oxygen for the reaction. This can slow down rusting but might not completely prevent it, as some oxygen can re-dissolve from the air over time.
  • Bottle C: The oil layer acts as a physical barrier. It prevents oxygen from the air from dissolving in the water and reaching the iron nails, effectively stopping the rusting process.

Conclusion:

This experiment demonstrates the factors affecting rust formation on iron. By controlling the presence of oxygen and moisture, we can influence the rate of rusting. Techniques like boiling water and using oil coatings can be used to minimize rust on iron objects.

3. Prepare crystals of alum. 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.

Ans: 


Type by Jyotishna Hazarika


Colour By: Himashree Bora.