Chapter 6- Physical and Chemical Changes – Long Questions and Answers(Solved Worksheet)
Table of Contents
Question: What is the fundamental difference between a physical change and a chemical change, and how can you identify them?
Answer:
- Difference: A physical change alters the form or appearance of a substance without changing its chemical composition, while a chemical change transforms a substance into another by altering its chemical composition.
- Identification: Physical changes are often reversible, cause no new substances to form, and involve changes in shape, size, or phase. Chemical changes are typically irreversible, result in the formation of new substances, and exhibit indicators such as colour change, gas production, precipitate formation, and temperature change.
Also Check – 8 Differences between Physical and Chemical Changes
Question: Explain why the melting of ice cream is considered a physical change, despite its transformation from a solid to a liquid.
Answer: The melting of ice cream is a physical change because it involves a change in its state from a solid to a liquid while maintaining its chemical composition (H2O). It is reversible; you can refreeze the melted ice cream back into its original form, demonstrating the characteristics of a physical change.
Question: Describe a scenario in which recognizing a chemical change in cooking is essential. How does understanding this concept improve culinary skills?
Answer: Recognizing a chemical change in cooking is crucial when baking bread. During yeast fermentation, the dough undergoes a chemical change, producing carbon dioxide gas and causing the dough to rise. Understanding this process allows bakers to create light and fluffy bread, showcasing the importance of chemical changes in culinary skills.
Question: Why is it significant to know that some physical changes are reversible, even if not perfectly, in daily life? Provide an example where this knowledge could be applied.
Answer: Understanding reversible physical changes is important for practical applications like recycling plastics. While plastic recycling may not result in the same quality, it showcases how materials can be reshaped and repurposed, contributing to sustainability efforts.
Question: In the chalk dust experiment, what does the reconstitution of chalk dust into a chalk-like form demonstrate about physical changes? Why might the reconstituted chalk be less effective?
Answer: The reconstitution of chalk dust into a chalk-like form demonstrates the reversibility aspect of physical changes. It shows that some materials can return to their original or near-original form through physical manipulation. However, the reconstituted chalk might be less effective because it lacks binders and additives present in manufactured chalk, impacting its structural integrity and functionality.
Question: Explain why stretching a rubber band is considered a physical change, and what property remains unchanged throughout this process?
Answer: Stretching a rubber band is a physical change because it changes the shape and size of the rubber band without altering its chemical composition or inherent properties. Throughout this process, the inherent property of the rubber band, which is being rubber, remains unchanged.
Question: Describe the chemical changes involved when baking bread, and why is this process classified as a chemical change?
Answer: Baking bread involves chemical changes, such as the conversion of dough into bread through yeast fermentation. This process is classified as a chemical change because it results in the formation of new substances, including carbon dioxide gas and ethanol, through the action of yeast on the dough’s components.
Question: How does the ripening of fruits exemplify a chemical change, and what specific chemical processes occur during this ripening process?
Answer: The ripening of fruits exemplifies a chemical change due to various chemical processes that occur. These processes include the breakdown of starches into sugars, changes in flavour and texture, and the production of aromatic compounds. The transformation of fruit’s chemical composition leads to its ripened state.
Question: In the context of the water cycle, explain the physical changes associated with water changing from a liquid to a gas and back to a liquid. How does this cycle illustrate physical changes?
Answer: In the water cycle, water undergoes physical changes. When water evaporates from bodies of water, it changes from a liquid to a gas (vaporisation), and when it condenses in the atmosphere to form clouds, it changes from a gas back to a liquid (condensation). This cycle illustrates physical changes because the substance, which is water, changes states between liquid and gas without altering its chemical composition.
Question: Explain why the change in colour observed when heating a used hack-saw blade is considered a physical change.
Answer: The change in colour of the hack-saw blade when exposed to a flame is considered a physical change because it alters one of the blade’s physical properties (colour) without changing its chemical composition. Physical changes involve modifications in the form or appearance of a substance while retaining its chemical identity. In this case, the colour change is reversible, as it may revert back or change again when the blade is removed from the flame, highlighting the concept of reversibility in physical changes.
Question: How does rusting of iron illustrate a chemical change, and what are the primary factors contributing to this process?
Answer: Rusting of iron exemplifies a chemical change because it involves the reaction of iron with oxygen and moisture (water) from the atmosphere to form a new substance, rust. The primary factors contributing to rusting include exposure to oxygen and water. When iron reacts with these elements, it undergoes a chemical transformation, resulting in the formation of rust, which is distinct from the original iron material. Unlike physical changes, rusting is irreversible and has practical implications, such as weakening iron objects.
Question: Describe the chemical reactions that occur when magnesium burns, and why is this process considered a chemical change?
Answer: When magnesium burns, it undergoes a chemical change through a combustion reaction with oxygen from the air. The reaction can be summarised as follows:
First Reaction: Magnesium (Mg) + Oxygen (O2) → Magnesium Oxide (MgO)
This process is considered a chemical change because it results in the formation of a new substance, magnesium oxide (MgO), with different chemical properties compared to the original magnesium. Additionally, when the resulting magnesium oxide is mixed with water, it further reacts to form magnesium hydroxide, reinforcing the concept of chemical changes.
Question: How does the observation of a blue litmus paper turning red after testing the ash (magnesium oxide) relate to the chemical changes observed during the burning of magnesium?
Answer: The observation of a blue litmus paper turning red after testing the ash (magnesium oxide) demonstrates the basic nature of magnesium oxide when mixed with water. This change in litmus paper colour is an indication of the formation of magnesium hydroxide [Mg(OH)2] as a result of the reaction between magnesium oxide and water. This observation is relevant to the chemical changes observed during the burning of magnesium because it highlights that chemical reactions can produce substances with different properties, such as the basicity of magnesium oxide when it reacts with water.
Question: Explain the chemical reaction that occurs when iron is introduced into a copper sulphate solution. Why does the colour of the solution change, and what is the brown deposit observed on the iron object?
Answer: When iron is introduced into a copper sulphate solution, a displacement reaction takes place. The more reactive iron displaces copper from the solution, resulting in the formation of iron sulphate (green solution) and the deposition of pure copper on the iron object. The blue colour of the copper sulphate solution changes to green due to the formation of iron sulphate. The brown deposit on the iron object is pure copper, which has been displaced from the copper sulphate solution by the iron.
Question: Describe the chemical reaction between vinegar (acetic acid) and baking soda (sodium hydrogen carbonate). What gas evolved during this reaction, and how can you confirm its presence?
Answer: When vinegar reacts with baking soda, it produces carbon dioxide gas along with other substances. The evolution of carbon dioxide gas is indicated by the hissing sound and the formation of bubbles during the reaction. To confirm the presence of carbon dioxide, you can pass the evolved gas through lime water. If the gas is carbon dioxide, it will turn the lime water milky, indicating the formation of calcium carbonate (CaCO3) and water (H2O).
Question: What are some common manifestations of chemical changes in our daily lives, and why are these changes significant?
Answer: Chemical changes are commonly observed in various aspects of daily life:
- Burning phenomena, such as the burning of magnesium ribbon, coal, wood, and leaves, signify chemical changes accompanied by the release of heat.
- Explosive reactions, as seen in fireworks, involve chemical changes that release heat, light, sound, and gases.
- Food spoilage and the foul odours associated with it indicate chemical changes, signifying the degradation of food.
- Discoloration in fruits and vegetables, like apple browning, results from chemical changes due to reactions with the environment. Chemical changes are significant because they are fundamental to biological processes, environmental interactions, and industrial and pharmaceutical applications.
Question: Is neutralisation a chemical change? Explain your answer.
Answer: Yes, neutralisation is a chemical change. During neutralisation, an acid and a base react to form water and a salt. This reaction results in the formation of entirely new substances, water, and salt, with different chemical properties than the original acid and base. Therefore, neutralisation is classified as a chemical change because it involves the creation of new chemical compounds.
Question: What is the core concept behind the displacement reaction observed when iron interacts with a copper sulphate solution, and why is this experiment important in understanding the reactivity series of metals?
Answer: The core concept behind the displacement reaction is that a more reactive metal (iron) can displace a less reactive metal (copper) from its salt solution. Iron displaces copper in the copper sulphate solution, forming iron sulphate and depositing copper on the iron object. This experiment is important in understanding the reactivity series of metals because it vividly demonstrates that metals have varying reactivity levels. More reactive metals can replace less reactive metals from their compounds, showcasing the hierarchy of reactivity among different metals, a fundamental concept in chemistry.
Question: Explain the chemical equation for the rusting of iron. What are the key reactants, products, and the type of chemical reaction involved?
Answer: The chemical equation for the rusting of iron is:
- Reactants: Iron (Fe), Oxygen (O2), Water (H2O)
- Products: Iron(III) Hydroxide [Fe(OH)3]
- Type of Chemical Reaction: Rusting involves a redox reaction, where iron loses electrons (oxidation) and oxygen gains electrons (reduction). The equation for rusting is 4Fe + 3O2 + 6H2O → 4Fe(OH)3.
Question: What are the factors that influence the rusting of iron, and why does environmental exposure play a significant role in this process?
Answer: Several factors influence the rusting of iron, including environmental exposure and humidity. Environmental exposure is significant because iron objects exposed to moist and oxygen-rich environments are more susceptible to rusting. Additionally, the presence of environmental salts, especially in marine environments, accelerates the rusting process. Higher humidity levels expedite rust formation due to increased moisture availability for the reaction.
Question: Describe the prevention strategies for rusting, including protective coatings, galvanization, and the use of stainless steel. How do these methods work to prevent or slow down rusting?
Answer: Rust prevention strategies include:
- Protective Coatings: Applying paint or grease forms a barrier that prevents the iron surface from interacting with oxygen and moisture.
- Galvanization: Coating iron with zinc creates a protective layer. Zinc reacts with oxygen before iron does, protecting the iron.
- Stainless Steel: Mixing iron with chromium creates stainless steel, which is highly resistant to rusting. Chromium forms a protective oxide layer that prevents rust formation. These methods work by physically blocking or altering the chemical interactions that lead to rusting.
Question: Explain the process of crystallisation, including the steps involved and how it differs from simple evaporation. Why is crystallisation preferred for purifying substances?
Answer: Crystallisation is a separation and purification technique that involves forming solid crystals from a saturated solution. The process includes:
- Formation of a solution by dissolving the substance.
- Creation of a saturated solution.
- Cooling to induce crystal formation.
- Separation of crystals from the solution. Crystallisation differs from simple evaporation as it results in more pure substances with well-defined crystals. Impurities are often left behind in the solution. Crystallisation is preferred for purifying substances because it yields purer results with larger, well-formed crystals.
Question: Is the process of crystallisation a physical change or a chemical change? Provide an experiment example to illustrate this.
Answer: Crystallisation is a physical change. It transforms a substance’s physical form from a liquid solution to solid crystals without altering its chemical identity. An experiment example illustrating this is the crystallisation of copper sulphate:
- A solution of copper sulphate is prepared.
- It is heated to create a saturated solution.
- Upon cooling, pure copper sulphate crystals form.
- This process doesn’t change the chemical nature of copper sulphate but changes its physical state, demonstrating a physical change.
Also Check – Class 7- Chapter 5 – Physical and Chemical Changes – 4 Worksheets Solved and Unsolved
Also Check – NCERT Exemplar Solutions- Class 7 Science Chapter – 6- Physical and Chemical Changes
Also Check -Chapter 6- Physical and Chemical Changes-Class 7-Class 7 Question and Answer (MCQs)
Also Check – Chapter 6- Physical and Chemical Changes-Class 7-Class 7 Question and Answer (Short)
Also Check – NCERT Solutions for Class 7- Science -Chapter 6- Physical and Chemical Changes
Also Check – Class 7 science -Chapter 6-Physical and Chemical Changes- Complete Notes
Also Check – Crystallisation – Class 7
Also Check – Difference between Physical and Chemical Change
