CBSE Class 7 Science Chapter 6: Physical and Chemical Changes - NCERT Solutions

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NCERT Solutions PDF Class 7 PDF

This resource provides detailed NCERT Solutions for Class 7 Science, Chapter 6, focusing on Physical and Chemical Changes. Students will explore the fundamental differences between physical changes, where the substance's form alters but not its identity, and chemical changes, which result in the formation of new substances. The solutions cover key processes such as photosynthesis, dissolving, burning, melting, rusting, and digestion, explaining how to classify them. It also delves into methods to prevent rusting and the chemical reactions involved, like the reaction of baking soda with lemon juice. These solutions are designed to clarify complex concepts with step-by-step explanations, aiding students in mastering the chapter's content and preparing effectively for their examinations.

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Quick info

BoardCBSE
ClassClass 7
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 6

Chapter summary

Chapter 6 of the Class 7 Science NCERT Solutions focuses on distinguishing between physical and chemical changes. It explains that physical changes are temporary and do not form new substances, while chemical changes are permanent and produce new substances with different properties. The solutions cover examples like melting, dissolving, burning, rusting, and photosynthesis, providing clear classifications and explanations for each process. Methods to prevent rusting and the chemical reactions involved are also discussed, offering a comprehensive understanding of the chapter's core concepts.

Learning outcomes

  • Understand the difference between physical and chemical changes.
  • Classify various everyday processes as physical or chemical changes.
  • Identify new substances formed during chemical changes.
  • Explain the process of rusting and methods to prevent it.
  • Recognize that some processes involve both physical and chemical changes.

Topics covered

Paper topics

  • Physical Changes
  • Chemical Changes
  • Photosynthesis
  • Dissolving
  • Burning
  • Melting
  • Rusting of Iron
  • Prevention of Rusting
  • Digestion of Food
  • Baking Soda
  • Lime Water Reaction
  • Candle Burning

Important topics

  • Distinguishing Physical and Chemical Changes
  • Examples of Physical Changes
  • Examples of Chemical Changes
  • Rusting of Iron
  • Methods to Prevent Rusting
  • Chemical Reactions in Everyday Life

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Questions and Solutions

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
Solution:

Changes can be classified based on whether a new substance is formed or not.

Physical Changes are those where a substance changes its form or appearance, but not its chemical identity. No new substance is formed. The changes are often temporary.

  • (b) Dissolving sugar in water: Sugar dissolves to form a solution, but the sugar molecules themselves do not change. The sugar can be recovered by evaporating the water.
  • (d) Melting of wax: Wax changes from solid to liquid when heated and back to solid when cooled. Its chemical composition remains the same.
  • (e) Beating Aluminium to make Aluminium foil: Aluminium is shaped into a thin foil, but it remains aluminium. No new substance is formed.

Chemical Changes are those where a substance undergoes a chemical reaction to form one or more new substances with entirely different properties. These changes are usually permanent.

  • (a) Photosynthesis: Plants use sunlight, water, and carbon dioxide to produce glucose (food) and oxygen. This is a process where new substances are formed.
  • (c) Burning of coal: Coal reacts with oxygen to produce ash, carbon dioxide, and other gases, which are new substances.
  • (f) Digestion of food: Food is broken down into simpler substances that the body can absorb. This involves chemical reactions to form new compounds.

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) (b) Formation of manure from leaves is a physical change. (True/False) (c) Iron pipes coated with zinc do not get rusted easily. (True/False) (d) Iron and rust are the same substances. (True/False) (e) Condensation of steam is not a chemical change. (True/False)
Solution:

Let's evaluate each statement:

(a) Cutting a log of wood into pieces is a chemical change. False. Cutting wood changes its size and shape but does not form a new substance. It is a physical change.

(b) Formation of manure from leaves is a physical change. False. Leaves decompose to form manure, which is a new substance with different properties. This is a chemical change.

(c) Iron pipes coated with zinc do not get rusted easily. True. Zinc coating protects iron from rusting by forming a barrier and also by acting as a sacrificial metal.

(d) Iron and rust are the same substances. False. Rust is iron oxide, a new substance formed when iron reacts with oxygen and moisture. Iron is a metallic element.

(e) Condensation of steam is not a chemical change. True. Steam (gaseous water) changes into liquid water upon cooling. Water is still H2O, so no new substance is formed; it's a physical change of state.

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.
Solution:

Completing the statements with the correct terms:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium carbonate (CaCO3).

(b) The chemical name of baking soda is Sodium bicarbonate (NaHCO3).

(c) Two methods by which rusting of iron can be prevented are greasing and galvanization.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called chemical 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.
Solution:

This is a chemical change because the reaction between baking soda (sodium bicarbonate) and lemon juice (which contains citric acid) produces new substances, accompanied by the evolution of a gas.

The reaction can be represented as:

Baking Soda + Lemon Juice \rightarrow Salt + Carbon Dioxide + Water

Specifically, sodium bicarbonate reacts with citric acid to produce sodium citrate, carbon dioxide gas (which causes the bubbling), and water.

The formation of new substances (salt, carbon dioxide, water) and the release of a gas are clear indicators of a chemical change.

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.
Solution:

When a candle burns, the following changes occur:

Physical Changes:

  • The solid wax near the flame melts due to the heat. This molten wax then rises up the wick. This is a change of state from solid to liquid, which is a physical change.
  • When the molten wax cools down, it solidifies again, returning to its original state.

Chemical Changes:

  • The vaporized wax from the molten wax undergoes combustion. It reacts with oxygen in the air to produce new substances like carbon dioxide gas and water vapor, along with heat and light. This is a chemical change because new substances are formed.

Another familiar process where both physical and chemical changes occur is boiling of an egg:

  • Physical Change: The egg white and yolk become more solid and change their texture and appearance when heated. However, the fundamental chemical composition of the egg proteins doesn't change into something entirely new in terms of basic elements.
  • Chemical Change: The heat causes chemical reactions within the egg proteins, leading to denaturation and cross-linking, which changes their structure and properties permanently, making the egg cooked. New chemical bonds are formed, and the protein structure is irreversibly altered.

Common mistakes

  • Confusing temporary physical changes with permanent chemical changes.
  • Incorrectly classifying processes like manure formation or wood cutting.
  • Not recognizing that condensation of steam is a physical change.
  • Assuming iron and rust are the same substance.

Revision tips

  • Create a table to list examples of physical and chemical changes discussed in the chapter.
  • Focus on understanding the key characteristics that differentiate physical from chemical changes.
  • Review the examples of rusting and its prevention methods thoroughly.
  • Practice classifying new processes as either physical or chemical changes.

Practice MCQs

Q1. Which of the following is a chemical change?

Q2. What is formed when carbon dioxide is passed through lime water?

Q3. Which of these is NOT a method to prevent rusting of iron?

Q4. The chemical name of baking soda is:

Q5. Which of the following is an example of a physical change?

Frequently asked questions

What is the main difference between physical and chemical changes?

In a physical change, the form or appearance of a substance changes, but its chemical composition remains the same, and no new substance is formed. In a chemical change, a new substance with different properties is formed.

Can you give examples of physical changes from Chapter 6?

Yes, examples include dissolving sugar in water, melting of wax, beating aluminium into foil, and condensation of steam.

What are some examples of chemical changes mentioned in the chapter?

Examples include photosynthesis, burning of coal, digestion of food, and the reaction of baking soda with lemon juice.

How can rusting of iron be prevented?

Rusting of iron can be prevented by methods such as greasing, painting, galvanization (coating with zinc), and alloying with other metals.

Does burning a candle involve both physical and chemical changes?

Yes, when a candle burns, the wax melts (physical change) and then vaporizes and burns, reacting with oxygen to form new substances like carbon dioxide and water (chemical change).

What happens when carbon dioxide is passed through lime water?

When carbon dioxide is passed through lime water, it turns milky due to the formation of a white precipitate of calcium carbonate.

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