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Q70 (IAS/2014) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Basic chemical principles Official Key

Which of the following is/are the example/examples of chemical change? 1. Crystallization of sodium chloride 2. Melting of ice 3. Souring of milk Select the correct answer using the code given below.

Result
Your answer: —  ·  Correct: B
Explanation

A physical change is one in which a substance or object undergoes a change in its physical properties and no new substance is formed, while a chemical change is one in which one or more new substances are formed.[3]

Let's analyze each option:

1. **Crystallization of sodium chloride**: Dissolved sodium chloride (common salt) may be separated from water by the physical process of evaporation.[4] Crystallization involves only a change in physical state without forming a new substance, making it a physical change.

2. **Melting of ice**: When ice melts, it can be refrozen into ice.[5] This is a reversible change involving only a change in state from solid to liquid water, with no new substance formed. It is a physical change.

3. **Souring of milk**: Lactobacillus bacterium feeds on the sugar in milk (lactose), multiplies, and ferments the milk to form curd. These bacteria produce lactic acid, which makes curd sour.[6] Since lactic acid (a new substance) is formed, this is a chemical change.

Therefore, only statement 3 (souring of milk) is an example of a chemical change.

Sources
  1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > In a Nutshell > p. 68
  2. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > In a Nutshell > p. 68
  3. [3] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > In a Nutshell > p. 68
  4. [4] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.3.2 Compounds > p. 124
  5. [5] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > 5.5 Are Changes Permanent? > p. 66
  6. [6] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 2: The Invisible Living World: Beyond Our Naked Eye > Table 2.4: Testing for curd formation using milk in different conditions > p. 22
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Q. Which of the following is/are the example/examples of chemical change? 1. Crystallization of sodium chloride 2. Melting of ice 3. Sourin…
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Origin: From standard books Fairness: High fairness Books / CA: 10/10 · 0/10
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This is a foundational 'Sitter' directly from NCERT Class VII. The strategy is binary: Apply the 'New Substance Test'. If the chemical identity changes (Milk -> Lactic Acid), it is chemical. If only the state/shape changes (Water -> Ice), it is physical. Do not over-intellectualize basic NCERT definitions.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Is the crystallization of sodium chloride an example of a chemical change or a physical change?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.3.2 Compounds > p. 124
Presence: 5/5
“8.11), combined chemically in a fixed ratio. The ratio of the number of atoms of hydrogen to oxygen in water has been found to be 2:1. Are common salt and sugar elements or compounds? Let us find out. Sodium, a soft metal, and chlorine, a hazardous gas, combine to form a harmless yet taste-enhancing substance that is essential for our lives. This substance is known as sodium chloride, which is made up of particles of sodium and chlorine in a 1:1 ratio. We learnt that dissolved sodium chloride (common salt) may be separated from water by the physical process of evaporation.”
Why this source?
  • Explicitly states dissolved sodium chloride may be separated from water by the physical process of evaporation.
  • Implies crystals of NaCl form from solution without creating a new chemical species.
Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > In a Nutshell > p. 68
Presence: 5/5
“• A physical change is one in which a substance or object undergoes a change in its physical properties and no new substance is formed.• A chemical change is one in which one or more new substances are formed. It involves a chemical reaction and can be represented by a chemical equation.• Combustion, cooking, and rusting are examples of chemical changes.• Substances that undergo combustion are combustible substances. Heat and/or light are given out during combustion.• The lowest temperature at which a substance can catch fi re is called its ignition temperature.• Some changes can be reversed and some cannot.• Some changes are desirable and some are not.• Rocks undergo physical and chemical changes due to weathering to form soil.• Erosion caused by fl owing water and wind is a physical change.”
Why this source?
  • Provides the defining test for a physical change: no new substance is formed.
  • Allows classification of crystallization as physical if NaCl remains chemically unchanged.
Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > 2.4.3 Chemicals from Common Salt > p. 29
Presence: 4/5
“By now you have learnt that the salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food. You must have observed in the above Activity that it is a neutral salt. Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up.”
Why this source?
  • Describes large crystals/rock salt formed when seas dried up, indicating salt crystallizes from evaporating water.
  • Connects natural formation of solid salt deposits to a physical drying/evaporation process.
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