NCERT Class 7 Science — Chapter 2: Physical and chemical changes

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When the ash obtained from burning magnesium is dissolved in water, the resulting solution is:

Official previous-year question

Verified 1 Aug 2026.

Options

AAcidic, turning blue litmus red.
BNeutral, having no effect on litmus.
CBasic, turning red litmus blue.
DAmphoteric, reacting with both acids and bases.

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Solution

The correct option is

Basic, turning red litmus blue.

Explanation

Metals generally react with oxygen to form metallic oxides. These metallic oxides are basic in nature. When dissolved in water, they form metal hydroxides, which release hydroxide ions ($OH^-$), resulting in an alkaline solution.

Detailed Analysis

Combustion of Magnesium: When magnesium ribbon is burnt in the presence of oxygen, it burns with a dazzling white flame and forms a white powder (ash). This ash is Magnesium Oxide ($MgO$).
Reaction: $2Mg + O_2 \rightarrow 2MgO$
Reaction with Water: When this magnesium oxide ash is dissolved in water, it reacts to form Magnesium Hydroxide [$Mg(OH)_2$].
Reaction: $MgO + H_2O \rightarrow Mg(OH)_2$
Nature of Solution: Magnesium Hydroxide is a base. According to the properties of indicators, bases turn red litmus paper blue. Acids, conversely, turn blue litmus red.

Key Takeaway: Oxides of metals are basic in nature and turn red litmus blue, whereas oxides of non-metals are acidic in nature and turn blue litmus red.

Your note

More from Chapter 2: Physical and chemical changes

With reference to the burning of a magnesium ribbon, consider the following statements: 1. It burns with a dull blue flame. 2. The ash formed is chemically different from the magnesium ribbon. 3. The reaction produces a new substance called magnesium oxide. Which of the statements given above is/are correct?

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With reference to the reaction between Copper Sulphate and Iron, consider the following statements: 1. The colour of the solution changes from blue to green. 2. The green colour is due to the formation of Copper Oxide. 3. A brown deposit forms on the iron object, which is Rust (Iron Oxide). Which of the statements given above is/are correct?

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Which of the following pairs is correctly matched regarding the reaction of Copper Sulphate with Iron?

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With reference to the "Lime Water Test", consider the following statements: 1. It is a standard test for Carbon Dioxide gas. 2. The milky appearance is due to the formation of Calcium Oxide. 3. The chemical name of lime water is Calcium Hydroxide. Which of the statements given above is/are correct?

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Which of the following is NOT a characteristic indicator that a chemical change has taken place?

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The correct option is

Basic, turning red litmus blue.

Explanation

Detailed Analysis

Combustion of Magnesium: When magnesium ribbon is burnt in the presence of oxygen, it burns with a dazzling white flame and forms a white powder (ash). This ash is Magnesium Oxide ($MgO$).
Reaction: $2Mg + O_2 \rightarrow 2MgO$
Reaction with Water: When this magnesium oxide ash is dissolved in water, it reacts to form Magnesium Hydroxide [$Mg(OH)_2$].
Reaction: $MgO + H_2O \rightarrow Mg(OH)_2$
Nature of Solution: Magnesium Hydroxide is a base. According to the properties of indicators, bases turn red litmus paper blue. Acids, conversely, turn blue litmus red.

Key Takeaway: Oxides of metals are basic in nature and turn red litmus blue, whereas oxides of non-metals are acidic in nature and turn blue litmus red.