Worksheet Class 10 Chemistry

 10 High Order Thinking Skills (HOTS) questions based on the first three chapters of Class 10 Chemistry (NCERT):

1. Chemical Reactions and Equations
2. Acids, Bases and Salts
3. Metals and Non-metals

Here are 10 HOTS questions (application + reasoning type):

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Chapter 1: Chemical Reactions and Equations

1. A reddish-brown coating forms on an iron object left in moist air. Write a balanced chemical equation for this reaction and explain which type of chemical reaction it is.
2. Why does silver chloride turn grey in sunlight? Write the chemical reaction involved and identify the type of reaction.
3. Two salts, silver nitrate and sodium chloride, are dissolved in water. Predict the type of chemical reaction that takes place when they are mixed and explain why a white precipitate forms.

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Chapter 2: Acids, Bases and Salts

4. Fresh milk has a pH of 6. How does the pH change as it turns into curd? Explain the reason.
5. Why does baking soda (NaHCO₃) provide relief from acidity, whereas washing soda (Na₂CO₃) is not used for the same purpose, even though both are bases?
6. Tooth enamel is made up of calcium phosphate, Ca₁₀(PO₄)₆(OH)₂, which is the hardest substance in our body. Why does it get corroded when pH in the mouth falls below 5.5?
7. A farmer treats the soil of his field with quicklime, slaked lime, or chalk powder. Under what soil conditions is this treatment required and why?

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Chapter 3: Metals and Non-Metals

8. Aluminium foils are used to wrap food items, while silver and copper are not used for the same purpose. Explain why.
9. During electrolytic refining of copper, a thick plate of impure copper is made the anode while a thin plate of pure copper is made the cathode. Justify this arrangement.
10. When copper vessels are exposed to moist air, they slowly lose their shiny brown surface and acquire a green coating. Identify the chemical compound formed and write a balanced chemical equation for the reaction.

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Answers 

answers to the 10 HOTS questions from the first 3 chapters of Class 10 Chemistry (NCERT):

Chapter 1: Chemical Reactions and Equations

Q1. A reddish-brown coating forms on an iron object left in moist air. Write a balanced chemical equation for this reaction and explain which type of chemical reaction it is.

This is rusting of iron, a slow oxidation process.

4Fe + 6H_2O + 3O_2 \;\;\rightarrow\;\; 4Fe(OH)_3

Type: Oxidation and corrosion (slow chemical change).

Q2. Why does silver chloride turn grey in sunlight? Write the chemical reaction involved and identify the type of reaction.

Silver chloride decomposes in sunlight to form silver metal (grey) and chlorine gas.

2AgCl \xrightarrow{sunlight} 2Ag + Cl_2

Q3. Two salts, silver nitrate and sodium chloride, are dissolved in water. Predict the type of chemical reaction that takes place when they are mixed and explain why a white precipitate forms.

AgNO_3(aq) + NaCl(aq) \;\;\rightarrow\;\; AgCl(s) \downarrow + NaNO_3(aq)

Type: Double displacement and precipitation reaction.

Chapter 2: Acids, Bases and Salts

Q4. Fresh milk has a pH of 6. How does the pH change as it turns into curd? Explain the reason.

The pH decreases (below 6) because lactic acid is produced during curd formation.

Acid increases → pH decreases.

Q5. Why does baking soda (NaHCO₃) provide relief from acidity, whereas washing soda (Na₂CO₃) is not used for the same purpose, even though both are bases?

NaHCO₃ is a mild base, neutralizes excess HCl in the stomach without harming tissues.

NaHCO_3 + HCl \;\;\rightarrow\;\; NaCl + H_2O + CO_2

Q6. Tooth enamel is made up of calcium phosphate, Ca₁₀(PO₄)₆(OH)₂, which is the hardest substance in our body. Why does it get corroded when pH in the mouth falls below 5.5?

Acidic food/bacteria lower the pH below 5.5 → acids attack enamel → calcium phosphate dissolves → tooth decay.

Q7. A farmer treats the soil of his field with quicklime, slaked lime, or chalk powder. Under what soil conditions is this treatment required and why?

Treatment required when soil is too acidic (low pH).

Quicklime (CaO), slaked lime (Ca(OH)₂), or chalk powder (CaCO₃) neutralize the excess acid → make soil suitable for cultivation.

Chapter 3: Metals and Non-Metals

Q8. Aluminium foils are used to wrap food items, while silver and copper are not used for the same purpose. Explain why.

Aluminium reacts with oxygen to form a thin protective oxide layer (Al₂O₃), which prevents further corrosion and is safe.

Copper reacts with moist air → forms green coating of basic copper carbonate (toxic).

Silver tarnishes (forms black Ag₂S layer).

Hence, aluminium is preferrence 

Q9. During electrolytic refining of copper, a thick plate of impure copper is made the anode while a thin plate of pure copper is made the cathode. Justify this arrangement.

At anode: Impure copper dissolves into solution as Cu²⁺ ions.

Cu \;\;\rightarrow\;\; Cu^{2+} + 2e^-

Cu^{2+} + 2e^- \;\;\rightarrow\;\; Cu

This ensures pure copper is obtained.

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Q10. When copper vessels are exposed to moist air, they slowly lose their shiny brown surface and acquire a green coating. Identify the chemical compound formed and write a balanced chemical equation for the reaction.

Green coating = basic copper carbonate (CuCO₃·Cu(OH)₂).

2Cu + H_2O + CO_2 + O_2 \;\;\rightarrow\;\; CuCO_3 \cdot Cu(OH)_2