Question 103 marksShort AnswerSection SECTION-A

State the kind of chemical reactions in the following examples:

(i) Digestion of food in stomach

(ii) Combustion of coal in air

(iii) Heating of limestone

Types of Chemical Reactions

Official answer

(i) Digestion of food in stomach: This is a Decomposition reaction (and also an exothermic process). During digestion, complex food substances like carbohydrates are broken down into simpler substances like glucose.


(ii) Combustion of coal in air: This is a Combination reaction (and also an exothermic reaction). Carbon combines with oxygen to form a single product, carbon dioxide, releasing a large amount of heat:

C(s)+O2(g)→CO2(g)+Heat\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + \text{Heat}


(iii) Heating of limestone: This is a Thermal decomposition reaction. Calcium carbonate (limestone) decomposes on heating to give calcium oxide (quick lime) and carbon dioxide gas:

CaCO3(s)→HeatCaO(s)+CO2(g)\text{CaCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{CaO(s)} + \text{CO}_2\text{(g)}

Answer keywords:decompositioncombinationthermal decompositionexothermicbroken downlimestone

Marking scheme

  1. 1.Correctly identifying digestion of food as a decomposition/exothermic reaction: 1 Mark
  2. 2.Correctly identifying combustion of coal as a combination/exothermic reaction: 1 Mark
  3. 3.Correctly identifying heating of limestone as a thermal decomposition reaction: 1 Mark

Hint

Think about whether the process involves breaking down a substance (decomposition) or combining substances to form a single product (combination).

Quick oral answer

Digestion is a decomposition reaction because complex food molecules break down into simpler ones. Combustion of coal is a combination reaction because carbon and oxygen combine to form carbon dioxide. Heating of limestone is a thermal decomposition reaction because heat breaks down calcium carbonate into calcium oxide and carbon dioxide.

Common mistakes

  1. 1.Confusing digestion with displacement reactions.
  2. 2.Forgetting to specify 'thermal' for the decomposition of limestone, which is highly recommended since heat is the driving energy source.

Analysis and explanation

  1. Digestion: The NCERT text states that during digestion, food is broken down into simpler substances (e.g., carbohydrates to glucose). Breaking down of a complex substance into simpler substances is the definition of a decomposition reaction.
  2. Combustion of Coal: Coal is primarily carbon. When it burns in air, carbon reacts with oxygen to form carbon dioxide. Since two reactants combine to form a single product, it is a combination reaction. It also releases heat, making it exothermic.
  3. Heating of Limestone: Limestone is calcium carbonate (CaCO3\text{CaCO}_3). Heating it causes it to break down into calcium oxide (CaO\text{CaO}) and carbon dioxide (CO2\text{CO}_2). Because this decomposition is carried out by heating, it is specifically called a thermal decomposition reaction.

Interesting facts

The calcium oxide (quick lime) produced from the thermal decomposition of limestone is a key raw material in the industrial manufacture of cement.

Question details

Q10 of 36 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2014
Section
Section SECTION-A
Marks
3
Type
Short Answer
Topic
Types of Chemical Reactions
Bloom's level
Understanding
Syllabus topic
Chemical reactions: types of chemical reactions: combination, decomposition, displacement, double displacement, precipitation, endothermic exothermic reactions, oxidation and reduction.

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2014?

This question carries 3 marks in the CBSE Class 10 Science 2014 examination.

What topic does this question cover in Science?

This question covers the topic "Types of Chemical Reactions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2014 paper?

This is a Short Answer question from Section SECTION-A in the CBSE Class 10 Science 2014 paper.