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

Q: 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

(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: $$\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + \text{Heat}$$ (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: $$\text{CaCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{CaO(s)} + \text{CO}_2\text{(g)}$$

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:

$\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + \text{Heat}$

(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:

$\text{CaCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{CaO(s)} + \text{CO}_2\text{(g)}$

decompositioncombinationthermal decompositionexothermicbroken downlimestone

Marking Scheme

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

Analysis & Explanation

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.
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.
Heating of Limestone: Limestone is calcium carbonate ( $\text{CaCO}_3$ ). Heating it causes it to break down into calcium oxide ( $\text{CaO}$ ) and carbon dioxide ( $\text{CO}_2$ ). Because this decomposition is carried out by heating, it is specifically called a thermal decomposition reaction.

Common Mistakes

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

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.

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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.