Which one of the following is an example of endothermic reaction?
- A.CaO(s) + H2O(l) → Ca(OH)2(aq)
- B.CaCO3(s) → CaO(s) + CO2(g)
- C.C(s) + O2(g) → CO2(g)
- D.CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Correct answer
Explanation
The correct answer is B, CaCO3(s) → CaO(s) + CO2(g). Breaking calcium carbonate into quicklime and carbon dioxide needs a steady supply of heat, so the reaction absorbs energy.
An endothermic reaction takes in heat from its surroundings, while an exothermic reaction gives heat out. Decomposition by heat, called thermal decomposition, is usually endothermic. Limestone is heated to about 900 °C in lime kilns and cement plants for exactly this reason: without that heat the carbonate does not break down.
A is wrong: adding water to quicklime gives slaked lime with so much heat that the water may boil, a classic exothermic reaction.
C is wrong: burning carbon, as in coal, releases a large amount of heat, so it is exothermic.
D is wrong: burning methane, the main part of natural gas, also releases heat, which is why it is used as a fuel.
Exam tip: combustion and slaking of lime are exothermic; heating limestone is endothermic.
Practice Questions
View allConsider the following statements :
Statement-I : Giant stars live much longer than dwarf stars.
Statement-II : Compared to dwarf stars, giant stars have a greater rate of nuclear reactions.
Which one of the following is correct in respect of the above statements?
- A.Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- B.Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
- C.Statement-I is correct, but Statement-II is incorrect
- D.Statement-I is incorrect, but Statement-II is correct
Show answer
Massive giant stars burn their fuel so fast that they live much shorter lives than dwarf stars, so Statement I is false and Statement II is true.
Read the full explanation →Which one of the following is a filter feeder?
- A.Catfish
- B.Octopus
- C.Oyster
- D.Pelican
Show answer
The oyster feeds by drawing water over its gills and straining out plankton and organic particles, which is filter feeding.
Read the full explanation →With reference to radioisotope thermoelectric generators (RTGs), consider the following statements :
1. RTGs are miniature fission reactors.
2. RTGs are used for powering the onboard systems of spacecrafts.
3. RTGs can use Plutonium-238, which is a by-product of weapons development.
Which of the statements given above are correct ?
- A.1 and 2 only
- B.2 and 3 only
- C.1 and 3 only
- D.1, 2 and 3
Show answer
RTGs power spacecraft using heat from the natural decay of plutonium-238; they are not reactors and involve no fission. In an RTG, a radioactive isotope gives off heat as it decays on its own.
Read the full explanation →Consider the following :
1. Cashew
2. Papaya
3. Red sanders
How many of the above trees are actually native to India ?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
Of the three, only red sanders is native to India; cashew and papaya were brought from the Americas. Red sanders (Pterocarpus santalinus) is endemic to the southern Eastern Ghats, mainly the Seshachalam and nearby hills of Andhra Pradesh.
Read the full explanation →Consider the following :
1. Butterflies
2. Fish
3. Frogs
How many of the above have poisonous species among them ?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
Each of the three groups includes species that carry toxins in their bodies to deter predators. Butterflies: the Monarch butterfly feeds as a caterpillar on milkweed and stores its bitter cardiac glycosides, so birds that eat it fall sick; its bright orange colour is a warning.
Read the full explanation →