Air

Free sample questions

Air — sample Science questions with answers

1. Question 1

   Which of the following gases has the highest percentage by volume in the composition of clean air?

   • ANitrogen, approximately $78\%$
   • BOxygen, approximately $21\%$
   • CCarbon dioxide, approximately $0.03\%$
   • DArgon, approximately $0.97\%$

   Answer: A. Nitrogen, approximately $78\%$

   Clean air consists of approximately $78\%$ nitrogen, $21\%$ oxygen, $0.97\%$ noble gases (such as argon) and other materials, and $0.03\%$ carbon dioxide.

2. Question 2

   Which natural process helps maintain the balance of oxygen content in the atmosphere by releasing ${\text{O}}_2$ gas?

   • ACellular respiration by animals and plants
   • BPhotosynthesis by green plants
   • CDecomposition of dead organisms by bacteria
   • DCombustion of fossil fuels by vehicles

   Answer: B. Photosynthesis by green plants

   Photosynthesis is the only natural process that uses carbon dioxide (${\text{CO}}_2$) gas and releases oxygen (${\text{O}}_2$) gas into the atmosphere.

3. Question 3

   The presence of three main factors is necessary for combustion to occur. Which of the following correctly shows the combination of these factors?

   • AHeat, fuel, and oxygen
   • BHeat, nitrogen, and carbon dioxide
   • CCarbon dioxide, water, and fuel
   • DOxygen, water, and heat

   Answer: A. Heat, fuel, and oxygen

   Combustion can only occur if there is fuel, a supply of oxygen (${\text{O}}_2$), and sufficient heat to reach the flash point of the fuel.

4. Question 4

   In an experiment to determine the percentage of oxygen in air using a candle in a gas jar over water, the water level is found to rise after the candle goes out. What is the approximate fraction of the rise in water level?

   • A$\frac{1}{2}$ of the gas jar volume
   • B$\frac{1}{5}$ of the gas jar volume
   • C$\frac{3}{4}$ of the gas jar volume
   • D$\frac{4}{5}$ of the gas jar volume

   Answer: B. $\frac{1}{5}$ of the gas jar volume

   Oxygen makes up about $21\%$ (or nearly $\frac{1}{5}$) of the volume of air. When oxygen is used up during candle burning, water rises to fill that space.

5. Question 5

   Gas $X$ is used in fire extinguishers because of its property of not supporting combustion and being denser than air. What is gas $X$?

   • ACarbon dioxide (${\text{CO}}_2$)
   • BNitrogen (${\text{N}}_2$)
   • COxygen (${\text{O}}_2$)
   • DHelium ($\text{He}$)

   Answer: A. Carbon dioxide ($\text{CO}_2$)

   Carbon dioxide (${\text{CO}}_2$) is denser than air and does not support combustion, making it very suitable for smothering fire and preventing oxygen from entering.