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Nature of Matter: Elements, Compounds, and Mixtures - Is air a mixture?

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Air is a homogeneous mixture of several gases, primarily Nitrogen (N2N_2), Oxygen (O2O_2), Argon (ArAr), and Carbon Dioxide (CO2CO_2).

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Evidence that air is a mixture: Its composition is not fixed and varies from place to place (e.g., higher CO2CO_2 and pollutants in industrial areas).

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The components of air retain their individual physical and chemical properties; for example, the Oxygen (O2O_2) in air still supports combustion.

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Air can be separated into its constituent gases by physical methods such as the fractional distillation of liquid air.

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No energy change (evolution or absorption of heat/light) occurs when the gases of the atmosphere are mixed together.

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Air does not have a fixed chemical formula, unlike compounds such as Water (H2OH_2O) or Carbon Dioxide (CO2CO_2).

📐Formulae

Approximate Composition of Air by Volume:\text{Approximate Composition of Air by Volume:}

N2≈78%,O2≈21%,Ar≈0.9%,CO2≈0.03%N_2 \approx 78\%, \quad O_2 \approx 21\%, \quad Ar \approx 0.9\%, \quad CO_2 \approx 0.03\%

Separation Process: Liquid Air→distillationN2(B.P. −196∘C)+O2(B.P. −183∘C)\text{Separation Process: Liquid Air} \xrightarrow{\text{distillation}} N_2 (\text{B.P. } -196^\circ C) + O_2 (\text{B.P. } -183^\circ C)

💡Examples

Problem 1:

State two reasons why air is considered a mixture and not a compound.

Solution:

  1. The composition of air is variable and not fixed. 2. The components of air can be separated by physical processes like fractional distillation.

Explanation:

Compounds have a fixed ratio of elements by mass (e.g., H2OH_2O is always 1:81:8 by mass), whereas mixtures like air can have varying percentages of gases depending on the environment.

Problem 2:

When we burn a candle in a closed jar of air, the oxygen is consumed. Does this prove air is a mixture?

Solution:

Yes, it proves that air is a mixture.

Explanation:

In a mixture, the individual components retain their chemical properties. The fact that Oxygen (O2O_2) in air reacts independently to support combustion while Nitrogen (N2N_2) remains unreacted shows that they are not chemically bonded into a single compound.