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๐ฌ Think about it
You can't see air, but it's all around you. Is it just one thing, or many things mixed? Think of it like a fruit juice — it looks like one liquid, but it's really water, sugar, and pulp together. Let's check what air is made of.
Is air a mixture, and what kind?
- Yes, air is a uniform mixture of gases.
- It is mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour.
- Oxygen is needed by most living beings and helps burning.
- Nitrogen is about 78% of air and does not help burning.
Example:
When warm air touches a cool bottle, its water vapour turns into tiny water droplets.
How can we prove air has carbon dioxide in it?
- Make lime water from calcium oxide and water.
- Leave the clear lime water open to the air.
- The lime water slowly turns milky.
- So carbon dioxide from the air must be present.
Why Lime Water Turns Milky
Calcium hydroxide (lime water)
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Add carbon dioxide from air
Gas in the air reacts with lime water.
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Calcium carbonate forms
Tiny white insoluble particles appear.
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Lime water turns milky
In this Activity, we will make lime water and use it to test for carbon dioxide in the air.
Safety first
Perform the calcium oxide step carefully — it reacts vigorously and releases heat.
Materials needed
Glass tumbler, water, calcium oxide (quick lime), filter, petri dish, stirrer.
Procedure
1. Fill a glass tumbler half with water.
2. Add a small amount of calcium oxide (quick lime) slowly to it.
3. It reacts vigorously with water to form calcium hydroxide and releases heat.
4. Stir continuously to make a solution of calcium hydroxide (lime water).
5. Filter it and observe its colour.
6. Leave this colourless solution in a petri dish for a few hours.
7. Keep stirring the solution at regular intervals.
8. Observe whether it turns milky.
2. Add a small amount of calcium oxide (quick lime) slowly to it.
3. It reacts vigorously with water to form calcium hydroxide and releases heat.
4. Stir continuously to make a solution of calcium hydroxide (lime water).
5. Filter it and observe its colour.
6. Leave this colourless solution in a petri dish for a few hours.
7. Keep stirring the solution at regular intervals.
8. Observe whether it turns milky.
Observation
The colourless lime water slowly turns milky on standing in the air.
Explanation
Carbon dioxide from the air reacts with calcium hydroxide to form calcium carbonate (insoluble white particles) and water. The white particles make the solution look milky. This proves carbon dioxide is present in the air.
โ Summary
- Lime water made
- Left open to air
- Turns milky
- Air has COโ
Besides gases, what else floats in the air?
- Tiny dust particles are suspended in the air.
- You see them shining in a sunbeam in a dark room.
- They are not a real part of air — they are pollutants.
- Their number changes with time and place.
Example:
Shiny specks dancing in a beam of sunlight entering through a small window gap.
In this Activity, we will collect dust from the air on a black sheet to show that dust is suspended in air.
Materials needed
A black sheet of paper, an open window or garden spot, a magnifying glass.
Procedure
1. Take a black sheet of paper free from any visible dust particles.
2. Place it undisturbed near an open window, or in the garden, for a few hours.
3. Observe what settles on its surface.
4. Use a magnifying glass to examine the particles more closely.
2. Place it undisturbed near an open window, or in the garden, for a few hours.
3. Observe what settles on its surface.
4. Use a magnifying glass to examine the particles more closely.
Observation
Tiny particles settle on the surface of the black sheet.
Explanation
This shows that dust particles are suspended in the air. They are not an integral part of air and are considered pollutants. Their nature and number vary from time to time and place to place.
โ Summary
- Black sheet kept out
- Dust settles on it
- Dust is suspended
- Dust is a pollutant
What are the major pollutants in air?
- Particulate matter such as dust and soot.
- Gases like carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide.
- The Air Quality Index (AQI) describes how clean the air is.
๐ก A step further
- Major air pollutants include particulate matter (dust, soot).
- They also include gases like carbon monoxide and ozone.
- Nitrogen dioxide and sulfur dioxide are pollutant gases too.
- The air quality index (AQI) is a tool to describe air quality.
Important Points
- Air is a uniform mixture, mainly nitrogen and oxygen.
- Lime water turning milky proves carbon dioxide is in the air.
- Dust in air is a pollutant, not a true part of air.
Important Definitions
- Lime water — a solution of calcium hydroxide that turns milky with carbon dioxide.
- Air Quality Index (AQI) — a tool used to describe how clean or polluted the air is.
- Pollutant — an unwanted substance, like dust, suspended in air and not a true part of it.
