Properties of air
A football pumped with air feels heavier than a flat one. A cycle tyre, a balloon and a car tyre are all full of something we cannot see: air. Air takes up space and has weight.
Air is a homogeneous mixture (एकजिनसी मिश्रण) of gases. It has mass and weight and occupies space.
In the figure : In the figure air has weight: the stick tips towards the inflated balloon (A), which is heavier than the empty balloon (B).
Rule 1: Air has weight and occupies space — An inflated balloon is heavier than an empty one, so air has weight. It also occupies space, as it swells the balloon.
- Easy : Balloon with air tips the stick down.
- Medium : A pumped football takes more room than a flat one.
- Exam-type : Compare two balloons of the same size and material: the inflated one is heavier, so the stick tilts towards it.
Rule 2: Other uses of air — Air helps in the transmission of sound, and absorbs heat to keep the earth warm.
- Easy : We hear a bell because sound travels through air.
- Medium : Without air absorbing heat the earth would be much colder at night.
- Exam-type : Air is a mixture because its gases are not chemically combined.
Air: mixture of gases, has weight, takes space, carries sound, keeps the earth warm.
Thinking air has no weight because we cannot see it. A balloon proves it has weight.
Memory clue: Air = A Mixture with Weight (AMW).
Spot the difference
| Property | Air |
|---|---|
| Type | Homogeneous mixture of gases |
| Weight | Has weight |
| Space | Occupies space |
Practice questions
- 1What is air?
- 2Does air have weight?
- 3Does air occupy space?
- 4How does air help sound?
- 5How does air keep the earth warm?
- 6What does the balloon experiment show?
- 7Why is air called a mixture and not a compound?
- 8Sana says an empty balloon and a blown balloon weigh the same. How can she check this?
Show answers (try them yourself first)
- 1A homogeneous mixture of gases.
- 2Yes
- 3Yes
- 4Sound travels through air.
- 5By absorbing heat.
- 6Air has weight, as the inflated balloon is heavier.
- 7Its gases are only mixed, not chemically combined.
- 8Hang both on a balanced stick; it tilts towards the inflated balloon.
Air pressure and humidity
A straw lets you drink because of air pressure. Wet clothes dry slowly on rainy days because the air is already full of vapour. Dew forms on grass on cool mornings. The air around us is always working.
Atmospheric pressure (वातावरणीय दाब) is the pressure exerted by moving air molecules. Humidity (आर्द्रता) is the amount of water vapour in the air. Bernoulli's principle: as the speed of a fluid increases, its pressure decreases.
In the figure : In the figure atmospheric pressure decreases as we go higher: the air is thick near the sea level (A) and thin at the peak (C).
Rule 1: Atmospheric pressure — It acts equally in all directions. At sea level it is about 1,01,400 N/m². It is measured with a barometer and decreases with height.
- Easy : Pressure is highest at sea level.
- Medium : At a mountain peak the pressure is less than at sea level because the air is thin.
- Exam-type : A barometer reading falls as we climb.
In the figure : In the figure of Bernoulli's principle blowing air between two sheets of paper makes the pressure there (A) lower than outside (B), so the sheets move together.
Rule 2: Bernoulli's principle — Where the velocity of air is more, the pressure is less. So the sheets move together when air is blown between them.
- Easy : Blow between two papers: they come together.
- Medium : Fast air → low pressure; slow air → higher pressure.
- Exam-type : Outside pressure (B) is greater than inside (A), and it pushes the sheets together.
In the figure : In the figure wind blows from a region of higher pressure (A) to a region of lower pressure (B).
Rule 3: Wind — Wind blows from a region of high pressure to a region of low pressure.
- Easy : High → low pressure: wind direction.
- Medium : If place A has more pressure than place B, wind blows from A to B.
- Exam-type : The bigger the difference in pressure, the stronger the wind.
In the figure : In the figure water vapour (C) in the air condenses on the cold glass of ice (A) into drops (B) on the outside.
Rule 4: Humidity and dew — Humidity depends on the capacity of air to hold vapour. Dew forms when cool air loses vapour.
- Easy : Drops on a cold glass of water: vapour from the air.
- Medium : Dew on grass in a cool morning: cool air loses vapour.
- Exam-type : Wet clothes dry slowly on humid days since the air already holds much vapour.
Pressure falls with height. Fast air = low pressure. Wind: high → low pressure.
Thinking air presses only downwards. Atmospheric pressure acts in all directions.
Memory clue: Fast = Fall (pressure falls when speed rises); Wind: High to Low (H→L).
Spot the difference
| Point | Rule |
|---|---|
| Atmospheric pressure | Decreases with height |
| Bernoulli's principle | Speed up, pressure down |
| Wind | High to low pressure |
Practice questions
- 1What is atmospheric pressure?
- 2Which instrument measures air pressure?
- 3What is the air pressure at sea level?
- 4State Bernoulli's principle.
- 5Towards which region does wind blow?
- 6What is humidity?
- 7Why do two sheets of paper come together when we blow between them?
- 8Place A has pressure 1000 and place B has 990 (same units). Which way does wind blow, and why do drops form on a cold glass?
Show answers (try them yourself first)
- 1The pressure exerted by moving air molecules.
- 2Barometer
- 3About 1,01,400 N/m².
- 4When the speed of a fluid increases, its pressure decreases.
- 5From higher pressure to lower pressure.
- 6The amount of water vapour in the air.
- 7Fast air lowers the pressure between them; the outside pressure pushes them together.
- 8From A to B; vapour in the air condenses on the cold surface.
Properties of water
Water takes the shape of a glass, a bottle or a bucket, yet a 200 mL glass of water remains 200 mL in any of them. Ice floats on water in a drink. Sugar vanishes into tea. Water has many special properties.
A fluid (प्रवाही) has no shape of its own but flows. Water is a fluid with a definite volume. A universal solvent (सार्वत्रिक द्रावक) dissolves many substances.
In the figure : In the figure water has no shape of its own, so it takes the shape of the vessel, but its volume (here 200 mL) stays the same.
Rule 1: Fluid with definite volume — Water takes the shape of the vessel but keeps its volume.
- Easy : 300 mL water in a bottle stays 300 mL when poured into a bowl.
- Medium : 500 mL water poured equally into 2 glasses gives 500 ÷ 2 = 250 mL each.
- Exam-type : Two glasses of 250 mL and 150 mL poured together give 250 + 150 = 400 mL: volume does not change.
In the figure : In the figure water expands when it freezes (Q rises above the dotted line), so a full glass bottle cracks in a freezer. Ice is lighter than water, so it floats.
Rule 2: Expansion on freezing and solvent — Water expands on freezing, so ice floats. Water is a universal solvent: solute + solvent = solution. Uses: transport, electricity, coolant and cleaning.
- Easy : Sugar (solute) + water (solvent) = sugar solution.
- Medium : Water is used as a coolant in a vehicle radiator.
- Exam-type : Never fill a glass bottle fully with water and keep it in the freezer; the ice expands and may crack it.
Water: no shape, fixed volume, expands on freezing, universal solvent.
Thinking water has a fixed shape or that its volume changes with the vessel. Shape changes; volume does not.
Memory clue: Solute + Solvent = Solution; Ice is Icy-light, so it floats.
Spot the difference
| Property | Water |
|---|---|
| Shape | Takes the shape of the vessel |
| Volume | Definite |
| On freezing | Expands; ice floats |
Practice questions
- 1What is a fluid?
- 2Does water have a definite volume?
- 3Why does ice float on water?
- 4What is a universal solvent?
- 5Write the equation of a solution.
- 6Name two uses of water.
- 7Two glasses of 250 mL and 150 mL are poured into one jug. What is the volume of water in the jug?
- 8Why is it unsafe to keep a completely filled glass bottle of water in the freezer?
Show answers (try them yourself first)
- 1A substance with no shape of its own that flows.
- 2Yes
- 3Water expands on freezing, so ice is lighter than water.
- 4A liquid that dissolves many substances, like water.
- 5Solute + Solvent = Solution
- 6Transport and cleaning (also electricity, coolant).
- 7250 + 150 = 400 mL
- 8Freezing water expands and may crack the bottle.
Density and the anomalous behaviour of water
Why does a stone sink and a wooden stick float? Why can you float more easily in the sea than in a river? The answer lies in density, which tells how much matter is packed in a given space.
Density (घनता) = mass ÷ volume (unit g/cc). The anomalous behaviour of water (पाण्याचे असंगत वर्तन) is that below 4 °C it expands on cooling instead of contracting.
In the figure : In the figure density of water against temperature (a sketch) is the greatest at 4 °C (B). Below 4 °C the water expands as it cools, which is its anomalous behaviour.
Rule 1: The anomaly of water — The density of water is maximum at 4 °C. Below 4 °C water expands, so the density decreases.
- Easy : Densest at 4 °C.
- Medium : Below 4 °C, water is lighter (less dense).
- Exam-type : At 4 °C water has maximum density, so it settles at the bottom of a lake.
In the figure : In the figure in a lake the ice (A) forms on the top, and the water below it (B) stays liquid, so the fish (C) survive.
Rule 2: Why lakes freeze from the top — Cold water at 4 °C sinks, and water below 4 °C stays on top and freezes there. The water below stays liquid, so aquatic life survives.
- Easy : Ice forms on the top of a lake.
- Medium : The water below stays liquid at about 4 °C.
- Exam-type : If water had no anomaly, lakes would freeze from the bottom and fish would die.
In the figure : In the figure density = mass ÷ volume. A block of 180 g raises the water from 100 cc to 160 cc, so its volume is 60 cc and its density is 3 g/cc.
Rule 3: Calculating density — Density = mass ÷ volume. The volume of an irregular solid = rise in water level when it is dipped.
- Easy : 120 g in 40 cc: density = 120 ÷ 40 = 3 g/cc.
- Medium : A 120 g stone raises water from 100 cc to 150 cc. Volume = 150 − 100 = 50 cc. Density = 120 ÷ 50 = 2.4 g/cc.
- Exam-type : A piece of wood has mass 50 g and volume 100 cc: density = 50 ÷ 100 = 0.5 g/cc. Water is 1 g/cc, so the wood floats.
In the figure : In the figure a potato sinks in plain water (P) but floats in salt water (Q), because dissolved salt raises the density of water.
Rule 4: Sinking and floating — An object floats if its density is less than that of the liquid, and sinks if it is more. Dissolved salt raises the density of sea water.
- Easy : Density less than water: floats.
- Medium : Density more than water: sinks.
- Exam-type : It is easier to float in sea water because its density is more.
Density = mass ÷ volume. Water is densest at 4 °C. Ice floats.
Thinking heavier always sinks. It depends on density, not weight alone: a big ship floats.
Memory clue: D = M ÷ V (Density = Mass over Volume); 4 °C = most dense.
Spot the difference
| Object | Density compared to water | Result |
|---|---|---|
| Wood (0.5 g/cc) | Less than 1 | Floats |
| Stone (2.4 g/cc) | More than 1 | Sinks |
| Ice | Less than water | Floats |
Practice questions
- 1Write the formula of density.
- 2At what temperature is the density of water maximum?
- 3What is the anomalous behaviour of water?
- 4Why do fish survive in a frozen lake?
- 5Find the density: mass 120 g, volume 40 cc.
- 6A 120 g stone raises water from 100 cc to 150 cc. Find its density.
- 7A body has mass 50 g and volume 100 cc. Will it sink or float in water (1 g/cc)?
- 8A potato sinks in plain water. What would you add to make it float, and why?
Show answers (try them yourself first)
- 1Density = mass ÷ volume
- 24 °C
- 3Below 4 °C it expands on cooling instead of contracting.
- 4Ice forms on the top and the water below stays liquid.
- 5120 ÷ 40 = 3 g/cc
- 6Volume 50 cc; density 120 ÷ 50 = 2.4 g/cc.
- 7Density 0.5 g/cc; it floats.
- 8Salt; it raises the density of water above that of the potato.
Soil
The soil of a field in Maharashtra looks black, the one near a river is brown, and a beach is full of sand. Plants grow differently in each. Farmers study soil carefully before sowing.
Soil (माती) is the upper layer of the earth made of rock particles and decayed matter. The three main types are sandy, silt and clay. pH shows how acidic or alkaline the soil is.
In the figure : In the figure sandy soil has large particles and drains water fast, silt is in the middle and clay has very small particles and holds the most water. (A sketch with sample numbers.)
Rule 1: Types of soil — Colour shows fertility. Sandy soil drains fast, is easy to plough but less fertile. Silt is medium and fertile. Clay has small particles and holds water. Soil structure affects fertility.
- Easy : Sandy: drains fast, less fertile.
- Medium : Silt: medium, fertile.
- Exam-type : Clay holds water; it suits crops that need much water, such as rice.
In the figure : In the figure the pH of soil: acidic below 6.5, neutral from 6.5 to 7.5 and alkaline above 7.5. In the sketch A is acidic, B is neutral and C is alkaline.
Rule 2: Soil pH and care — Acidic: pH below 6.5. Neutral: 6.5–7.5. Alkaline: above 7.5. Uses of soil: pots (plasticity), china clay, shadu, terracotta, Multani. Rotate crops. World Soil Day is 5 December.
- Easy : pH 7.0 is neutral.
- Medium : pH 5.8 is acidic (below 6.5).
- Exam-type : pH 8.2 is alkaline (above 7.5).
Sand: fast drain. Silt: fertile. Clay: holds water. pH: below 6.5 acidic, 6.5–7.5 neutral, above 7.5 alkaline.
Thinking one type of soil suits all crops. Choose the crop to match the soil and rotate crops.
Memory clue: Sand = Swift drain; Silt = Super fertile; Clay = Catches water.
Spot the difference
| Soil type | Particles | Water |
|---|---|---|
| Sandy | Large | Drains fast |
| Silt | Medium | Medium; fertile |
| Clay | Very small | Holds the most |
Practice questions
- 1Name the three types of soil.
- 2Which soil drains water fastest?
- 3Which soil holds the most water?
- 4What does the colour of soil show?
- 5State the pH range of neutral soil.
- 6Soil A has pH 5.8, soil B has pH 7.0 and soil C has pH 8.2. Classify each.
- 7When is World Soil Day celebrated?
- 8A farmer's soil drains water very fast and plants wilt. Which soil is it likely to be, and what could he do?
Show answers (try them yourself first)
- 1Sandy, silt and clay.
- 2Sandy soil
- 3Clay
- 4Its fertility.
- 56.5 to 7.5
- 6A acidic, B neutral, C alkaline.
- 75 December
- 8Sandy soil; he can mix clay or silt/organic matter to hold more water.
Quick revision
- Air has weight; pressure falls with height; Bernoulli's principle.
- Water is densest at 4 °C; ice floats.
- Soil types: sandy, silt, clay; pH tests.