1. Atmospheric pressure — हमारे चारों ओर की हवा द्वारा डाला गया दबाव
2. Homogeneous mixture — समान रूप से मिला मिश्रण जिसमें हर जगह एक-सा संयोग हो
3. Humidity — हवा में उपस्थित जल-वाष्प की मात्रा
4. Condensation — वाष्प का ठंडा होकर पानी की बूंदों में बदलना
5. Dew — सुबह/रात को घास-पत्तों पर बनने वाली नन्ही जल बूंदें
6. Bernoulli’s principle — हवा की गति बढ़े तो दबाव घटता है, और विपरीत होने पर बढ़ता है
7. Pressure — किसी क्षेत्र पर लगाया गया बल
8. Mass — वस्तु में पदार्थ की मात्रा
9. Volume — वस्तु द्वारा घेरा गया स्थान
10. Density — द्रव्यमान ÷ आयतन (सघनता)
11. Buoyancy — द्रव/जल में तैरने की प्रवृत्ति (उत्थापन)
12. Solute — जो पदार्थ घुलता है (जैसे नमक)
13. Solvent — जिसमें घुलता है (जैसे पानी)
14. Solution — घुलने के बाद बना मिश्रण
15. Anomalous behaviour (of water) — पानी का 4°C पर सबसे अधिक सघन होना
16. pH — घोल की अम्लीय/क्षारीयता बताने वाला मान
17. Plasticity (soil) — मिट्टी को मनचाहा आकार देने की क्षमता
18. Texture (soil) — मिट्टी में कणों के आकार का अनुपात
19. Scattering of light — सूक्ष्म कणों द्वारा प्रकाश का चारों ओर फैलना
20. Universal solvent — ऐसा विलायक जो बहुत-से पदार्थ घोल देता है (पानी)

• Air has mass & weight: Inflated balloon side goes down; hence air, a gas mixture, has mass.
• Atmospheric pressure: Gas molecules strike surfaces → pressure. At sea level ≈ \(\,1{,}01{,}400\ \text{N m}^{-2}\,\). Acts equally in all directions.
• Bernoulli’s principle: Higher air velocity → lower pressure; lower velocity → higher pressure. Explains straw fountain and many aero effects.
• Wind: Air flows from high to low pressure → breeze/wind.
• Humidity & dew: Cool air holds less vapour → condensation → dew. Warm afternoon air holds more moisture → feels “dry”.
• Scattering of light: Dust/smoke/moisture in air scatter light; makes sunrises/sunsets glow and lets us “see” air effects.
• Sound in air: Air is the medium; in winter, higher density can carry distant sounds better.
• Water basics: No fixed shape, definite volume & mass; forms droplets due to surface tension; can seep through tiny gaps.
• Expansion on freezing: Water \(\to\) ice: volume ↑, density ↓ → ice floats. Never freeze a completely filled glass bottle.
• Density: \(\rho=\dfrac{\text{mass}}{\text{volume}}\). Water at \(1\,\text{L}=1\,\text{kg}\Rightarrow \rho=1\,\text{kg L}^{-1}=1\,\text{g cm}^{-3}\).
• Anomalous behaviour: Water’s density is maximum at \(4^{\circ}\text{C}\). Below that it expands as it cools.
• Solutions: Solute (salt) + solvent (water) → solution. Sea water denser than rainwater due to dissolved salts → easier to float/swim.
• Soil colour & hints: Varies (black/red/yellow/grey); indicates texture, fertility, drainage, organic & mineral content.
• Soil texture types: Sandy (large grains, drains fast, less fertile), Silt (medium, richer organics), Clay (fine, high water holding).
• Soil structure: Columnar/laminar/granular/blocky. Good structure ⇒ oxygen supply & drainage → healthy roots.
• Soil uses: Plant growth, water conservation (bunds/lakes), plasticity → pots/bricks/idols; special soils: China clay, Shadu, Terracotta, Multani.
• Soil testing: Check colour, texture, organic matter, pH & conductivity to guide fertilization and crop planning.
• Soil pH: Acidic \(<6.5\), Neutral \(6.5{-}7.5\), Alkaline \(>7.5\). Extremes reduce fertility.
• Fertility loss causes: Wrong pH, poor drainage, repeated same crop, saline water, excessive fertilizers/pesticides. Remedy: crop rotation, organics.

Remember: Without air, Earth’s average surface temperature would be very low; air (with water vapour & \( \text{CO}_2\)) regulates temperature by trapping some outgoing heat.

1. Q1. The pressure exerted by air in all directions is called? Atmospheric pressure.
2. Q2. Air is a __________ mixture of gases. Homogeneous.
3. Q3. Measure of water vapour in air? Humidity.
4. Q4. Turning of vapour into liquid water droplets? Condensation.
5. Q5. Morning water drops on grass are called? Dew.
6. Q6. Rule: higher air speed → lower pressure. Bernoulli’s principle.
7. Q7. Quantity: mass per unit volume. Density.
8. Q8. The best “universal solvent”. Water.
9. Q9. Substance that dissolves (e.g., salt). Solute.
10. Q10. Medium in which solute dissolves. Solvent.
11. Q11. Water’s maximum density occurs at? \(4^{\circ}\text{C}\).
12. Q12. Mixture obtained after dissolving. Solution.
13. Q13. Sea water’s density vs rainwater? Higher.
14. Q14. Soil that drains very fast. Sandy soil.
15. Q15. Soil with maximum water holding. Clay soil.
16. Q16. Ability of soil to be shaped. Plasticity.
17. Q17. Soil test that tells acidity/alkalinity. pH test.
18. Q18. Sound needs a __________ to travel. Medium.
19. Q19. Unit of density (school level). \(\text{g cm}^{-3}\) (or \( \text{kg m}^{-3}\)).
20. Q20. Value of sea-level air pressure (approx.). \(1{,}01{,}400\ \text{N m}^{-2}\).

1. Q1. How do we know air has mass? An inflated balloon makes the balance tilt; added air increases mass.
2. Q2. Why does a syringe piston spring back when the inlet is closed? Inside pressure becomes lower than outside; atmospheric pressure pushes it in.
3. Q3. Is atmospheric pressure the same in every direction? Yes, it acts equally in all directions.
4. Q4. What happens to air pressure when temperature increases? At fixed volume, pressure rises; in open air, warm air expands, becomes lighter → low-pressure region.
5. Q5. Why does a straw fountain form when you blow across the top? Fast air lowers pressure (Bernoulli); higher pressure below pushes water up.
6. Q6. Why do water droplets appear outside a cold glass? Air near it cools; vapour condenses into droplets (condensation).
7. Q7. When are we likely to get dew? At night/early morning when air cools and can’t hold much vapour.
8. Q8. Why do wet clothes dry slowly in the monsoon? High humidity reduces evaporation rate.
9. Q9. Does sound travel in space? No, because there’s no air (no medium).
10. Q10. Why is ice lighter than water? On freezing, volume increases → density decreases → it floats.
11. Q11. Write the formula for density. \(\rho=\dfrac{\text{mass}}{\text{volume}}\).
12. Q12. What is the density of pure water at room conditions (school level)? \(\approx 1\ \text{g cm}^{-3}\).
13. Q13. Why is sea swimming easier than lake swimming? Sea water is denser (salts) → more buoyancy.
14. Q14. Which soil is richest in organic matter among the three? Silt soil usually has more organics.
15. Q15. Two benefits of good soil structure? Better aeration and drainage → healthier roots.
16. Q16. pH value of neutral soil? Around \(7\) (range \(6.5{-}7.5\)).
17. Q17. One cause of soil fertility loss. Excess chemical fertilizers/pesticides or repeated same crop.
18. Q18. Name a special soil used for crockery. China clay (Kaolin).
19. Q19. Is there atmospheric pressure on the Moon? Essentially none; Moon has no appreciable atmosphere.
20. Q20. Why should a water-filled glass bottle not be kept in a freezer? Water expands on freezing → bottle may crack/burst.

1. Q1. Explain why the balanced stick tilts when one balloon is inflated. Inflated balloon contains more air → greater mass on that side → stick tilts, proving air has weight.
2. Q2. How does Bernoulli’s principle explain the straw fountain? Blowing increases air speed at the top → pressure drops → higher atmospheric pressure on water surface pushes water up.
3. Q3. Why do we feel winds? Air moves from high-pressure to low-pressure areas; temperature differences create pressure differences → winds.
4. Q4. Describe scattering of light in air. Dust/smoke/moisture particles deflect light in many directions; that’s why shafts of light are visible in dusty rooms.
5. Q5. State two properties showing that water is a fluid. Takes shape of container (no fixed shape) and can flow/seep through small gaps.
6. Q6. Give evidence that ice is less dense than water. Ice floats; also, the water level rises after freezing in a container → volume increased.
7. Q7. What is the anomalous behaviour of water? Water’s density increases down to \(4^{\circ}\text{C}\), then decreases on further cooling—so it expands below \(4^{\circ}\text{C}\).
8. Q8. Calculate density if mass \(=1\,\text{kg}\) and volume \(=1\,\text{L}\). \(\rho=\frac{1\,\text{kg}}{1\,\text{L}}=1\,\text{kg L}^{-1}=1\,\text{g cm}^{-3}\).
9. Q9. Why is the sea denser than rainwater? Dissolved salts add mass without much volume change → density increases.
10. Q10. List the three basic soil textures and one trait each. Sandy—fast drainage; Silt—moderate drainage, richer organics; Clay—high water holding.
11. Q11. How does soil structure affect roots? Granular/blocky structures create pores for air and water → better root growth.
12. Q12. Why is crop rotation recommended? Restores nutrients (e.g., legumes add nitrogen), breaks pest cycles, maintains soil health.
13. Q13. What does a pH below \(6.5\) indicate? Remedy? Acidic soil; apply suitable amendments (e.g., lime) and add organic matter.
14. Q14. Give two everyday uses based on water’s properties. Hydroelectric power from falling water; cooling car engines (coolant).
15. Q15. Why can we hear train whistles farther on winter mornings? Cooler, denser air transmits sound efficiently with less energy loss.
16. Q16. Define solute, solvent with an example. Solute: salt; Solvent: water; together they form a salt solution.
17. Q17. How does humidity affect evaporation? High humidity lowers evaporation; low humidity speeds it up.
18. Q18. Give one property of sandy soil that helps ploughing. Large, loose particles make it easy to turn and aerate.
19. Q19. Mention two special clays and uses. China clay—for crockery/tiles; Shadu—for idols/statues.
20. Q20. How does oil atop water in a jar help the ‘water-level’ experiment? Oil reduces evaporation; any fall in level is mainly due to root absorption.

A) Fill in the blanks (choose from the box)

1. (a) The capacity of air to hold moisture depends upon the ______ of the air. Temperature.
2. (b) Water does not have a ______ but has definite ______ and ______. Shape; volume; mass.
3. (c) While freezing, the ______ of water is lowered. Density.
4. (d) ______ soil has pH 7. Neutral.

B) Why is it said that…

1. (a) Air is a homogeneous mixture of various gases. Because nitrogen, oxygen, carbon dioxide, water vapour, etc. are uniformly mixed so that any sample of air has nearly the same composition.
2. (b) Water is a universal solvent. It dissolves a very large variety of substances (salts, sugars, gases), so most natural/biological solutions form in water.
3. (c) There is no alternative to water for cleaning purposes. Water is abundant, non-toxic, fluid, and dissolves/removes many kinds of dirt and salts; hence ideal for washing/bathing.

C) What will happen if…

1. (a) The amount of water vapour in the air increases. Humidity rises, evaporation slows, we feel sticky; fog/dew/rain become more likely if air gets saturated.
2. (b) Only one crop is grown repeatedly in the soil. Specific nutrients get depleted, pests build up, yield falls; crop rotation is needed to restore fertility.

D) With whom should I pair up?

E) State true or false

1. (a) Sandy soil has low capacity for holding water. True.
2. (b) The substance in which a solute dissolves is called a solvent. True.
3. (c) The pressure exerted by air is called atmospheric pressure. True.

F) Explain the picture (Water ⇄ Ice)

Explain why ice floats and why freezing starts at the top. When water freezes, its volume increases and density decreases, so ice floats. In ponds/lakes the top layer cools first and freezes; water is densest at \(4^{\circ}\text{C}\), so deeper water stays liquid, allowing aquatic life to survive.

G) Write answers in your own words

1. (a) How is light scattered by the air? Tiny particles (dust, smoke, droplets) in air deflect incoming light in different directions, making the beam visible and colouring the sky at sunrise/sunset.
2. (b) Explain various properties of water. Water is fluid (no fixed shape), has definite volume/mass, dissolves many substances (universal solvent), expands on freezing (ice floats), and flows to generate energy and cool engines.
3. (c) Why is the density of seawater more than that of rainwater? Dissolved salts in seawater increase mass per unit volume, so \(\rho_{\text{sea}}>\rho_{\text{rain}}\).
4. (d) What is the importance of good soil structure? It improves aeration and drainage, enables deeper roots, and supports soil life—thus enhancing fertility and plant growth.
5. (e) What are the various uses of soil? Supports crops/forests, stores water (bunds/lakes), and—due to plasticity—helps make pots, bricks, idols; special clays serve in tiles/crockery/cosmetics.
6. (f) Need and importance of soil testing for farmers. Testing reveals texture, organics, pH, nutrients/salts; guides correct fertilization, amendments, crop choice, and prevents overuse of chemicals.
7. (g) Importance of air in transmission of sound. Air is the medium carrying sound waves to our ears; without air (in vacuum) sound cannot travel.
8. (h) Why should a glass bottle completely filled with water never be kept in a freezer? Water expands on freezing; since glass can’t stretch, internal pressure cracks or bursts the bottle.

Handy formulas: \( \displaystyle \rho=\frac{m}{V} \), units: \( \text{g cm}^{-3} \) or \( \text{kg m}^{-3} \). Sea-level air pressure \( \approx 1{,}01{,}400\ \text{N m}^{-2} \). Water is densest at \(4^{\circ}\text{C}\).

Observe • Experiment • Explain — that’s science!