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1. Introduction to Matter
Everything in our universe—from the air we breathe and the food we eat to stars, plants, and even a single drop of water—is made up of material that scientists call matter.
Key Definitions
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Matter: Anything that occupies space and has mass.
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Panch Tatva: Early Indian philosophers classified matter into five basic elements: Air, Earth, Fire, Sky, and Water. Ancient Greek philosophers held a similar view.
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Modern Classification: Scientists today classify matter based on two criteria:
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Physical Properties (Physical nature and appearance).
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Chemical Nature (Composition and reactions).
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1.1 Physical Nature of Matter
This section explores what matter is actually made of and how small its components are.
1.1.1 Matter is Made Up of Particles
Historically, there were two schools of thought: one believed matter was continuous (like a block of wood), and the other believed it was particulate (made of particles like sand).
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The Particle Theory: Matter is not continuous; it is made up of tiny particles.
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Evidence (The Salt Experiment): When salt is dissolved in water, the water level does not rise significantly. This is because the tiny particles of salt get into the spaces between the particles of water.
1.1.2 How Small are These Particles?
The particles of matter are incredibly small—so small they are "beyond our imagination."
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Evidence (The Potassium Permanganate Experiment):
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If you take just 2–3 crystals of potassium permanganate and dissolve them in 100 mL of water, the water turns deep purple.
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Even after diluting this solution several times (taking 10 mL and adding it to 90 mL of clear water repeatedly), the color persists.
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Conclusion: A single crystal contains millions of tiny particles that keep dividing themselves into smaller and smaller units.
Quick Summary Table
| Feature | Description |
| Basic Requirement | To be matter, an object must have Mass and Volume. |
| Structure | Matter is particulate (made of particles), not a continuous block. |
| Size | Particles are microscopic and exist in millions even in a tiny crystal. |
| SI Unit of Mass | Kilogram (kg). |
| SI Unit of Volume | Cubic metre ($m^3$). |
Study Tip: Remember that "Smell" as a sense is not matter, but the "Smell of Perfume" is matter because it involves actual particles of gas diffusing through the air.
1.2 Characteristics of Particles of Matter
The physical behavior of everything around us is dictated by the microscopic properties of its particles. These characteristics explain why a stone is hard, why smell travels across a room, and why salt disappears in water.
1.2.1 Particles of Matter Have Space Between Them
Matter is not a solid "block" without gaps; rather, it is composed of particles with varying amounts of empty space between them.
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The Evidence: When we make tea, coffee, or lemonade (nimbu paani), the particles of one type of matter (like sugar or lemon juice) get into the spaces between the particles of the other (water).
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Observation: This is why dissolving salt in water does not significantly increase the volume of the water—the salt particles simply "fill the gaps."
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Key Insight: The amount of space varies by state:
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Solids: Minimum space.
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Liquids: Moderate space.
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Gases: Maximum space.
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1.2.2 Particles of Matter are Continuously Moving
Particles are never still; they possess Kinetic Energy.
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Diffusion: The intermixing of particles of two different types of matter on their own is called diffusion.
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The Temperature Effect: As temperature rises, particles move faster because their kinetic energy increases. Therefore, diffusion becomes faster on heating.
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Practical Examples:
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Incense Stick: You can smell an unlit incense stick only by going close, but once lit, the moving gas particles carry the scent across the room.
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Food: The smell of hot sizzling food reaches you from several meters away, whereas cold food requires you to be close.
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Ink in Water: A drop of ink spreads on its own in a beaker of water due to the constant motion of both ink and water particles.
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1.2.3 Particles of Matter Attract Each Other
There is an internal force acting between the particles of matter that keeps them bound together.
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The Force of Attraction: This force varies in strength depending on the substance.
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Comparison of Strength:
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Iron Nail: Extremely strong attraction (hard to break).
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Chalk: Weak attraction (breaks easily with a hammer).
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Rubber Band: Moderate attraction (can be stretched but eventually snaps).
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The Diver Example: A diver can cut through water in a swimming pool because the force of attraction between water particles is enough to keep them together as a liquid, but weak enough to be overcome by the diver’s physical force.
Summary Table: Particle Characteristics
| Characteristic | Physical Phenomenon | Key Factor |
| Spaces Between | Dissolving sugar in water | Explains solubility and compressibility. |
| Continuous Motion | Smell of hot food traveling | Driven by Kinetic Energy; increases with heat. |
| Mutual Attraction | Solids maintaining their shape | Defined by the Intermolecular Force strength. |
Concept Check: Q: Why does a wooden table count as matter?
A: Because it occupies space (Volume) and has mass, and its particles are held together by strong forces of attraction.
Based on the NCERT Science textbook for Class 9, here are the solutions to the in-text questions from page 3.
Q1. Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.
Answer: The following are classified as matter because they have mass and occupy space:
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Chair (Solid)
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Air (Gas)
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Almonds (Solid)
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Lemon water (Liquid)
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Smell of perfume (The smell of perfume is considered matter because it involves the presence of actual perfume particles dispersed in the air).
Note: Love, hate, thought, and cold are feelings or sensations and do not have mass or volume, so they are not matter.
Q2. Give reasons for the following observation: The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
Answer: This happens due to the relationship between temperature and the kinetic energy of particles:
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Particles of matter are continuously moving. At higher temperatures, particles have more kinetic energy and move faster.
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In hot sizzling food, the rate of diffusion (intermixing of particles with air) is very high, allowing the aroma to travel long distances quickly.
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In cold food, the particles have low kinetic energy and a much lower rate of diffusion, so the smell does not travel far.
Q3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer: This observation highlights two key properties of particles of matter:
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Particles of matter have spaces between them: The diver can move through the water because there is enough space between the liquid particles for them to be displaced.
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Forces of attraction: The force of attraction between water particles is intermediate—strong enough to keep the liquid together, but weak enough to be overcome by the physical force of the diver.
Q4. What are the characteristics of the particles of matter?
Answer: The four fundamental characteristics of particles of matter are:
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They are very tiny: Particles are small beyond our imagination (as seen in the potassium permanganate experiment).
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They have spaces between them: This allows substances like salt or sugar to dissolve in water without increasing the total volume significantly.
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They are continuously moving: Particles possess kinetic energy, which increases as the temperature rises.
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They attract each other: Particles are held together by a force of attraction; the strength of this force varies from one substance to another (e.g., stronger in an iron nail than in a piece of chalk).
1.3 States of Matter
Matter around us exists in three distinct physical states. These states arise due to variations in the characteristics of the particles—specifically their spacing, motion, and force of attraction.
1.3.1 The Solid State
Solids are characterized by structural rigidity and resistance to changes in shape or volume.
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Key Properties:
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Definite Shape & Boundaries: Solids maintain a fixed shape regardless of the container.
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Fixed Volume: They occupy a specific amount of space.
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Negligible Compressibility: Particles are already so close together that they cannot be easily pushed closer.
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Rigidity: They have a tendency to maintain their shape when subjected to outside force.
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Exceptional Cases (Conceptual Clarity):
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Rubber Band: It is a solid because it regains its shape once force is removed; it only breaks under excessive force.
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Sponge: It is a solid, but it can be compressed because of minute holes that trap air. When pressed, the air is expelled.
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Salt/Sugar: Though they take the shape of a jar, the individual crystals maintain a fixed shape.
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1.3.2 The Liquid State
Liquids are "fluids"—they have the ability to flow and change shape.
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Key Properties:
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No Fixed Shape: They take the shape of the container they are poured into.
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Fixed Volume: Unlike gases, a liter of water remains a liter regardless of the container size.
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Low Compressibility: While more compressible than solids, it is still very low.
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Fluidity: Particles can slide over one another, allowing the substance to flow.
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Diffusion in Liquids:
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Solids, liquids, and even gases can diffuse into liquids.
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Life-Sustaining Fact: Gases from the atmosphere (like oxygen and carbon dioxide) diffuse into water, which is essential for the survival of aquatic plants and animals.
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1.3.3 The Gaseous State
Gases are highly energetic and have the most "freedom" among the three states.
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Key Properties:
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No Fixed Shape or Volume: They expand to fill any container completely.
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High Compressibility: Because of large spaces between particles, gases can be compressed into small volumes (e.g., LPG for cooking or CNG for vehicles).
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High Rate of Diffusion: Particles move at very high speeds, allowing smells (like the aroma of food) to spread across a house in seconds.
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Pressure: Gas particles hit the walls of their container randomly and at high speed. This force exerted per unit area is what we call gas pressure.
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Comparative Summary Table
| Property | Solid | Liquid | Gas |
| Shape | Fixed | Not Fixed | Not Fixed |
| Volume | Fixed | Fixed | Not Fixed |
| Inter-particle Space | Minimum | Moderate | Maximum |
| Force of Attraction | Maximum | Moderate | Minimum |
| Compressibility | Negligible | Low | Very High |
| Particle Motion | Only Vibrate | Slide over each other | Random, high speed |
Did You Know? The reason you can easily move your hand through air (gas) but need a karate expert to break a block of wood (solid) is due to the difference in the force of attraction between the particles and the available space between them!
How do you think temperature affects these three states differently?
Q1. The mass per unit volume of a substance is called density (density = mass/volume). Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Answer: Density depends on how closely the particles are packed. The order of increasing density is:
Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron
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Note: Exhaust is denser than air because it contains smoke particles and heavier gases. Cotton is a solid but has many air pores, making its overall density lower than water.
Q2. (a) Tabulate the differences in the characteristics of states of matter.
Answer:
| Property | Solid | Liquid | Gas |
| Shape | Fixed/Definite | Indefinite (takes shape of container) | Indefinite |
| Volume | Fixed | Fixed | Indefinite |
| Rigidity/Fluidity | Rigid | Fluid (flows) | Fluid (flows) |
| Compressibility | Negligible | Low | Very High |
| Intermolecular Space | Minimum | Moderate | Maximum |
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
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Rigidity: The tendency of a substance to maintain its shape when subjected to outside force. Solids are highly rigid.
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Compressibility: The property by which the volume of a substance can be decreased by applying pressure. Gases have the highest compressibility.
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Fluidity: The ability of a substance to flow. Liquids and gases are fluids.
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Filling a gas container: Because gas particles move randomly in all directions at high speeds, they occupy all the available space in a container.
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Shape: The external boundary of an object. Only solids have a fixed shape.
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Kinetic Energy: The energy possessed by particles due to their motion. It is maximum in gases and minimum in solids.
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Density: Mass per unit volume. Generally, Solids > Liquids > Gases.
Q3. Give reasons:
(a) A gas fills completely the vessel in which it is kept.
Answer: The force of attraction between gas particles is negligible, and they possess high kinetic energy. This allows them to move randomly in all directions and occupy the entire volume of the container.
(b) A gas exerts pressure on the walls of the container.
Answer: Particles of gas move randomly at high speeds. During this movement, they hit each other and the walls of the container. The force exerted by these particles per unit area on the walls creates gas pressure.
(c) A wooden table should be called a solid.
Answer: A wooden table has a definite shape, distinct boundaries, and a fixed volume. It is rigid and cannot be compressed, which are all defining characteristics of the solid state.
(d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
Answer: In air (gas), the particles have large spaces between them and very weak forces of attraction, allowing our hand to displace them easily. In wood (solid), the particles are tightly packed with very strong forces of attraction, requiring a massive amount of force to break them apart.
Q4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer: While solids are usually denser than their liquid forms, ice has a lower density than water. This is because when water freezes to form ice, its molecules arrange themselves in a cage-like structure with large empty spaces between them. This increases the volume for the same mass, making ice lighter than the liquid water it displaces, allowing it to float.
Can Matter Change its State?
Matter does not always stay in one form. By altering physical conditions—specifically temperature and pressure—we can force a substance to transition between solid, liquid, and gaseous states.
1.4.1 Effect of Change of Temperature
When we heat a substance, we provide kinetic energy to its particles.
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Melting (Fusion): As a solid is heated, particles vibrate more vigorously. Eventually, they overcome the strong forces of attraction and break free from their fixed positions. The temperature at which a solid becomes a liquid at atmospheric pressure is its Melting Point.
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Example: The melting point of ice is $273.15\text{ K}$ ($0^\circ\text{C}$).
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Boiling (Vaporization): When a liquid is heated, particles gain enough energy to break all bonds of attraction, entering the gaseous state. Boiling is a bulk phenomenon.
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Example: The boiling point of water is $373\text{ K}$ ($100^\circ\text{C}$).
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Latent Heat: This is the "hidden" heat used to change the state of matter without increasing its temperature.
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Latent Heat of Fusion: Heat required to change $1\text{ kg}$ of solid into liquid at its melting point.
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Latent Heat of Vaporization: Heat required to change $1\text{ kg}$ of liquid into gas at its boiling point.
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1.4.2 Effect of Change of Pressure
The physical state of matter is largely determined by the distance between its particles. By applying pressure, we can bring these particles closer together.
Liquefaction of Gases
Gases have the largest intermolecular spaces. If we increase pressure and reduce temperature, the gas particles come so close together that they turn into a liquid.
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LPG (Liquefied Petroleum Gas): Used for cooking.
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CNG (Compressed Natural Gas): Used as a clean fuel for vehicles.
Solid Carbon Dioxide (Dry Ice)
Solid $CO_2$ is a unique example of the effect of pressure.
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It is stored under extremely high pressure.
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If the pressure is decreased to $1\text{ atmosphere}$, it converts directly into a gas without passing through the liquid state.
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Because it provides cooling without leaving any liquid residue, it is known as Dry Ice.
Key Definitions & Processes
| Process | Transition | Description |
| Sublimation | Solid $\rightarrow$ Gas | Change of state directly from solid to gas without becoming liquid (e.g., Camphor, Ammonium Chloride). |
| Deposition | Gas $\rightarrow$ Solid | Direct change of gas to solid without becoming liquid. |
| Condensation | Gas $\rightarrow$ Liquid | Particles lose energy and come closer together to form a liquid. |
| Solidification | Liquid $\rightarrow$ Solid | Also known as freezing; particles lose energy and settle into fixed positions. |
Note on Units:
To convert Celsius to Kelvin: $T(K) = T(^\circ\text{C}) + 273$
To convert Kelvin to Celsius: $T(^\circ\text{C}) = T(K) - 273$
Summary Table: Interconversion of States
The state of a substance is a result of the tug-of-war between Temperature (which increases particle movement) and Pressure (which brings particles together).
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Increasing Temperature / Decreasing Pressure: Favors the Gaseous state.
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Decreasing Temperature / Increasing Pressure: Favors the Solid state.
Based on the
Questions & Answers
1. Convert the following temperature to the celsius scale:
(a) 300 K
(b) 573 K
Answer: To convert Kelvin to Celsius, we use the formula: $T(^\circ\text{C}) = T(K) - 273$.
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(a) 300 K: $300 - 273 = 27^\circ\text{C}$
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(b) 573 K: $573 - 273 = 300^\circ\text{C}$
2. What is the physical state of water at:
(a) 250°C
(b) 100°C
Answer:
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(a) 250°C: At this temperature, which is far above the boiling point of water ($100^\circ\text{C}$), water exists in the gaseous state (steam).
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(b) 100°C: This is the boiling point of water. At this temperature, water can exist in both liquid and gaseous states. The transition is occurring as the liquid absorbs latent heat to turn into vapor.
3. For any substance, why does the temperature remain constant during the change of state?
Answer: During a change of state, the heat energy supplied to the substance is used to overcome the forces of attraction between the particles. This energy is known as Latent Heat (hidden heat). Because the energy is being consumed to break molecular bonds rather than increasing the kinetic energy of the particles, the thermometer does not show a rise in temperature until the entire substance has completely changed its state.
4. Suggest a method to liquefy atmospheric gases.
Answer: Atmospheric gases can be liquefied by increasing pressure and decreasing temperature.
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Process: High pressure is applied to compress the gas particles, bringing them closer together. Simultaneously, the temperature is lowered to reduce the kinetic energy of the particles. When the particles are close enough and moving slowly enough, the inter-molecular forces of attraction bind them together, turning the gas into a liquid.
Quick Summary Table
| Concept | Key Takeaway |
| Kelvin to Celsius | Subtract 273 from the Kelvin value. |
| Latent Heat | Heat used for state change, not temperature rise. |
| Liquefaction | Requires High Pressure + Low Temperature. |
Study Material: Evaporation – The Silent Cooling Phenomenon
While boiling is a "loud" and obvious change of state that happens at a specific temperature, evaporation is the quiet, everyday hero. It allows clothes to dry on a line and keeps your body cool during a workout, all without ever reaching the boiling point.
1.5 Evaporation: The Surface Phenomenon
Unlike boiling, which involves the entire bulk of a liquid, evaporation is a surface phenomenon.
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The Science: In any liquid, particles move at different speeds. A small fraction of particles at the surface possess higher kinetic energy.
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The Escape: These high-energy surface particles break away from the forces of attraction of their neighbors and escape into the air as vapor.
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Definition: The phenomenon of change of a liquid into vapors at any temperature below its boiling point is called evaporation.
1.5.1 Factors Affecting Evaporation
Why do clothes dry faster on a windy day or when spread out? The rate of evaporation isn't constant; it depends on four major factors:
| Factor | Effect on Evaporation | Why? |
| Surface Area | Increases | More surface area means more particles are exposed at the top, ready to "jump" into the air. |
| Temperature | Increases | Higher temperature gives more particles enough kinetic energy to overcome attractive forces. |
| Humidity | Decreases | Humidity is the amount of water vapor already in the air. If the air is "full," it can't accept more vapor easily. |
| Wind Speed | Increases | Wind blows away the water vapor particles near the surface, making room for more to escape. |
1.5.2 How Does Evaporation Cause Cooling?
This is the most "magical" part of the process. In an open vessel, a liquid keeps evaporating. But where do the particles get the energy to escape?
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Energy Debt: As high-energy particles leave the surface, the remaining liquid loses energy.
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Absorption: To recover this lost energy, the liquid absorbs heat from its surroundings.
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The Result: Because heat is being taken away from the surroundings, the surroundings become cold.
Real-World Examples of Evaporative Cooling
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The "Acetone" Test: When you pour nail polish remover (acetone) on your palm, the particles evaporate rapidly by taking energy from your hand, making your palm feel ice-cold.
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Earthen Pots (Matka): These pots have tiny pores. Water seeps through and evaporates from the outer surface, taking heat from the water inside and keeping it cool.
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Summer Clothing: We wear cotton clothes in summer because cotton is a great absorber of sweat. It brings the sweat into contact with the air, leading to evaporation which cools our body.
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Desert Coolers: These work best on hot, dry days because the high temperature and low humidity lead to a high rate of evaporation, resulting in significant cooling.
Critical Distinction: Evaporation vs. Boiling
Pro-Tip: Remember that Boiling happens throughout the liquid (bulk phenomenon) at a fixed temperature, while Evaporation happens only at the surface at any temperature below the boiling point.
Check Your Understanding
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Why do we see water droplets on the outside of a glass containing ice-cold water?
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Answer: Water vapor in the air touches the cold glass, loses energy (condensation), and turns back into liquid droplets.
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Why does a saucer allow you to sip hot tea faster than a cup?
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Answer: A saucer has a larger surface area, increasing the rate of evaporation and cooling the tea much faster.
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How would you explain the cooling effect of a desert cooler using the factors we just discussed?
Based on the principles of evaporation found in the NCERT Science textbook, the cooling effect of a desert cooler is a perfect real-world application of how temperature, humidity, and energy absorption work together.
Here is the breakdown of why it works so well:
1. The Role of Humidity
A desert cooler is designed for "hot and dry" days. As we discussed, evaporation increases when humidity is low. On a dry day, the air has a high capacity to hold more water vapor. This allows the water in the cooler pads to evaporate rapidly. If it were a humid or rainy day, the air would already be "full" of moisture, and the cooler wouldn't be nearly as effective.
2. The Role of Temperature
The "hot" part of a hot dry day provides the necessary thermal energy. High temperatures increase the kinetic energy of the water particles on the cooling pads, allowing them to overcome their liquid bonds and convert into vapor more quickly.
3. The Mechanism of Cooling (Latent Heat)
This is the core "magic" of the device. As the water on the pads evaporates:
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It needs energy to change from a liquid to a gas (the Latent Heat of Vaporization).
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It absorbs this heat energy from the incoming hot air and the surface of the pads.
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Because the air loses its heat energy to the water, the air itself becomes significantly colder before it is blown into the room by the fan.
In Summary
A desert cooler "trades" the heat of the air for the evaporation of water. By exploiting low humidity and high temperatures, it forces rapid evaporation, which effectively "strips" the heat away from the air, leaving you with a chilled breeze.
Fun Fact: This is exactly why you feel cold when you step out of a swimming pool on a windy day—your body is acting like the cooling pad, and the wind is accelerating the evaporation of water from your skin!
Exercise Questions and Answers:
Based on the
Chapter Exercises: Questions & Answers
1. Convert the following temperatures to the Celsius scale.
(a) 293 K
(b) 470 K
Answer:
To convert Kelvin ($K$) to Celsius ($^\circ\text{C}$), subtract 273 from the given value.
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(a) 293 K: $293 - 273 = 20^\circ\text{C}$
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(b) 470 K: $470 - 273 = 197^\circ\text{C}$
2. Convert the following temperatures to the Kelvin scale.
(a) 25°C
(b) 373°C
Answer:
To convert Celsius ($^\circ\text{C}$) to Kelvin ($K$), add 273 to the given value.
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(a) 25°C: $25 + 273 = 298\text{ K}$
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(b) 373°C: $373 + 273 = 646\text{ K}$
3. Give reason for the following observations.
(a) Naphthalene balls disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several metres away.
Answer:
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(a) Naphthalene balls undergo sublimation, where a solid changes directly into a gas without passing through the liquid state. As they turn into vapor, they disappear into the air completely.
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(b) Perfume particles are highly volatile and move at high speeds. They diffuse rapidly into the air particles, spreading throughout the room and reaching our nostrils even from a distance.
4. Arrange the following substances in increasing order of forces of attraction between the particles—water, sugar, oxygen.
Answer:
The strength of attraction is lowest in gases and highest in solids.
Oxygen (Gas) < Water (Liquid) < Sugar (Solid)
5. What is the physical state of water at—
(a) 25°C
(b) 0°C
(c) 100°C
Answer:
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(a) 25°C: Liquid (standard room temperature).
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(b) 0°C: Both Solid and Liquid (ice starts melting or water starts freezing).
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(c) 100°C: Both Liquid and Gas (water starts boiling and turning into steam).
6. Give two reasons to justify—
(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.
Answer:
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(a) Water: It has no fixed shape (takes the shape of the container) and it has the ability to flow (fluidity).
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(b) Iron Almirah: It has a fixed shape and a fixed volume; it is rigid and cannot be compressed easily.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer:
Ice at $273\text{ K}$ ($0^\circ\text{C}$) is more effective because it must absorb extra energy—the Latent Heat of Fusion—from the surroundings to melt into water. Water at the same temperature has already absorbed this heat, so it takes less energy from the surroundings than ice does.
8. What produces more severe burns, boiling water or steam?
Answer:
Steam produces more severe burns than boiling water. This is because steam (at $100^\circ\text{C}$) contains more energy in the form of Latent Heat of Vaporization compared to boiling water at the same temperature. When steam hits the skin, it releases this extra "hidden" heat as it condenses.
9. Name A, B, C, D, E, and F in the following diagram showing change in state.
Answer:
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A: Fusion (or Melting) — Solid to Liquid
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B: Vaporisation — Liquid to Gas
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C: Condensation — Gas to Liquid
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D: Solidification (or Freezing) — Liquid to Solid
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E: Sublimation — Solid to Gas
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F: Deposition — Gas to Solid
