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Chapter 1 Matter In Our Surroundings
Physical Nature Of Matter
Everything around us, from the air we breathe to the smallest particle of sand, is composed of material that scientists call matter.
Matter is defined by two fundamental properties: it occupies space (volume) and has mass.
Historically, matter was classified by early Indian philosophers into five basic elements or "Panch Tatva": air, earth, fire, sky, and water. Ancient Greek thinkers had similar ideas.
Modern science classifies matter based on both its physical properties and chemical nature. This chapter focuses on the physical classification.
Matter Is Made Up Of Particles
Scientific understanding of matter's nature evolved from two main ideas: matter being continuous (like a solid block) or particulate (made of tiny pieces like sand).
An activity of dissolving salt or sugar in water demonstrates that matter is particulate. The salt/sugar particles spread out and fit into the spaces between water particles, causing little or no change in the water level.
This observation supports the idea that matter is composed of tiny particles with spaces between them.
How Small Are These Particles Of Matter?
Experiments involving the serial dilution of a small amount of potassium permanganate solution illustrate the incredibly small size of matter particles.
Even after multiple dilutions, the solution retains its color, showing that a single crystal of potassium permanganate must contain millions of tiny particles that continue to divide.
This suggests that the particles of matter are extremely small, beyond simple imagination.
Characteristics Of Particles Of Matter
Particles Of Matter Have Space Between Them
As seen in dissolving salt/sugar, Dettol, or potassium permanganate in water, the particles distribute uniformly within the water.
Making beverages like tea or lemonade also involves particles of one substance fitting into the spaces between particles of another.
This phenomenon confirms that there are significant spaces between the particles of matter in different states.
Particles Of Matter Are Continuously Moving
Observations like the smell of an incense stick spreading through a room or ink/honey spreading in water demonstrate that particles of matter are in constant motion.
This inherent movement is due to the particles possessing kinetic energy.
When temperature increases, particles move faster, indicating that kinetic energy also increases with temperature.
The spontaneous mixing of particles of two different types of matter is called diffusion.
Diffusion occurs because particles move into the spaces between other particles.
Heating increases the rate of diffusion because the particles gain more kinetic energy and move faster.
Particles Of Matter Attract Each Other
Various activities (like forming human chains, trying to break an iron nail, chalk, or rubber band, or trying to cut the surface of water) show that particles of matter are held together by a force of attraction.
This force of attraction keeps the particles bonded together.
The strength of this attractive force varies between different substances. Solids generally have stronger forces than liquids, which have stronger forces than gases.
States Of Matter
Matter primarily exists in three physical states: solid, liquid, and gas.
These states differ based on the arrangement, movement, and forces of attraction between their constituent particles.
The Solid State
Solids have a definite shape, distinct boundaries, and a fixed volume.
They possess very low compressibility, meaning they are difficult to squeeze.
Solids are generally rigid, meaning they resist changes to their shape when external force is applied.
While typically rigid, some substances like a rubber band can change shape under force but regain their original shape when the force is removed (up to a limit).
Individual crystals of sugar or salt retain their shape regardless of the container they are placed in, confirming their solid nature.
A sponge is considered a solid because it has a definite shape and volume, but its compressibility is due to trapped air within its pores which is expelled when pressed.
Solids generally do not diffuse into other solids significantly under normal conditions.
The Liquid State
Liquids have no fixed shape but possess a fixed volume.
They take the shape of the container they are kept in.
Liquids are not rigid but are considered fluids because they can flow and change shape.
Liquids exhibit diffusion, allowing solids, other liquids, and gases to dissolve in them.
For example, atmospheric gases like oxygen and carbon dioxide dissolve in water, supporting aquatic life.
The rate of diffusion is generally higher in liquids compared to solids.
This higher diffusion rate is attributed to the particles in the liquid state moving more freely and having larger spaces between them than in solids.
The Gaseous State
Gases have neither a definite shape nor a definite volume.
They completely fill the vessel in which they are contained.
Gases are highly compressible compared to solids and liquids.
High compressibility allows large volumes of gas to be stored in small cylinders (e.g., LPG, CNG, oxygen cylinders).
Particles in the gaseous state move about randomly and at high speeds.
This rapid, random movement leads to particles colliding with each other and with the walls of the container.
The force exerted by these particles per unit area on the walls of the container is what creates the pressure exerted by the gas.
Gases diffuse very quickly into other gases due to the high speed of their particles and the large spaces between them. This explains why the smell of food or perfume travels rapidly through air (a mixture of gases).
When compressing a gas in a syringe, the piston moves easily because the particles are far apart and can be pushed closer together.
Can Matter Change Its State?
Matter can change from one state to another.
A common example is water, which can exist as a solid (ice), a liquid (water), or a gas (water vapour).
Changes in the state of matter occur due to changes in temperature or pressure.
Effect Of Change Of Temperature
Increasing the temperature of a solid increases the kinetic energy of its particles.
The increased kinetic energy causes particles to vibrate more vigorously from their fixed positions.
Eventually, the particles gain enough energy to overcome the forces of attraction holding them in place, causing the solid to melt and change into a liquid.
The minimum temperature at which a solid melts into a liquid at atmospheric pressure is called its melting point.
The melting point is an indicator of the strength of the forces between particles in a solid.
The melting point of ice is 273.15 K (0°C).
The process of melting (solid to liquid change) is also known as fusion.
During melting, the temperature of the substance remains constant even as heat energy is continuously supplied until all the solid has melted.
This absorbed heat energy, which does not cause a rise in temperature but is used to overcome the forces of attraction and change the state, is called latent heat (latent means hidden).
The latent heat of fusion is the amount of heat energy required to convert 1 kg of a solid into its liquid state at its melting point and atmospheric pressure.
Particles in water at 0°C (273 K) have more energy than particles in ice at the same temperature due to the latent heat of fusion absorbed.
Similarly, heating a liquid increases the kinetic energy of its particles.
At a specific temperature, particles gain enough energy to break free from the attractive forces and change into the gaseous state.
The temperature at which a liquid starts boiling at atmospheric pressure is its boiling point.
Boiling is considered a bulk phenomenon because particles from the entire volume of the liquid gain enough energy to vaporize.
The boiling point of water is 373 K (100°C).
The latent heat of vaporisation is the heat energy required to convert 1 kg of a liquid into its gaseous state at its boiling point and atmospheric pressure.
Particles in steam (water vapour) at 100°C (373 K) have more energy than water at the same temperature due to the latent heat of vaporisation absorbed.
Some substances can change directly from the solid state to the gaseous state without becoming a liquid, and vice versa.
The direct conversion of a solid to a gas is called sublimation (e.g., camphor, ammonium chloride).
The direct conversion of a gas to a solid is called deposition.
Temperature conversion between Celsius (°C) and Kelvin (K):
To convert Celsius to Kelvin, add 273.15 (often rounded to 273): $K = °C + 273$
To convert Kelvin to Celsius, subtract 273.15 (often rounded to 273): $°C = K - 273$
Effect Of Change Of Pressure
The state of matter is also influenced by pressure, in addition to temperature.
The distance between particles in gases is large.
Applying pressure to a gas forces the particles closer together.
Increasing pressure and decreasing temperature can cause gases to liquefy.
Solid carbon dioxide (dry ice) is stored under high pressure.
When the pressure on solid CO₂ is reduced to 1 atmosphere, it converts directly to gaseous CO₂ (sublimation), without melting into a liquid.
Thus, the physical state (solid, liquid, or gas) of a substance is determined by a combination of both temperature and pressure.
Units of pressure: The SI unit of pressure is Pascal (Pa). Atmosphere (atm) is also a common unit. $1 \text{ atm} \approx 1.01 \times 10^5 \text{ Pa}$. Atmospheric pressure at sea level is considered normal atmospheric pressure.
Evaporation
A change of state from liquid to vapour can occur even below the liquid's boiling point. This phenomenon is called evaporation.
Examples include water left in an open container slowly disappearing or wet clothes drying.
Evaporation is a surface phenomenon. At any given temperature, particles in a liquid have different kinetic energies.
Particles at the surface of the liquid with higher kinetic energy can overcome the forces of attraction holding them in the liquid state and escape into the vapour state.
Factors Affecting Evaporation
The rate of evaporation is influenced by several factors:
- Surface Area: A larger surface area exposed to the atmosphere increases the rate of evaporation. This is why wet clothes are spread out to dry.
- Temperature: Increasing the temperature provides more kinetic energy to the particles, allowing more particles to escape into the vapour state, thus increasing the rate of evaporation.
- Humidity: Humidity is the amount of water vapour in the air. If the air is already saturated with water vapour (high humidity), the rate of evaporation decreases because the air cannot hold much more water vapour.
- Wind Speed: An increase in wind speed increases the rate of evaporation. Wind carries away the water vapour particles from the surface of the liquid, decreasing the concentration of water vapour in the surroundings and promoting further evaporation. Clothes dry faster on a windy day.
How Does Evaporation Cause Cooling?
Evaporation is a process that causes cooling.
When a liquid evaporates, the particles that escape into the vapour state are those with the highest kinetic energy (most energetic).
To compensate for the energy lost by these escaping particles, the remaining liquid particles (and the surroundings) must supply energy.
The particles absorb energy from the surroundings (or the surface it is evaporating from) to regain the energy lost during evaporation.
This absorption of energy from the surroundings makes the surroundings feel cool.
Examples of cooling due to evaporation:
- When acetone, petrol, or perfume is put on the palm, it evaporates quickly, absorbing heat from the palm and making it feel cool.
- Sprinkling water on a hot roof or ground cools the surface because the water evaporates, absorbing a large amount of heat from the surface (due to the high latent heat of vaporisation of water).
- Wearing cotton clothes in summer helps us stay cool. Cotton absorbs sweat, which then evaporates from the fabric's surface, taking heat from our body and causing a cooling effect.
- Water kept in an earthen pot (matka) stays cool in summer because water seeps through the porous walls and evaporates from the outer surface, taking heat from the water inside the pot.
Water droplets observed on the outer surface of a glass containing ice-cold water are due to the condensation of water vapour present in the air. The cold glass cools the water vapour near its surface, causing it to lose energy and change into liquid water droplets.
More to Know: Other States of Matter
Scientists are currently discussing five states of matter:
- Solid
- Liquid
- Gas
- Plasma: This state consists of highly energetic and excited particles, typically in the form of ionized gases. Plasma is found in fluorescent tubes and neon signs when electricity is passed through the gas, causing it to ionize and glow. The Sun and stars are also composed of plasma due to their extremely high temperatures.
- Bose-Einstein Condensate (BEC): This state was predicted by Albert Einstein based on calculations by Indian physicist Satyendra Nath Bose. BEC is formed by cooling a gas of extremely low density (about one-hundred-thousandth the density of normal air) to super-low temperatures, near absolute zero. The particles in a BEC behave as a single quantum mechanical entity.
What You Have Learnt (Summary)
- Matter is made up of tiny particles.
- Matter around us exists in three main states: solid, liquid, and gas.
- The forces of attraction between particles are strongest in solids, intermediate in liquids, and weakest in gases.
- The space between particles and the kinetic energy of particles are lowest in solids, intermediate in liquids, and highest in gases.
- Particle arrangement is ordered in solids, layers can slip in liquids, and random in gases.
- States of matter can be interconverted by changing temperature or pressure.
- Sublimation is the direct change from solid to gas.
- Deposition is the direct change from gas to solid.
- Boiling is a bulk phenomenon where the whole liquid turns to vapour at the boiling point.
- Evaporation is a surface phenomenon where liquid turns to vapour below the boiling point.
- Evaporation rate depends on surface area, temperature, humidity, and wind speed.
- Evaporation causes cooling.
- Latent heat of vaporisation is the energy to change 1 kg liquid to gas at its boiling point.
- Latent heat of fusion is the energy to change 1 kg solid to liquid at its melting point.
Measurable Quantities and Units
Some important physical quantities and their SI units:
- Temperature: kelvin (K)
- Length: metre (m)
- Mass: kilogram (kg)
- Weight: newton (N)
- Volume: cubic metre ($m^3$)
- Density: kilogram per cubic metre ($kg\ m^{-3}$)
- Pressure: pascal (Pa)
Intext Questions
Page No. 3
Question 1. Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.
Answer:
Question 2. 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:
Question 3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer:
Question 4. What are the characteristics of the particles of matter?
Answer:
Page No. 6
Question 1. 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:
Question 2. (a) Tabulate the differences in the characterisitcs of states of matter.
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
Answer:
Question 3. Give reasons
(a) A gas fills completely the vessel in which it is kept.
(b) A gas exerts pressure on the walls of the container.
(c) A wooden table should be called a solid.
(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:
Question 4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer:
Page No. 9
Question 1. Convert the following temperature to celsius scale:
a. 300 K
b. 573 K.
Answer:
Question 2. What is the physical state of water at:
a. 250ºC
b. 100ºC ?
Answer:
Question 3. For any substance, why does the temperature remain constant during the change of state?
Answer:
Question 4. Suggest a method to liquefy atmospheric gases.
Answer:
Page No. 10
Question 1. Why does a desert cooler cool better on a hot dry day?
Answer:
Question 2. How does the water kept in an earthen pot (matka) become cool during summer?
Answer:
Question 3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer:
Question 4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer:
Question 5. What type of clothes should we wear in summer?
Answer:
Exercises
Question 1. Convert the following temperatures to the celsius scale.
(a) 293 K
(b) 470 K
Answer:
Question 2. Convert the following temperatures to the kelvin scale.
(a) 25°C
(b) 373°C
Answer:
Question 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:
Question 4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Answer:
Question 5. What is the physical state of water at—
(a) 25°C
(b) 0°C
(c) 100°C ?
Answer:
Question 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:
Question 7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer:
Question 8. What produces more severe burns, boiling water or steam?
Answer:
Question 9. Name A,B,C,D,E and F in the following diagram showing change in its state
Answer: