CBSE Class 9 Science Advanced Chemistry Matter in Our Surrounding Notes Set 01

Matter in Our Surrounding

Introduction

Books, stones, water and air are all around us. A book fills some space (its volume) and feels heavy (it has mass). Air also fills space and has mass: a balloon full of air weighs more than an empty one. Pushing or lifting any object needs a force, which shows that matter resists a change in its position.

Matter is anything that occupies space, has mass and offers resistance.

A. Physical Nature of Matter

(a) Matter is made up of particles

Everything around us is made of very small particles, which are atoms or molecules, and these particles are always moving. Two kinds of observation prove this: diffusion and Brownian motion.

Evidence 1: sugar in water. Mark the level of water in a beaker and stir some sugar into it. The sugar vanishes but the level does not rise. This is because sugar is made of separate particles, which settle in the empty gaps between the water particles. If sugar were one continuous block, the level would rise.

Evidence 2: pollen grains in water. In 1827 Robert Brown saw under a microscope that tiny pollen grains floating in water kept moving in zig-zag paths. The grains are pushed from all sides by the fast-moving water molecules, which are too small to be seen, but their effect on the grains can be seen.

Brownian motion is this random, zig-zag motion of tiny visible particles. It occurs in gases too: dust specks dancing in a beam of sunlight are hit by air molecules. It shows that matter is made of tiny particles and that these particles are always moving. Brownian motion gets faster when the temperature rises.

(b) Characteristics of particles of matter

1. Particles of matter are extremely small. Dissolve 2 to 3 crystals of potassium permanganate (\( \ce{KMnO4} \)) in 100 mL of water to get a deep purple solution. Take 10 mL of it and mix it with 90 mL of clear water, and repeat this 5 to 8 times. The solution is still coloured even after so much dilution. So a few crystals contain millions of tiny particles, which keep spreading into more and more water.

2. There are spaces between the particles. Dissolve 20 g of sugar in 100 mL of water marked on a beaker. The level of the solution stays at the same mark, so the sugar particles have gone into the gaps among the water particles, which therefore are not packed tightly.

3. Particles are constantly moving. This is shown by diffusion, which is the mixing of the particles of two different substances on their own. In it, particles travel from where they are plentiful (higher concentration) to where they are fewer (lower concentration). Perfume opened in one corner of a room is smelt everywhere soon, as its particles mix with the moving air particles.

Experiment: put a gas jar of red-brown bromine vapour at the bottom and invert a jar of colourless air over it. After some time the upper jar also turns red-brown because bromine vapour has diffused into the air.

The particles have kinetic energy, which is why they move, and they move faster when the temperature is higher.

4. Particles attract each other. The force between particles of the same substance is called cohesive force, and the force between particles of different substances is called adhesive force. If chalk, an ice cube and an iron nail are struck with a hammer, the chalk breaks easily, the ice needs a stronger blow, and the nail does not break. So the attraction is weak in chalk, stronger in ice and very strong in iron.

Rigid and fluid

A rigid substance does not bend and keeps its shape under an outside force; solids are rigid. A fluid is a substance that can flow. A liquid is a fluid which takes the shape of its container and has a well defined surface, while a gas is a fluid which spreads to fill the whole container. Liquids and gases together are called fluids.

(c) Classification of matter

On the basis of physical state, matter is of three kinds: solid, liquid and gas.

Solids (ice, wood, coal, iron) have a definite shape, mass and volume. They are almost incompressible, have high density, have very strong forces between particles, expand very little on heating and diffuse into each other very slowly.

Liquids (water, alcohol, milk, mercury) have a definite mass and volume but no definite shape. They are slightly more compressible than solids, have lower density than solids, have weaker forces between particles, expand much more than solids on heating, and two different liquids diffuse into each other easily to form a uniform mixture.

Gases (\( \ce{O2} \), \( \ce{N2} \), \( \ce{H2} \)) have a definite mass, but neither a definite shape nor a definite volume. They are highly compressible because the spaces between particles are very large, have extremely low density, have negligible forces between particles, expand greatly on heating and diffuse into each other very quickly.

Comparison of the three states of matter

Uses of the high compressibility of gases. LPG (liquefied petroleum gas) is stored in cylinders for cooking at home, oxygen is supplied to hospitals in cylinders (as compressed gas, or as liquid oxygen in large tanks), and CNG (compressed natural gas) is used as fuel in vehicles.

Gas particles move randomly at high speed. They collide with each other and with the walls of the container, and these collisions produce the pressure of the gas.

Plasma

Plasma is a state of matter made of very energetic, ionised gas particles. It forms when electricity passes through a gas and ionises it. It glows with a colour that depends on the gas. Neon signs (neon gas) and fluorescent tubes (mercury vapour and argon; the book names helium) contain plasma, and the Sun and the stars glow because they are made of plasma.

Bose-Einstein condensate (B.E.C.)

It is formed when a gas of extremely low density, about one hundred-thousandth of the density of normal air, is cooled to an extremely low temperature.

Solved Examples

Example 1. What is the difference between fluidity and viscosity ?
Answer:
Fluidity is the tendency of a substance to flow.
Viscosity is the resistance a fluid offers to flow, so the two are opposite properties.

Example 2. Give reasons why the rate of diffusion of liquid in another liquid is greater than rate of diffusion of a solid in a liquid ?
Answer:
The particles of a liquid have more kinetic energy than the particles of a solid, so they move and mix faster.

B. Interconversion of States of Matter

The change of matter from one state to another, and back to the original state, by changing the conditions is called interconversion of states of matter. The states can be changed by changing (a) the temperature or (b) the pressure.

(a) Changing the temperature

(i) Solid and liquid. A solid becomes a liquid on heating and a liquid becomes a solid on cooling. Ice at 0 °C turns into water at 0 °C on taking in heat, and water at 0 °C turns into ice at 0 °C when it freezes.

Activity: change of state from ice to water. Take a beaker half-filled with ice cubes on a wire gauze over a tripod stand, with a Celsius thermometer hung from an iron stand so that its bulb is in the ice, and a glass stirrer. The temperature of ice is 0 °C (273 K). Heat gently with a Bunsen burner, stir continuously and note the temperature five or six times. The temperature stays at 0 °C (273 K) until all the ice has melted.

Melting (fusion) is the change of a solid into a liquid by absorbing heat. Freezing (solidification) is the change of a liquid into a solid by giving out heat. The fixed temperature at which a solid melts at 1 atm pressure is its melting point, and the fixed temperature at which a liquid freezes is its freezing point. The two are numerically equal; for ice and water both are 0 °C (about 273 K).

Why the temperature does not rise. Heat makes the particles of the solid vibrate faster. The heat supplied finally overcomes the attraction between particles, so they leave their fixed positions and move freely, and the solid melts. All the heat is used up in this change.

Latent heat of fusion is the heat needed to change 1 kg of solid into liquid at its melting point and atmospheric pressure. For ice it is 3.34 × 10⁵ J/kg. (Latent means hidden.) Water at 0 °C has more energy than ice at 0 °C because it has absorbed this heat.

(ii) Liquid and gas. A liquid becomes a gas on heating and a gas becomes a liquid on cooling. At 1 atm, water changes into steam at 100 °C by absorbing heat, and steam at 100 °C changes into water by giving out heat.

Activity: change of state from water to steam. Set up the same apparatus with water in the beaker. Heat it gently, stir, and note the temperature until the water boils and for a few minutes after. The temperature rises till 100 °C (373 K) and stays at 100 °C (373 K) while the water keeps changing into steam.

Boiling (vaporisation) is the change of a liquid into a gas by absorbing heat. Condensation (liquefaction) is the change of a gas into a liquid by giving out heat. The fixed temperature at which a liquid changes rapidly into gas at atmospheric pressure is its boiling point, and the temperature at which a gas condenses is the condensation point. They are equal: for water vapour and water both are 100 °C (about 373 K).

On heating, the particles move faster until they have enough energy to break the attraction between them. The liquid then changes to gas. Latent heat of vaporisation is the heat needed to change 1 kg of liquid at its boiling point into vapour without any rise in temperature. For water it is about 22.6 × 10⁵ J/kg (the book gives 22.5 × 10⁵ in one place). Steam at 373 K has more energy than water at 373 K because it has absorbed this latent heat.

Temperature-time graph

If the readings of the experiments are plotted against time, we get the graph shown.

Between A and B the ice melts and the temperature stays at 0 °C. At B all the ice has become water. From B to C the temperature of the water rises. At C (100 °C) the water starts boiling, and from C to D the temperature stays constant, as the liquid changes into gas. At D all the water has become steam, and beyond D the temperature of steam rises along DE.

(iii) Solid and gas: sublimation. The change of a solid directly into vapour on heating, or of a vapour directly into solid on cooling, is called sublimation. The solid that sublimes is called the sublime, and the solid obtained on cooling the vapour is the sublimate. Examples are ammonium chloride (\( \ce{NH4Cl} \)), iodine, camphor, naphthalene (moth balls) and anthracene.

Specific heat. The specific heat of a substance is the heat needed to raise the temperature of a unit mass of it by 1 °C. In SI units it is the heat in joules needed to raise the temperature of 1 kg of the substance by 1 °C. It is written C (or S), and its SI unit is J/kg °C or J kg^–1 °C^–1. It changes slightly with temperature because the arrangement of molecules changes.

(b) Changing the pressure

States differ because of different spaces between particles. When a gas is compressed, the spaces become smaller and it can turn into a liquid. So high pressure and low temperature can liquefy a gas. For example, carbon dioxide (\( \ce{CO2} \)) is a gas at normal conditions, but it can be liquefied under a pressure about 70 times atmospheric pressure. Solid \( \ce{CO2} \) is called dry ice; it is extremely cold and is used to deep-freeze food and keep ice-cream cold.

Unit of pressure. The atmosphere (atm) is a unit of pressure, and the SI unit is the pascal (Pa), with 1 atm = 1.01 × 10⁵ Pa. Lowering the pressure lowers the boiling point of a liquid, which makes it change to gas more easily.

Solved Examples

Example 3. Why is ice at 273 K more effective in cooling than water at the same temperature.
Answer:
To melt, ice has to absorb the latent heat of fusion (about 3.34 × 10⁵ J/kg) from its surroundings, which cools them.
Water at 273 K is already liquid and has no such heat to absorb, so it cools much less than ice.

Example 4. What produces more severe burns, boiling water or steam ?
Answer:
Water at 373 K changing to steam at 373 K takes in the latent heat of vaporisation (about 22.6 × 10⁵ J/kg).
So steam holds more heat than boiling water at the same temperature, and gives all of it to the skin when it condenses. Steam therefore causes more severe burns.

C. Evaporation

The change of a liquid into vapour at any temperature below its boiling point is called evaporation. The particles of a liquid are always moving with different kinetic energies. A small number of particles at the surface with high energy break free of the attraction of the other particles and become vapour. The atmospheric pressure at sea level is 1 atm.

(a) Factors affecting evaporation

1. Temperature: the rate of evaporation increases with temperature, since more particles then have enough energy to escape.

2. Surface area: evaporation happens at the surface, so a larger area gives a faster rate. This is why wet clothes are spread out.

3. Humidity: humidity is the amount of water vapour in the air. The rate of evaporation is inversely related to humidity: it is high in dry air and low in humid air.

4. Wind speed: faster wind carries the vapour away, so the rate increases with wind speed.

5. Nature of the substance: liquids with a high boiling point evaporate slowly and those with a low boiling point evaporate quickly.

(b) Differences between evaporation and boiling

(c) Cooling caused by evaporation

An evaporating liquid takes its latent heat of vaporisation from whatever it touches, and cools it. Examples:

1. Cotton clothes in summer. We sweat more in summer. Sweat evaporates by taking heat from the body, leaving it cool. Cotton absorbs sweat well and so helps the evaporation.

2. Acetone on the palm. Acetone, ether or nail polish remover poured on the palm feels cool, since the heat needed for evaporation is taken from the palm.

3. Sprinkling water on a roof or ground after a hot day. The water evaporates by taking heat from the ground and the air, which cools the place.

(d) Water droplets on a glass of ice-cold water

Water vapour in the air loses energy when it touches the cold glass and turns into liquid water, which forms droplets on the outside of the glass.

Solved Examples

Example 5. Out of water and petrol which is a more volatile ?
Answer:
Petrol is more volatile, because the force of attraction between its particles is weaker than that between water particles.

Example 6. Why do we feel cold after taking a hot bath ?
Answer:
The hot water on the body evaporates by taking heat from the skin, so the body loses heat and we feel cold.