CBSE Class 9 Science Matter in our sorroundings Notes

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MATTER IN OUR SURROUNDINGS

MATTER IN OUR SURROUNDINGS

All matter in the universe exists in three states. There are two ways of classifying matter:

1. According to physical state: solid, liquid or gas.
2. According to composition: element, compound or mixture.

CHARACTERISTICS OF PARTICLES OF MATTER

Particles of matter have space between them. When we make tea, coffee or lemonade (nimbu paani), particles of one type of matter get into the spaces between the particles of the other. This shows that there is enough space between the particles of matter. Similarly, particles of sugar, salt, Dettol or potassium permanganate get evenly distributed in water.

Particles of matter are continuously moving. They possess kinetic energy. As the temperature rises, the particles move faster, so with increase in temperature the kinetic energy of the particles also increases.

Particles of matter attract each other. A force acts between the particles and keeps them together. The strength of this force of attraction varies from one kind of matter to another.

INTEXT QUESTIONS (PAGE 3)

Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.

Answer: Chair, air, smell, almonds, cold-drink and smell of perfume are matter, because they have some weight and occupy space.

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: Hot food evaporates easily. Its vapours diffuse between the air molecules and reach within a short time to a distant place. The case is different with cold food because it remains in solid form and does not mix with air molecules, so we have to go close to smell it.

A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Answer: It shows that matter has space between its particles. The diver pushes the water particles aside and moves through the gaps.

What are the characteristics of the particles of matter?

Answer: (1) Particles of matter have space between them. (2) Particles of matter are continuously moving. (3) Particles of matter attract each other. (4) Particles of matter are very small in size.

STATES OF MATTER

1. The solid state. Solids have a definite shape, distinct boundaries and a fixed volume, i.e. negligible compressibility. Solids tend to maintain their shape when an outside force acts on them. They may break under force, but it is difficult to change their shape, so they are rigid.

2. The liquid state. Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid. The rate of diffusion of liquids is higher than that of solids, because in the liquid state the particles move freely and have greater space between them than in the solid state.

3. The gaseous state. Gases are highly compressible compared with solids and liquids. The LPG cylinder that we get at home for cooking, or the oxygen supplied to hospitals in cylinders, is compressed gas. In the gaseous state the particles move about randomly at high speed. Because of this random movement, the particles hit each other and also the walls of the container.

INTEXT QUESTIONS (PAGE 6)

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: The order of density is gas < liquid < solid. Thus: air, exhaust from chimneys (gas); water, honey (liquid); cotton, chalk, iron (solid), in increasing order of density.

(a) Tabulate the differences in the characteristics of states of matter.

Answer: The differences are given in the table.

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Answer: Comments:

1. Rigidity: the property due to which an object retains its shape and size. Solids are rigid whereas liquids and gases are not.
2. Compressibility: the property due to which a substance can be compressed, i.e. its volume can be decreased. Gases are compressible, solids and liquids hardly so.
3. Fluidity: the property due to which a substance tends to flow. Gases and liquids are fluids; solids are not.
4. Filling a gas container: a gas can be filled in a container by compressing it under high pressure. The compressibility of gases helps in this.
5. Shape: the definite geometry of a substance. Solids have a definite shape whereas liquids and gases do not.
6. Kinetic energy: the energy possessed by the particles of a substance due to their motion. Gas particles have the highest kinetic energy. Increasing the temperature increases the kinetic energy of the particles.
7. Density: the mass per unit volume of a substance.

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: (a) The force of attraction between gas particles is very small, so the particles move freely in all directions. Therefore a gas completely fills the vessel in which it is kept.

(b) Gas particles move randomly in all directions at high speed. They hit each other and also hit the walls of the container with a force. Therefore a gas exerts pressure on the walls of the container.

(c) A wooden table has a definite shape and volume. It is rigid and cannot be compressed, i.e. it has the characteristics of a solid. Hence a wooden table should be called a solid.

(d) The particles of air have large spaces between them, whereas wood has little space between its particles and is rigid. For this reason we can easily move our hand in air, but to do the same through a block of wood we need a karate expert.

Liquids generally have lower density than solids. But you must have observed that ice floats on water. Find out why.

Answer: Though ice is a solid, it has a large number of empty spaces between its particles. These spaces are larger than the spaces between the particles of water. Thus the volume of ice is greater than that of water, and hence the density of ice is less than that of water. A substance with lower density than water floats on water. Therefore ice floats on water.

CHANGE OF STATE OF MATTER

The processes by which matter changes its state are vaporisation, condensation, freezing, melting, sublimation and evaporation.

Vaporisation (liquid to gas). The change of water into vapour is called vaporisation. When water is heated to 100 °C it starts boiling and turns into vapour. Since water boils at 100 °C, this is called the boiling point of water. In general, the temperature at which a liquid boils is called its boiling point. Different liquids boil at different temperatures.

Condensation (gas to liquid). The change of vapour into liquid because of a decrease in temperature is called condensation. It is the reverse of vaporisation. Distilled water is made by condensation of vapour: water is boiled to vaporise it and the vapour is cooled, i.e. condensed, to get distilled water. This process is called distillation.

Freezing (liquid to solid). The change of liquid into solid because of a decrease in temperature is called freezing. Water freezes into ice at 0 °C.

Melting (solid to liquid). The change of solid into liquid because of an increase in temperature is called melting. Ice melts into water at 0 °C.

LATENT HEAT

When water is heated to 100 °C it starts boiling and changes into vapour. But even with a continuous supply of heat, the temperature does not rise above 100 °C while the water is boiling. The heat supplied to boiling water is used to change the water into vapour and the temperature does not rise.

Similarly, when heat is supplied to ice, the temperature does not rise above 0 °C until all the ice melts. The heat supplied at 0 °C is used to change ice into water.

The heat which is used in these processes without a rise in temperature is called latent heat. Latent heat is the heat released or absorbed by a body during a change of state (a phase transition) without a change in temperature. Examples: melting of ice, boiling of water.

The particles of a solid or liquid are bound together by forces of attraction, because of which matter exists in a particular state. The heat supplied during a change of state is used to overcome the forces of attraction between the particles and is not used to increase their kinetic energy. Since the kinetic energy does not increase, there is no rise in temperature.

The word “latent” comes from the Latin word latere, which means “to lie hidden”. Joseph Black introduced the term latent heat around 1762.

There are two types of latent heat: latent heat of fusion and latent heat of vaporisation.

Latent heat of fusion (melting or freezing). The heat required to change a solid into a liquid without any change in temperature is called the latent heat of fusion. The change from solid to liquid is an endothermic process, because heat is taken in. When ice is melted, the temperature stays at 0 °C however much heat is supplied, until all the ice has melted; after that the temperature starts rising. The quantity of heat required to convert 1 kg of a solid to liquid, at its melting point, without any change in temperature is called its latent heat of fusion. The heat required is measured in joules (J). For ice, 3.34 × 10⁵ J of heat is required to convert 1 kg of ice into water at its melting point.

Latent heat of vaporisation. The heat required to change a liquid into a gas without any change in temperature is called the latent heat of vaporisation. This change is also endothermic. Water boils at 100 °C, and its temperature does not rise above 100 °C however much heat is supplied; the heat is used to change water into vapour. The quantity of heat required to convert 1 kg of a liquid into gas at its boiling point is its latent heat of vaporisation. For water it is about 22.5 × 10⁵ J/kg. Different liquids have different latent heats of vaporisation.

SUBLIMATION

The process in which a solid changes directly into vapour without becoming liquid, and a vapour changes directly into solid without becoming liquid, is called sublimation. Generally a solid first changes into liquid and then into gas as the temperature rises, but there are some substances which change into gas without becoming liquid, and which change from gas into solid without becoming liquid. Such substances are said to sublime. Examples: camphor, naphthalene balls, ammonium chloride, iodine, dry ice.

The solid obtained after cooling the vapour of a sublimable substance is called the sublimate. The cooling of the vapour to get the sublimate is also known as sublimation, although it is also called deposition. When camphor is heated it changes into vapour without becoming liquid, and when the vapour is cooled it changes into solid without becoming liquid.

Sublimation in everyday life. Dry ice (solid carbon dioxide) turns into vapour without becoming liquid. Because of this property it is used to give the illusion of smoke or cloud on stage and in films. Naphthalene balls are kept with clothes and documents to protect them from insects: they sublime, and their vapour keeps insects away. The size of naphthalene balls gradually decreases and they finally disappear when left in the open, because of sublimation. Naphthalene balls are also used in toilets as a disinfectant and air freshener.

INTEXT QUESTIONS (PAGE NO. 3)

Convert the following temperatures to the Celsius scale: (a) 300 K (b) 573 K.

Answer: (a) 300 K = (300 − 273) °C = 27 °C.

(b) 573 K = (573 − 273) °C = 300 °C.

What is the physical state of water at (a) 250 °C (b) 100 °C?

Answer: (a) At 250 °C water is in the gaseous state (steam).

(b) At 100 °C water can exist in both the liquid and the gaseous state. At this temperature, after taking in heat equal to the latent heat of vaporisation, the water starts changing from the liquid to the gaseous state.

For any substance, why does the temperature remain constant during the change of state?

Answer: During a change of state, all the heat supplied is used to overcome the forces of attraction between the particles. This heat does not increase the kinetic energy of the particles, so the temperature does not rise.

Suggest a method to liquefy atmospheric gases.

Answer: By applying high pressure and cooling the gas to a low temperature. Under these conditions the particles of the gas come closer, their kinetic energy decreases and the gas turns into liquid.

EVAPORATION

The change of a liquid into vapour without reaching its boiling point is called evaporation. Evaporation takes place only at the surface of a liquid, while vaporisation (boiling) takes place in the whole mass of the liquid. Evaporation takes place even at room temperature and speeds up as the temperature rises.

When water or any other liquid is left in the open, its volume gradually decreases. Water left in a pot in the open disappears in two or three days, and water left in a garden or on an open balcony disappears faster than water kept in a room. This happens because of evaporation.

Process of evaporation. The particles at the surface of a liquid gain kinetic energy from the surroundings. With this energy they overcome the force of attraction between the particles of the liquid and escape into the air as vapour. They also take some energy from their neighbouring particles, so the temperature of the remaining liquid falls. In this way evaporation causes cooling.

Factors affecting evaporation: temperature, pressure, surface area, humidity of the air and wind speed.

Rate of evaporation is affected by:

Temperature. Evaporation increases with increase in temperature and decreases with decrease in temperature. With a rise in temperature, more particles at the surface gain enough kinetic energy to overcome the force of attraction and escape into the air.

Pressure. Evaporation decreases with increase in pressure and increases with decrease in pressure. This means the rate of evaporation is indirectly proportional to the pressure. When the pressure above the surface is higher, the particles need more energy to escape, so the rate of evaporation falls; at lower pressure they need less energy and the rate increases.

Surface area. Evaporation increases with increase in surface area, since evaporation takes place only at the surface. The more surface of the liquid is exposed to the atmosphere, the more particles can escape. Examples: after rain, roads dry up more quickly than potholes. On roads the water is spread over a large area exposed to the atmosphere, so evaporation takes place quickly and the roads dry quickly. In potholes less water surface is exposed to air, so less water comes in contact with air and receives less heat, resulting in delayed evaporation. Wet clothes are spread over the laundry line to dry quickly: spreading exposes more surface area of water to the air, which speeds up evaporation. If wet clothes are left even in the sun without spreading, they take more time to dry because of less surface area exposed to air. Water in a plate evaporates faster than water in a tumbler because more surface is exposed to the atmosphere and receives more heat; in a glass less surface area is exposed, so fewer water molecules receive heat and evaporation is slower. Hence the rate of evaporation increases with increase in surface area and decreases with decrease in surface area.

Humidity. Humidity is the amount of water vapour present in the air. In weather reports, which are published in newspapers or given on TV channels, the humidity percent is given, which shows the percent of water vapour present in the air. Evaporation decreases with increase in humidity and increases with decrease in humidity. When the air already holds a lot of water vapour, its capacity to take in more is smaller, so the rate of evaporation falls. Dry air can hold more water vapour, so evaporation is faster in dry air. This is why clothes dry quickly in summer and winter but slowly in the rainy season, and why sweat does not dry quickly in the rainy season and we feel uneasy and damp.

Wind speed. Evaporation increases with increase in wind speed. Fast wind carries away the water vapour from near the surface, so more liquid can evaporate. That is why wet clothes dry faster on a windy day. Wet clothes are also given jerks before hanging them on the line, so that some water droplets are thrown out and the clothes dry sooner.

Evaporation in everyday life

1. Water from the surface of oceans, seas and other large water bodies evaporates continuously. The water vapour rises, collects as clouds and falls as rain, so evaporation is an essential part of the water cycle.
2. On summer days we sweat to regulate our body temperature. The sweat evaporates by taking heat from the body, which cools the skin and gives relief on hot days. Sweating is a natural mechanism to keep the body cool.
3. Water is kept in earthen pots to keep it cool. An earthen pot has many tiny pores; water evaporates through them and the evaporation cools the rest of the water in the pot.
4. Wet clothes on a line dry because of evaporation.
5. Sea water, which contains a lot of salt, is left in shallow ponds. The water evaporates by the heat of the sun, leaving the salt behind. This salt is collected and, after purification, used in food.
6. Evaporative coolers are widely used on hot summer days. Dry air is blown over wet husk (khus); water evaporates from the husk and cools it, and the cool air blown from it is sent into the room.

INTEXT QUESTIONS (PAGE 6)

Why does a desert cooler cool better on a hot dry day?

Answer: The rate of evaporation increases with increase in temperature and decrease in humidity. A desert cooler works on the principle of evaporation, so it cools better when the day is hot and dry.

How does the water kept in an earthen pot (matka) become cool during summer?

Answer: The surface of an earthen pot has tiny pores. The water in the pot evaporates through these pores, and evaporation causes cooling. The heat needed for evaporation is taken from the water in the pot, so the water becomes cool.

Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Answer: Acetone, petrol and perfume are volatile and evaporate very fast. For evaporation they take the required latent heat of vaporisation from the palm, which therefore cools and feels cold.

Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Answer: In a saucer the exposed surface area of the tea or milk is larger than in a cup, so evaporation is faster and the tea or milk cools quickly. It therefore becomes easier to sip.

What type of clothes should we wear in summer?

Answer: Cotton clothes. In summer we sweat more, and cotton is a good absorber of water. It absorbs sweat and exposes it to the air, so the sweat evaporates faster. The evaporation takes heat from the body and keeps us cool.

EXERCISE QUESTIONS (PAGE 12)

Convert the following temperatures to the Celsius scale: (a) 300 K (b) 573 K.

Answer: Kelvin is the SI unit of temperature, where 0 °C = 273 K. (a) 300 K = (300 − 273) °C = 27 °C.

(b) 573 K = (573 − 273) °C = 300 °C.

Convert the following temperatures to the Kelvin scale: (a) 25 °C (b) 373 °C.

Answer: (a) 25 °C = (25 + 273) K = 298 K.

(b) 373 °C = (373 + 273) K = 646 K.

Give reasons 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: (a) Naphthalene undergoes sublimation easily, i.e. it changes from solid directly to gas. So naphthalene balls disappear with time without leaving any solid.

(b) Gas particles have high speed and large spaces between them. The particles of perfume diffuse into the air at a very fast rate and reach our nostrils, so we can smell the perfume from a distance.

Arrange the following substances in increasing order of forces of attraction between the particles: water, sugar, oxygen.

Answer: Oxygen < water < sugar.

Sugar is a solid, so the forces of attraction between its particles are strong. Water is a liquid, so the forces are weaker than in sugar. Oxygen is a gas, so the forces are the weakest.

What is the physical state of water at (a) 25 °C (b) 0 °C (c) 100 °C?

Answer: (a) At 25 °C water is in the liquid state.

(b) At 0 °C water can exist as both solid and liquid. After taking in heat equal to the latent heat of fusion, ice starts changing into water.

(c) At 100 °C water can exist as both liquid and gas. After taking in heat equal to the latent heat of vaporisation, water starts changing into water vapour.

Give two reasons to justify (a) water at room temperature is a liquid; (b) an iron almirah is a solid at room temperature.

Answer: (a) At room temperature (25 °C) water is a liquid because (i) it has no definite shape but has a fixed volume, i.e. it takes the shape of the container in which it is kept, and (ii) it flows.

(b) An iron almirah is a solid at room temperature because (i) it has a definite shape and volume, and (ii) it is rigid.

Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer: When ice melts it absorbs energy equal to the latent heat of fusion from the surroundings, in addition to cooling them. Water at the same temperature has no such latent heat to absorb, so ice cools more effectively.

What produces more severe burns, boiling water or steam?

Answer: Steam. Steam at 100 °C contains extra heat (the latent heat of vaporisation) that was absorbed when water changed into steam. When steam condenses on the skin, it releases this extra heat in addition to the heat of boiling water, so it causes more severe burns.

Name A, B, C, D, E and F in the diagram showing the changes of state.

Answer: A = melting (fusion), in which solid changes into liquid. B = evaporation (vaporisation), in which liquid changes into gas. C = condensation, in which gas changes into liquid. D = freezing (solidification), in which liquid changes into solid. E = sublimation, in which solid changes directly into gas. F = sublimation (deposition), in which gas changes directly into solid.

(v) Shape The property of having a definite geometry is called shape of a particular substance. Solids have a definite shape whereas gases and liquids do not have.

• Vaporization

• Condensation

• Freezing

• Melting

• Sublimation

BOILING POINTS

CONDENSATION:(CHANGE OF GAS INTO LIQUID)-

The change of vapor into water because of decrease in temperature is called condensation. Condensation is the reverse process of vaporization. When the temperature of vapor decreases it changes into water, this process is known as condensation.
Distilled water is manufactured by the condensation of vapor. The process of making of distilled water is known as distillation. In distillation first water is boiled to vaporize and the vapor is cooled, i.e. condensed to get distilled water.

When water is heated up-to 1000C it starts boiling and changes into vapor. But we see thateven after continuous supply of heat temperature does not rise above the 100C while boiling of water. Temperature supplied after 100C to boiling water is used to change the water into vapor and temperature of water does not rise.On the other hand we see that when heat is supplied to ice, temperature does not rise above the 00C, until all ice melts. In this process also the heat supplied to ice after 00C is used to change of ice into water and temperature of ice does not rise.