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National Council of Educational Research and Training (NCERT) Book for Class IX
Chapter: Chapter 1 – Matter in Our Surroundings
Class IX NCERT Science Text Book Chapter 1 Matter in Our Surroundings is given below.
As we look at our surroundings, we see a large variety of things with different shapes, sizes and textures. Everything in this universe is made up of material which scientists have named “matter”. The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand– each thing is matter. We can also see as we look around that all the things mentioned above occupy space, that is, volume* and have mass.
Since early times, human beings have been trying to understand their surroundings. Early Indian philosophers classified matter in the form of five basic elements – the “Panch Tatva”– air, earth, fire, sky and water. According to them everything, living or nonliving, was made up of these five basic elements. Ancient Greek philosophers had arrived at a similar classification of matter.
Modern day scientists have evolved two types of classification of matter based on their physical properties and chemical nature.
In this chapter we shall learn about matter based on its physical properties. Chemical aspects of matter will be taken up in subsequent chapters.
1.1 Physical Nature of Matter
1.1.1 MATTER IS MADE UP OF PARTICLES
For a long time, two schools of thought prevailed regarding the nature of matter. One school believed matter to be continuous like a block of wood, whereas, the other thought that matter was made up of particles like sand. Let us perform an activity to decide about the nature of matter – is it continuous or particulate?
Activity ______________ 1.1
In order to answer these questions we need to use the idea that matter is made up of particles. What was there in the spoon, salt or sugar, has now spread throughout water. This is illustrated in Fig. 1.1.
1.1.2 HOW SMALL ARE THESE PARTICLES OF MATTER?
Activity ______________ 1.2
This experiment shows that just a few crystals of potassium permanganate can colour a large volume of water (about 1000 L). So we conclude that there must be millions of tiny particles in just one crystal of potassium permanganate, which keep on dividing themselves into smaller and smaller particles. Ultimately a stage is reached when the particles cannot divide further into smaller particles.
The same activity can be done using 2 mL of Dettol instead of potassium permanganate. The smell can be detected even on repeated dilution.
The particles of matter are very small – they are small beyond our imagination!!!!
1.2 Characteristics of Particles of Matter
1.2.1 PARTICLES OF MATTER HAVE SPACE BETWEEN THEM
In activities 1.1 and 1.2 we saw that particles of sugar, salt, Dettol, or potassium permanganate got evenly distributed in water. Similarly, when we make tea, coffee or lemonade (nimbu paani ), particles of one type of matter get into the spaces between particles of the other. This shows that there is enough space between particles of matter.
1.2.2 PARTICLES OF MATTER ARE CONTINUOUSLY MOVING
Activity ______________ 1.3
Activity ______________ 1.4
Activity ______________ 1.5
From the above three activities (1.3, 1.4 and 1.5), we can conclude the following:
Particles of matter are continuously moving, that is, they possess what we call the kinetic energy. As the temperature rises, particles move faster. So, we can say that with increase in temperature the kinetic energy of the particles also increases.
In the above three activities we observe that particles of matter intermix on their own with each other. They do so by getting into the spaces between the particles. This intermixing of particles of two different types of matter on their own is called diffusion. We also observe that on heating, diffusion becomes faster. Why does this happen?
1.2.3 PARTICLES OF MATTER ATTRACT EACH OTHER
Activity ______________ 1.6
Activity ______________ 1.7
Activity ______________ 1.8
The above three activities (1.6, 1.7 and 1.8) suggest that particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another.
1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, colddrink, smell of perfume.
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.
3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
4. What are the characteristics of the particles of matter?
1.3 States of Matter
Observe different types of matter around you. What are its different states? We can see that matter around us exists in three different states– solid, liquid and gas. These states of matter arise due to the variation in the characteristics of the particles of matter. Now, let us study about the properties of these three states of matter in detail.
1.3.1 THE SOLID STATE
Activity _____________ 1.9
All the above are examples of solids. We can observe that all these have a definite shape, distinct boundaries and fixed volumes, that is, have negligible compressibility. Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
Consider the following:
(a) What about a rubber band, can it change its shape on stretching? Is it a solid?
(b) What about sugar and salt? When kept in different jars these take the shape of the jar. Are they solid?
(c) What about a sponge? It is a solid yet we are able to compress it. Why?
All the above are solids as:
1.3.2 THE LIQUID STATE
(a) water, cooking oil, milk, juice, a cold drink.
(b) containers of different shapes. Put a 50 mL mark on these containers using a measuring cylinder from the laboratory.
We observe that 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.
Refer to activities 1.4 and 1.5 where we saw that solids and liquids can diffuse into liquids. The gases from the atmosphere diffuse and dissolve in water. These gases, especially oxygen and carbon dioxide, are essential for the survival of aquatic animals and plants.
All living creatures need to breathe for survival. The aquatic animals can breathe under water due to the presence of dissolved oxygen in water. Thus, we may conclude that solids, liquids and gases can diffuse into liquids. The rate of diffusion of liquids is higher than that of solids. This is due to the fact that in the liquid state, particles move freely and have greater space between each other as compared to particles in the solid state.
1.3.3 THE GASEOUS STATE
Have you ever observed a balloon seller filling a large number of balloons from a single cylinder of gas? Enquire from him how many balloons is he able to fill from one cylinder. Ask him which gas does he have in the cylinder.
We have observed that gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas (CNG) is used as fuel these days in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.
We come to know of what is being cooked in the kitchen without even entering there, by the smell that reaches our nostrils. How does this smell reach us? The particles of the aroma of food mix with the particles of air spread from the kitchen, reach us and even farther away. The smell of hot cooked food reaches us in seconds; compare this with the rate of diffusion of solids and liquids. Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.
In the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.
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.
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.
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.
4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
1.4 Can Matter Change its State?
We all know from our observation that water can exist in three states of matter–
What happens inside the matter during this change of state? What happens to the particles of matter during the change of states? How does this change of state take place? We need answers to these questions, isn’t it?
1.4.1 EFFECT OF CHANGE OF TEMPERATURE
On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.
The melting point of a solid is an indication of the strength of the force of attraction between its particles.
The melting point of ice is 273.16 K*. The process of melting, that is, change of solid state into liquid state is also known as fusion. When a solid melts, its temperature remains the same, so where does the heat energy go?
You must have observed, during the experiment of melting, that the temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the
state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat. The word latent means hidden. The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion. So, particles in water at 00 C (273 K) have more energy as compared to particles in ice at the same temperature.
When we supply heat energy to water, particles start moving even faster. At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Boiling is a bulk phenomenon. Particles from the bulk of the liquid gain enough energy to change into the vapour state.
For water this temperature is 373 K (100 0C = 273 + 100 = 373 K).
Can you define the latent heat of vaporisation? Do it in the same way as we have defined the latent heat of fusion. Particles in steam, that is, water vapour at 373 K (100 0 C) have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.
So, we infer that the state of matter can be changed into another state by changing the temperature.
We have learnt that substances around us change state from solid to liquid and from liquid to gas on application of heat. But there are some that change directly from solid state to gaseous state and vice versa without changing into the liquid state.
Now, heat slowly and observe. What do you infer from the above activity?
A change of state directly from solid to gas without changing into liquid state (or vice versa) is called sublimation.
1.4.2 EFFECT OF CHANGE OF PRESSURE
We have already learnt that the difference in various states of matter is due to the difference in the distances between the constituent particles. What will happen when we start putting pressure and compress a gas enclosed in a cylinder? Will the particles come closer? Do you think that increasing or decreasing the pressure can change the state of matter?
Applying pressure and reducing temperature can liquefy gases. Have you heard of solid carbon dioxide (CO2)? It is stored under high pressure. Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere* without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice.
Thus, we can say that pressure and temperature determine the state of a substance, whether it will be solid, liquid or gas.
1. Convert the following temperature to celsius scale: a. 300 K b. 573 K.
2. What is the physical state of water at:
a. 250ºC b. 100ºC ?
3. For any substance, why does the temperature remain constant during the change of state?
4. Suggest a method to liquefy atmospheric gases.
Do we always need to heat or change pressure for changing the state of matter? Can you quote some examples from everyday life where change of state from liquid to vapour takes place without the liquid reaching the boiling point? Water, when left uncovered, slowly changes into vapour. Wet clothes dry up. What happens to water in the above two examples?
We know that particles of matter are always moving and are never at rest. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.
1.5.1 FACTORS AFFECTING EVAPORATION
Let us understand this with an activity.
You must have observed that the rate of evaporation increases with–
1.5.2 HOW DOES EVAPORATION CAUSE COOLING?
In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold.
What happens when you pour some acetone (nail polish remover) on your palm? The particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool.
After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface.
Can you cite some more examples from daily life where we can feel the effect of cooling due to evaporation?
Why should we wear cotton clothes in summer?
During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.
Why do we see water droplets on the outer surface of a glass containing ice-cold water?
Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.
1. Why does a desert cooler cool better on a hot dry day?
2. How does the water kept in an earthen pot (matka) become cool during summer?
3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
5. What type of clothes should we wear in summer?
Now scientists are talking of five states of matter: Solid, Liquid, Gas, Plasma and Bose-Einstein Condensate.
Plasma: The state consists of super energetic and super excited particles. These particles are in the form of ionised gases. The fluorescent tube and neon sign bulbs consist of plasma. Inside a neon sign bulb there is neon gas and inside a fluorescent tube there is helium gas or some other gas. The gas gets ionised, that is, gets charged when electrical energy flows through it. This charging up creates a plasma glowing inside the tube or bulb. The plasma glows with a special colour depending on the nature of gas. The Sun and the stars glow because of the presence of plasma in them. The plasma is created in stars because of very high temperature.
Bose-Einstein Condensate: In 1920, Indian physicist Satyendra Nath Bose had done some calculations for a fifth state of matter. Building on his calculations, Albert Einstein predicted a new state of matter – the Bose- Einstein Condensate (BEC). In 2001, Eric A.
Cornell, Wolfgang Ketterle and Carl E. Wieman of USA received the Nobel prize in physics for achieving “Bose-Einstein condensation”.
The BEC is formed by cooling a gas of extremely low density, about one-hundred-thousandth the density of normal air, to super low temperatures. You can log on to www.chem4kids.com to get more information on these fourth and fifth states of matter.
|Density||kilogram per cubic metre||kg m–3|
1. Convert the following temperatures to the Celsius scale.
(a) 300 K (b) 573 K.
2. Convert the following temperatures to the Kelvin scale.
(a) 25°C (b) 373°C.
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.
4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
5. What is the physical state of water at—
(a) 25°C (b) 0°C (c) 100°C ?
6. Give two reasons to justify—
(a) water at room temperature is a liquid.
(b) an iron almirah is a solid at room temperature.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
8. What produces more severe burns, boiling water or steam?
9. Name A,B,C,D,E and F in the following diagram showing change in its state
Prepare a model to demonstrate movement of particles in solids, liquids and gases.
For making this model you will need
How to make?
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