Introduction
There are a large number of things around us which we see and feel. The nature and behaviour of different types of substances around us are different. These substances differ in their shape, size, colour, smell, texture, etc.
Since ancient times, human beings have been eager to understand surroundings. According to ancient Indian philosophers, all matter whether living or non-living is made up of five basic constituents commonly called panch tatvas, i.e., air, earth, fire, sky and water. A similar classification of matter was also evolved by ancient Greek philosophers.
However, scientists have classified matter into the following two types on the basis of their properties.
- Physical classification : On the basis of physical properties, matter has been classified as solids, liquids and gases.
- Chemical classification : On the basis of chemical properties, matter has been classified as elements, compounds and mixtures.
In this chapter, we will study the physical classification of matter while chemical classification of matter will be discussed in the next chapter.
Matter
Matter is the material of which the universe is composed. Matter may be defined as something that occupies space, possesses mass, and offers resistance to any stress (force applied on it).
A definite variety of matter, all samples of which have the same properties is commonly called a substance. For example, sugar is a substance because even the smallest particle of sugar is sweet.
Matter is made up of particles :
The ancient philosophers held the view that matter is continuous, like a block of wood. But this is not true. Actually, matter is made up of discrete particles. This can be easily verified by performing the following activity.
Evidence – 1
Take a beaker. Fill half of it with water. Mark the level of water in the beaker. Add some sugar to the water and dissolve it with the help of a glass rod. You will see that the sugar has disappeared, but there is no change in the level of water. Can you see where has the sugar gone? This can be explained by assuming that matter is not continuous, rather it is made up of particles. Sugar contains a large number of separate particles. These particles when dissolved in water occupy the vacant spaces between the particles of water. That is why, the water level in the beaker did not rise. Had sugar been continuous, like a block of wood, the water level in the beaker would have risen.
Experiment to show that matter is made of particles
Evidence-2
The best evidence for the existence and movement of particles in liquids was given by Robert Brown in 1827. Robert Brown suspended extremely small pollen grains in water. On looking through the microscope, it was found that the pollen grains were moving rapidly throughout water in a very irregular way (or zig-zag way). It was also observed that warmer the water, faster the pollen grains move on the surface of water.
The movement of pollen grains on the surface of water can be explained as follows : Water is made up of tiny particles which are moving very fast (The water molecules themselves are invisible under the microscope because they are very, very small). The pollen grains move on the surface of water because they are constantly being hit by the fast moving particles of water. So, though the water particles (or water molecules) are too small to be seen, but their effect on the pollen grains can be seen clearly. The random motion of visible particles (pollen grains) caused by the much smaller invisible particles of water is an example of Brownian motion (after the name of the scientist Robert Brown who first observed this phenomenon.)
Brownian motion as observed through a microscope. This figure shows the path of a single pollen grain as it is being bombarded by the tiny, invisible particles of water (in which it is placed)
The zig-zag movement of the small particles suspended in a liquid (or gas) is called Brownian motion. Brownian motion increases on increasing the temperature. The existence of Brownian motion gives us two consolations about the nature of matter :
(i) That matter is made up of tiny particles, and
(ii) That the particles of matter are constantly moving.
Exercise
1. The best evidence for the existence and movement of particles in liquids was given by :
(A) Robert Hook
(B) Robert Brown
(C) Robert Charles
(D) None of the above
Answer2. The zig-zag movement of the small particles suspended in a liquid is called :
(A) Random movement
(B) Brownian movement
(C) Colloidal properties
(D) Helical movement
Answer3. “Particles of matter are constantly moving”, this conclusion was given by the existence of :
(A) Tyndall effect
(B) Weight of particles
(C) Invisiblity of particles
(D) Brownian movement
Answer4. Brownian movement was first observed in :
(A) Pollen grains
(B) Wheat grains
(C) Water molecules
(D) None of the above
AnswerCharacteristics of particles of matter
The important characteristics of particles of matter (like atoms or molecules) are the following :
(i) The particles of matter are minutely small.
(ii) The particles of matter have spaces between them.
(iii) The particles of matter are constantly moving.
(iv) The particles of matter attract each other.
We will now perform some simple experiments to show all the characteristics of the particles of matter. The samples of matter (or materials) which we will be using for performing these experiments are potassium permanganate, water, sugar, incense stick (agarbatti), air, copper sulphate, a piece of chalk, ice and an iron nail.
The particles of matter are very-very small :
This can be explained by performing the following activity :
Step-1
Take some crystals of blue vitriol (copper sulphate) and dissolve them in 100 mL of water. The solution will turn blue.
Step-2
Take about 10 mL of the blue solution prepared in step 1. Add it to another 100 mL of fresh water.
Step-3
Take about 10 mL of the solution prepared in step 2. Add it to another 100 mL of fresh water.
In this way, repeat the exercise 5 to 8 times. You will find that the last solution is still blue though the intensity of the colour has gradually decreased.
Just a few crystals of copper sulphate can colour a huge volume of water
This indicates that a few crystals of copper sulphate are able to colour an enormously large volume of water. This also shows that a small crystal of copper sulphate contains millions of small particles.
Particles of matter have space between them :
Experiment
We take about 100 mL of water in a beaker and mark the level of water. Also take 50 gm of sugar. Dissolve the sugar by stirring and we get a sugar solution.
Conclusion
The level of sugar solution in the beaker is at the same mark where water level was initially in the beaker.
It shows that particles of sugar go into the spaces between various particles of water due to which there is no change in the volume.
The particles (or molecules) in water are not tightly packed, they have spaces between them.
When we dissolve sugar in water, there is no change in the volume of water
Exercise
1. The chemical name of blue vitriol is :
(A) Copper sulphide
(B) Copper sulphate
(C) Copper carbonate
(D) Copper sulphite
Answer2. The correct formula of blue vitriol is :
(A) CuSO4 · 2H2O
(B) CuSO4 · H2O
(C) CuSO4 · 5H2O
(D) CuSO4 · 3H2O
Answer3. Which statement is not correct:
(A) Particles of matter have spaces between them
(B) Particles of matter are very-very small
(C) Particles of matter are constantly moving
(D) Particles of matter are stationary
AnswerParticles of matter are constantly moving :
Experiment
To show that particles of matter are in continuous motion and to study the effect of temperature on their speed.
Take an agarbatti (incense stick) and place it in a corner of a room. You will get its smell only when you go closer to it. Light the agarbatti and place it in a corner of the room. You will get smell even in the opposite corner of the room to the one where you have placed the agarbatti.
This activity tells us that
(i) The particles of matter are in continuous motion.
(ii) The speed with which the particle move increases with the increase in temperature.
The particles of perfume in the incense stick get mixed up with particles of the air and spread out. Because of this spreading of particles, it is possible to get smell even when we are at a distance. This explains why we have to be close to have the smell of the agarbatti and we can get smell of the agarbatti when lighted even at a distance.
Particles of matter attract each other :
Evidences of Inter particle Force of Attraction
(a) A small piece of gold (of the size of 50 paise coin) can be drawn into wire of about one kilometer length without breaking.
Reason
The particles of gold attract each other and thus remain attached without flying apart.
(b) A glass tumbler has its shape because its particles have attraction for each other. But it can be broken on hammering (application of force from outside).
Reason
The attraction between the particles of glass is weaker than the attraction between the particles of gold (or iron, or copper, or aluminium, etc.)
(c) Ice and water differ in their properties. Ice is harder and colder than water.
Reason
The forces of attraction between the particles of ice are stronger than the forces of attraction between the particles of water.
(d) Under normal conditions, oxygen is a gas while water is a liquid.
Reason
The force of attraction between the molecules of oxygen is much weaker than the force of attraction between the molecules of water. In other words, it is stated that the force of attraction between the molecules of water is much stronger than the force of attraction between the molecules of oxygen.
Exercise
1. Speed of particles :
(A) Increases with increase in temperature
(B) Increases with decrease in temperature
(C) Decreases with increase in temperature
(D) None of the above
Answer2. Under normal conditions oxygen is a gas while water is a liquid, the reason behind is :
(A) The force of attraction between the molecules of oxygen is much stronger than the water molecules
(B) The force of attraction between the molecules of O2 is much weaker than the H2O molecules
(C) They have different properties
(D) None of the above
Answer3. A small piece of Au can be drawn into wire of about 1 km length without breaking because:
(A) The particles are very fine
(B) The particles are invisible
(C) The particles of gold attract each other and thus remain attached without flying apart
(D) None of the above
Answer