ICSE Class 8 Physics Chapter 04 More About Solids

More About Solids, Liquids, and Gases

Learning Outcomes

After studying this chapter, you will be able to understand:

Kinetic theory of matter, States of matter, Cohesion and adhesion, Surface tension, Pressure: atmospheric pressure, Liquid pressure, Archimedes' principle, Law of flotation

Did You Know?

The Greeks in 500 BC believed that all matter is composed of atoms. But Aristotle, the famous philosopher, in 350 BC stated that all matter is composed of four elements - earth, air, fire, and water. This view lasted for 2000 years until John Dalton (shown below) revived the idea of atoms around 1800 AD. He explained chemical reactions by proposing that all matter is made up of atoms. Although he did not know about their structure, he knew that the evidence pointed to something fundamental.

Introduction to Matter

Look at your surroundings. What do you see? You see so many things like books, stones, bus, petrol, water, lunchboxes, trees, people, etc., of different textures and nature. All these things constitute matter. Anything that has mass and occupies space is called matter. When we talk of matter we mean every substance in the universe, from the tiniest speck of dust to the largest star.

Look at the figure below. Can you name some things from the figure that have mass and occupy space? Does the air in the balloon also form part of matter? Yes! since it has mass and occupies space.

So, we know that everything around us is made up of matter. Have you ever wondered what is matter made up of? Matter is made up of numerous tiny particles called molecules that, in turn, are made up of atoms. For example, a drop of water is made up of millions of molecules of H2O. A molecule of water, in turn, is made up of two atoms of hydrogen and one atom of oxygen. Thus, water is made up of numerous atoms of hydrogen and oxygen. This means that all things that make up matter, like bus, books, water, air, etc. are made up of atoms. This serves as the foundation of all science. Can you believe that the whole lot of different things which you see around are made from just about 100 elements, in different proportions?

Do you know that even you are made of atoms? You might have studied that our body is made up of cells. If a cell is broken down, you will find that it is made of many chemicals which are a combination of elements, such as carbon, hydrogen, oxygen, sulphur, phosphorous which are again made up of atoms. Infact, all living things are an incredible organization of atoms.

In order to understand the nature of these atoms and molecules, their arrangement in a substance, their movement, etc., scientists gave the kinetic theory of matter.

Kinetic Theory Of Matter

The kinetic theory of matter was published in around 1860. This is the most-widely accepted theory about the behaviour of matter. This theory states that all matter is made up of atoms (often molecules), but with an enormous variety of arrangements and different motions. The main points that the kinetic theory of matter states are as follows:

1. All matter is made up of atoms or molecules.

2. The smallest molecules are of the order 10^-10 m in size.

3. These molecules are always in constant, random, zig-zag motion and possess kinetic energy due to their motion.

4. The kinetic energy of the molecules of a substance is directly proportional to the temperature of the substance.

5. There is a force of attraction between the molecules of matter. This force between similar kinds of molecules is called force of cohesion, while the force of attraction between different kinds of molecules is called force of adhesion.

6. The space between the molecules of a substance is called the inter-molecular space.

Teacher's Note

The kinetic theory helps explain why gases spread quickly through air - think of how perfume scent travels across a room due to random molecular motion.

7. The inter-molecular force of attraction is less when the inter-molecular space between the molecules is large.

Depending upon the arrangement of molecules in matter, the force of attraction between the molecules, and the inter-molecular spaces between them, all matter is divided into three categories: Solids, Liquids, and Gases. Let's study about them in detail.

Solids

Look at your pencil box. It is a solid. Solids have a definite volume and shape. In solids, molecules are very tightly packed because they have a very strong force of attraction between them.

Since the force of attraction is very strong, the molecules have least inter-molecular spaces between them. See Figure 4.1 (a). Since the inter-molecular space is almost negligible, the molecules cannot move easily from their positions. Because of this reason, solids are not easily compressible and expand very little on heating.

Liquids

Take a glass of water. Now take an empty pan and pour water in the pan from the glass. What do you observe? You will see that water, which was previously in the shape of the glass has now taken the shape of the pan. Thus, we say that liquids have a definite volume but not a definite shape. They take the shape of the container in which they are kept.

If you throw water on the floor you will see that it spreads on the floor. What does this show? This shows that the inter-molecular attraction between the molecules of a liquid is not as strong as in the case of solids. Because of this, the inter-molecular space in liquids is more in comparison to solids. See Figure 4.1 (b). Hence, on heating they expand more than solids. Since liquids have a definite volume, they cannot be compressed easily.

Gases

In gases the molecules are very loosely packed. The forces of attraction between the molecules are the least in gases in comparison to solids and liquids. See Figure 4.1 (c).

Hence, their expansion is very high when heated. They can also flow in any direction unlike liquids, which can flow only from a higher level to a lower level.

Since inter-molecular forces are very less in gases, they can change their shape and volume easily and can be compressed to fill any container, e.g., petroleum gas is compressed and filled in the cooking gas cylinders that we use in our kitchen to cook food.

Teacher's Note

You experience the three states daily - ice (solid), water (liquid), and steam (gas) - and their properties change based on temperature and molecular arrangement.

Force Of Cohesion And Adhesion

Have you ever dipped your wooden pencil in a glass of water? Try it out yourself and see what happens. When you take your pencil out after dipping it in water, you will see drops of water sticking to the pencil. We say that the pencil is wet. Now when another pencil is dipped in a glass of mercury, does it appear to be wet? No! It doesn't look as if you had dipped it in mercury.

Why does this difference between the two liquids occur? When we dip our pencil in water, the water molecules get attracted to the pencil molecules and stick to it. The attractive force acting between different types of molecules (of different substances) is called the force of adhesion. The strength of this force depends on the kind of molecules that are present and the distance between them. There is an adhesive force between the water molecules and wood (pencil) and that is why water wets the wood.

On the other hand, the force of attraction acting between similar kinds of molecules (within a substance) is called the force of cohesion. The force of cohesion acts between the molecules of water, oil, etc., and holds them together as drops. Force of cohesion also acts between the molecules of wood, paper, etc.

In case of water molecules the adhesive force between water and wood is much greater than the cohesive force between water molecules itself, that is why the molecules stick to the pencil. However, in mercury the cohesive forces are much greater than the adhesive force, that is why the molecules of mercury do not stick to the pencil.

We have just studied that molecules of a substance are attracted to one another due to the cohesive forces between the molecules. In a jar full of a liquid, say water, a molecule in the middle has other molecules all around. So, it experiences cohesive forces from all the sides. See Figure 4.2. Think of the molecule which is at the surface of the liquid. At the surface, the molecule has no molecule above it. How does the cohesive force act on such molecules? This is explained by a term called surface tension.

Surface Tension

Have you ever tried placing a razor or a needle gently on the water surface? Does it float or sink? It floats due to a property of the liquid surface called surface tension, which is caused by the molecular attractions in the liquid.

In Figure 4.2, the molecules that are down in the liquid experience cohesive forces from the neighbouring molecules on all sides. Thus, the forces balance out. However, at the surface the molecules have no liquid molecules above them. Hence, the net cohesive force that acts on the molecules is sideways and downwards. Because of this, the molecules on the surface experience an overall force pulling them inwards, keeping the molecules and the surface together. Due to this pull, the surface tends to occupy a minimum surface area and the surface contracts and behaves like a stretched elastic membrane.

The property of a liquid to contract its surface area due to the inter-molecular attractions is called surface tension.

Let us study an example to make the concept clear. Take your paint brush and dip it in a glass of water. Look at its hair. Now slowly take the brush out of water and now look at its hair. Do you see any difference? When the brush is under water, the hair are fluffed out. But, when the brush is lifted out, the surface film of water contracts due to surface tension and pulls the hair together [Fig. 4.3 (a)].

Look at Figure 4.3 (b). Do you see the water droplet spherical? Can you tell why this happens? Molecules on the inside are pulled in all directions. But, we know that the molecules on the surface are pulled in only one direction - inwards. Thus, the molecules on the outside tend to move inward to form a shape, which has the least surface area possible. The geometrical figure that has the least surface area, for a given volume, is a sphere. Hence, the water droplet is spherical.

Another effect seen in liquids due to cohesion and adhesion is the shape of the meniscus in the capillary tube.

Teacher's Note

Surface tension is why water beads up on a freshly waxed car - the cohesive forces create an elastic membrane at the surface.

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