Everything around us, from the air we breathe to the water we drink and the books we read, is made up of matter. Matter is defined as anything that has mass and occupies space. The study of matter and its states is the very foundation of chemistry and physics, because understanding how matter behaves helps us understand the universe itself.
In ancient times, philosophers believed that matter was made of four basic elements - earth, fire, air and water. However, modern science has replaced these ideas with a precise, evidence-based understanding. We now know that matter is made up of tiny particles, and these particles can exist in three main physical states: solid, liquid and gas. Matter can also change from one state to another on heating, cooling or applying pressure.
This chapter introduces the particulate nature of matter, explains the three states of matter in detail, and describes the conditions under which matter changes from one state to another. We will also study the terms evaporation, condensation, sublimation and diffusion, which describe how the particles of matter move and interact.
Matter is made up of extremely small particles called atoms and molecules. These particles are so small that they cannot be seen with the naked eye, but their existence can be proved through experiments. For example, when we dissolve a few drops of potassium permanganate in water, the colour spreads through the whole container. This shows that matter is made of particles which are too small to be seen but are present everywhere.
There are three characteristics of particles of matter that explain all the observed behaviour of matter:
Because the particles are always moving, they have kinetic energy. The amount of kinetic energy possessed by particles decides the state of matter. In solids, particles have the least kinetic energy and are tightly packed; in liquids they have moderate energy and can slide past one another; and in gases they have the maximum energy and move freely in all directions.
Solid state: In solids, the particles are very closely packed, leaving almost no space between them. The forces of attraction between particles are extremely strong, so the particles can only vibrate about their fixed positions. As a result, solids have a definite shape, a definite volume and a fixed melting point. They are generally incompressible. Ice, stone, wood and iron are examples of solids.
Liquid state: In liquids, the particles are close together but not as tightly packed as in solids. The intermolecular forces are weaker than in solids, and the particles can move around and slide past each other. A liquid has a definite volume but no definite shape - it takes the shape of the container it is kept in. Liquids are slightly compressible and can flow. Water, milk and oil are examples of liquids.
Gaseous state: In gases, the particles are very far apart, and the forces of attraction are negligible. The particles move freely in all directions with high speed and fill the entire container. A gas has neither a definite shape nor a definite volume. Gases are highly compressible. Air, oxygen, hydrogen and carbon dioxide are examples of gases.
The arrangement, spacing and movement of particles differ in each state. This difference arises because of the balance between the attractive forces between particles and the kinetic energy of the particles.
Diffusion is the process by which particles of matter mix with each other on their own due to continuous random motion. When a drop of ink is added to water, the colour spreads slowly and uniformly; this is diffusion. Diffusion is fastest in gases, slower in liquids and extremely slow in solids.
Diffusion of gases is very fast because gas particles have large empty spaces between them and move with very high speed. When we open a bottle of perfume, its fragrance spreads to the whole room. When we light an incense stick in a corner of a room, its smell reaches all parts of the room. Diffusion of a solid into another solid is very slow, but it still happens - for example, if a copper and a zinc plate are kept in contact for a long time, some particles of each slowly diffuse into the other.
Diffusion increases with increase in temperature, because heating increases the kinetic energy of particles, making them move faster. Diffusion is important in daily life - the breathing process depends on the diffusion of oxygen and carbon dioxide across the walls of the alveoli in the lungs.
Matter can change from one state to another when it is heated, cooled or subjected to pressure. These changes are physical changes because the chemical nature of the substance remains the same.
Melting (Fusion): When a solid is heated, it absorbs heat and its particles gain kinetic energy. At a particular temperature, the solid melts and becomes a liquid. The temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. Melting point is also called fusion point because the change is called fusion. For ice, the melting point is 0 degree Celsius. The heat absorbed during melting is called latent heat of fusion, and it is used only to break the bonds between particles, not to raise the temperature.
Boiling (Vaporisation): When a liquid is heated continuously, at a particular temperature it starts converting into vapour. This temperature is called its boiling point. The boiling point of water at sea level is 100 degree Celsius (373 K). The heat absorbed during boiling is called latent heat of vaporisation. It is the heat that changes a liquid into vapour at its boiling point without increasing its temperature.
Condensation: The change of a gas into its liquid form is called condensation. When water vapour comes in contact with a cool surface, it condenses to form liquid water. Cloud formation, fog and the formation of dew are all examples of condensation.
Sublimation: Some substances, when heated, change directly from the solid state to the gaseous state without passing through the liquid state. This change is called sublimation. Similarly, the vapour of such a substance can change directly to solid on cooling. Ammonium chloride, iodine, camphor and naphthalene are examples of substances that sublime. On heating, camphor turns directly into vapour, and on cooling the vapour forms solid crystals again.
Deposition: The change of a gas directly into a solid without passing through the liquid state is called deposition. The formation of frost on a cold winter morning is an example of deposition.
When a solid is heated, the kinetic energy of its particles increases. At the melting point, the heat supplied is used to overcome the forces of attraction between particles, and the solid melts. If the heating continues, the temperature rises further until the boiling point is reached, at which the liquid changes into gas.
We can represent the Celsius scale of temperature in Kelvin using the relation:
K = C + 273
So 0 degree Celsius is 273 K, and 100 degree Celsius is 373 K.
Applying pressure also changes the state of matter. If we apply pressure and reduce temperature to a gas, the particles come closer and the gas can be liquefied. For example, carbon dioxide gas can be compressed and cooled to form solid carbon dioxide, which is called dry ice. Dry ice directly changes into carbon dioxide gas on heating without melting, which is why it is used for storage of food items and in stage shows to create smoke effects.
The temperature at which a gas can be liquefied by applying pressure is increased if the gas is already compressed. This principle is used in LPG cylinders, where domestic cooking gas is stored as a liquid under high pressure.
Evaporation is the process of conversion of a liquid into its vapour at a temperature below its boiling point. It occurs only at the surface of the liquid. A wet cloth dries on a clothesline because water evaporates from it even at room temperature.
Factors affecting evaporation:
Evaporation is different from boiling. Boiling is a bulk phenomenon that occurs throughout the liquid at the boiling point, while evaporation is a surface phenomenon that occurs below the boiling point. Evaporation causes cooling because the fast-moving particles with higher kinetic energy escape from the liquid, leaving behind slower particles with lower energy, thus lowering the temperature of the liquid. This is why we feel cool when water evaporates from our skin after bathing, and why sweating keeps our body cool in summer.
| State of Matter | Shape | Volume | Compressibility | Particle Arrangement |
|---|---|---|---|---|
| Solid | Definite | Definite | Very low | Closely packed, only vibrate |
| Liquid | No definite shape | Definite | Slight | Close, can slide past each other |
| Gas | No definite shape | No definite volume | Very high | Very far apart, move freely |
| Change of State | Process Name | Example |
|---|---|---|
| Solid to Liquid | Melting (Fusion) | Ice melts to water at 0 C |
| Liquid to Gas | Boiling / Vaporisation | Water boils at 100 C |
| Gas to Liquid | Condensation | Dew drops on leaves |
| Solid to Gas (direct) | Sublimation | Camphor, iodine |
| Gas to Solid (direct) | Deposition | Frost on cold morning |
In this chapter we learned that matter is anything which has mass and occupies space, and it is made up of extremely small particles that are always in motion and have spaces between them. Matter exists in three physical states - solid, liquid and gas - which differ in particle arrangement, intermolecular forces and compressibility. We studied the various changes of state, including melting, boiling, condensation, sublimation and deposition, and understood the role of latent heat in these changes. Finally, we examined evaporation, its factors and the cooling effect it produces, which explains many everyday phenomena like sweating and the drying of clothes. A clear understanding of the particulate nature of matter forms the foundation of chemistry and physics, and is essential for higher classes.