Introduction to Matter and States

Matter In Our Surroundings

Everything around us is made up of matter. From the air we breathe and the food we eat to the stones, clouds, stars, plants, and animals – all are examples of matter. Even a tiny drop of water or a small particle of sand is matter. We can feel, see, or touch most forms of matter. We can also sense the presence of air, though we cannot see it.

Physical Nature Of Matter

For a long time, scientists debated about the nature of matter. One school of thought believed matter to be continuous, like a block of wood. The other school believed that matter was made up of particles, like sand. Experiments have shown that matter is indeed made up of particles.

Matter is made up of particles

We can demonstrate this through simple experiments. If we dissolve sugar or salt in water, the solid sugar or salt disappears, and the water level does not change significantly. This happens because the particles of sugar or salt get into the spaces between the particles of water. This activity shows that water, and indeed matter, is made up of tiny particles.

Illustration showing sugar particles dissolving in water by fitting into spaces between water particles

When salt is dissolved in water, the salt particles occupy the spaces between water particles.

How small are these particles of matter?

The particles of matter are very, very small – smaller than anything we can imagine. We can get an idea of how small these particles are by performing a simple experiment involving the dissolution of a few crystals of potassium permanganate in a large volume of water.

If you dissolve two or three crystals of potassium permanganate in 100 ml of water, the water turns purple. If you then take about 10 ml of this purple solution and add it to another 90 ml of clear water, the solution is still coloured, though lighter. Repeating this process of dilution several times (say, 5 to 8 times) shows that even after several dilutions, the water is still slightly coloured.

This suggests that even a few crystals of potassium permanganate must contain millions of tiny particles, which are able to spread out and colour large volumes of water upon successive dilutions. The particles of matter are constantly dividing themselves into smaller and smaller particles.

Matter has particulate nature

The particles of matter are incredibly small. Their size is in the range of nanometers ($10^{-9}$ meter) or even smaller, depending on the substance. These particles are atoms or molecules.

The idea that matter is composed of tiny particles dates back to ancient Indian and Greek philosophers, but it was firmly established through scientific experiments in the 19th and 20th centuries. The understanding of matter as composed of particles forms the basis of modern chemistry and physics.

Characteristics Of Particles Of Matter

The particles that make up matter have certain inherent characteristics that govern the properties and behaviour of different states of matter. These characteristics are:

Particles of matter have space between them.
Particles of matter are continuously moving.
Particles of matter attract each other.

Particles Of Matter Are Continuously Moving

Particles of matter are not stationary; they are in constant, random motion. This motion is due to the kinetic energy possessed by the particles.

Evidence for continuous movement

Several observations support the idea of continuous particle motion:

Diffusion: When substances mix spontaneously due to the movement of their particles. For example, if you open a bottle of perfume in one corner of a room, its fragrance spreads throughout the room. This is because the particles of perfume vapour move and mix with the moving particles of air. Similarly, the mixing of ink or a crystal of potassium permanganate in water is due to the movement of particles of both substances.
Brownian Motion: The random, zigzag movement of microscopic particles suspended in a fluid (liquid or gas) is due to the bombardment of these particles by the fast-moving particles of the fluid. This provides direct evidence for the existence and motion of the invisible particles of the fluid.
Mixing of Gases: When two gases are brought into contact, they readily mix with each other even without stirring, due to the random motion of their particles.

The motion of particles is minimum in solids, more in liquids, and maximum in gases. As temperature increases, the kinetic energy of the particles increases, and they move faster.

Diffusion

The spontaneous mixing of particles of different types of matter on their own is called diffusion. Diffusion occurs because particles are in continuous motion and there are spaces between them. The rate of diffusion increases with temperature because particles gain more kinetic energy and move faster. Diffusion is fastest in gases, slower in liquids, and slowest in solids.

Particles Of Matter Attract Each Other

Particles of matter are held together by forces of attraction between them. These forces are responsible for holding the particles close to each other, which is why matter exists in different states and maintains its form to some extent.

Evidence for attractive forces

The existence of these attractive forces can be demonstrated by simple observations:

Breaking Solids: Try breaking a piece of chalk, a block of ice, and an iron nail. It is relatively easy to break chalk, harder to break ice, and very difficult to break an iron nail. This suggests that the particles in chalk are held together by weaker forces compared to the particles in ice, and the particles in an iron nail are held together by very strong forces of attraction.
Holding a Stream of Water: If you let water run from a tap and try to break the stream with your finger, you will find that the stream remains together and flows around your finger. This shows that the particles of water are held together by sufficient forces of attraction that prevent the stream from breaking into individual drops easily.

The strength of these attractive forces varies from one substance to another and also depends on the state of matter. These forces are strongest between particles in the solid state, weaker in the liquid state, and weakest in the gaseous state. The nature and strength of these interparticle forces (often called intermolecular forces) play a key role in determining the physical properties of substances, such as melting point, boiling point, hardness, etc.

States Of Matter

Matter exists in different physical states depending on the conditions of temperature and pressure. These states are determined by the arrangement and motion of the particles and the strength of the attractive forces between them. The three common states of matter are solid, liquid, and gas.

Particle arrangement in solid, liquid, and gaseous states.

The Solid State

Solids have a definite shape and a definite volume. For example, a block of ice, a piece of stone, or a chair are solids.

Properties of Solids based on Particle Nature:

Particle Arrangement: Particles in a solid are closely packed and arranged in a regular, ordered lattice structure. The spaces between particles are very small.
Particle Motion: Particles in a solid cannot move from one position to another. They can only vibrate about their fixed mean positions. Their kinetic energy is lowest among the three states.
Interparticle Forces: The forces of attraction between particles in a solid are very strong, holding them tightly in their fixed positions.
Shape and Volume: Due to the fixed positions and strong forces, solids have a definite shape and a definite volume. They are generally rigid and incompressible.
Diffusion: The rate of diffusion in solids is extremely slow because the particles cannot move from their positions, although some diffusion can occur over very long periods (e.g., diffusion between two metal blocks pressed together).

The Liquid State

Liquids have a definite volume but do not have a definite shape. They take the shape of the container they are poured into. For example, water, milk, and oil are liquids.

Properties of Liquids based on Particle Nature:

Particle Arrangement: Particles in a liquid are closely packed but are not arranged in a regular, ordered structure. They are arranged in layers that can slide over each other. The spaces between particles are larger than in solids but smaller than in gases.
Particle Motion: Particles in a liquid can move from one position to another within the bulk of the liquid. They are in constant random motion, moving faster than particles in solids. They have more kinetic energy than solids.
Interparticle Forces: The forces of attraction between particles in a liquid are weaker than in solids but stronger than in gases. These forces are strong enough to hold the particles together (giving liquids a definite volume) but weak enough to allow them to move past each other.
Shape and Volume: Due to the ability of particles to move past each other, liquids do not have a definite shape but take the shape of the container. However, because the attractive forces are still strong enough to hold them together, they have a definite volume. Liquids are generally considered almost incompressible.
Diffusion: Diffusion occurs readily in liquids because particles can move around. The rate of diffusion is slower than in gases but much faster than in solids.

The Gaseous State

Gases have neither a definite shape nor a definite volume. They occupy the entire volume of the container they are placed in. For example, air, oxygen, and hydrogen are gases.

Properties of Gases based on Particle Nature:

Particle Arrangement: Particles in a gas are very far apart from each other compared to their size. There are large spaces between particles. They are arranged randomly.
Particle Motion: Particles in a gas are in constant, rapid, and random motion in all directions. They collide with each other and with the walls of the container. Their kinetic energy is highest among the three states.
Interparticle Forces: The forces of attraction between particles in a gas are very weak, almost negligible, except during collisions.
Shape and Volume: Due to the large spaces and weak forces, gases do not have a definite shape or a definite volume. They expand to fill the entire volume of the container. Gases are highly compressible.
Diffusion: Diffusion occurs very rapidly in gases due to the fast and random motion of particles and large spaces between them. Gases mix readily with each other.
Pressure: Gases exert pressure on the walls of the container due to the collisions of the fast-moving particles with the walls.

These three states of matter represent different arrangements and levels of energy of the same particles, dictated by the interplay between their kinetic energy (due to motion) and the potential energy (associated with interparticle forces). Changing temperature and pressure can alter this balance and cause matter to transition from one state to another.

Can Matter Change Its State?

Yes, matter can change its state from solid to liquid, liquid to gas, and vice versa, under appropriate conditions of temperature and pressure. These transitions between states are called phase changes.

The state of matter is determined by the balance between the kinetic energy of the particles (which tends to keep them moving and apart) and the strength of the interparticle forces of attraction (which tends to keep them together). By changing the temperature or pressure, we can alter this balance and cause a change of state.

Diagram showing phase transitions between solid, liquid, and gas states

Phase transitions between the three states of matter.

Effect Of Change Of Temperature

Changing the temperature of a substance can cause it to change its state.

Solid to Liquid (Melting or Fusion): When a solid is heated, the kinetic energy of its particles increases, and they vibrate more vigorously about their fixed positions. At a certain temperature, called the melting point, the particles gain enough energy to overcome the strong forces of attraction and break free from their fixed positions, starting to move more freely. The solid then melts and changes into a liquid.
The melting point of a solid is the temperature at which it changes from solid to liquid state at atmospheric pressure. The melting point is a characteristic property of a solid. For example, the melting point of ice is 0°C (273.15 K).

During melting, the temperature of the substance remains constant at its melting point until all the solid has converted into liquid. The heat energy supplied during this phase change is used to overcome the forces of attraction between particles and is called the latent heat of fusion.
Liquid to Gas (Boiling or Vaporisation): When a liquid is heated, the kinetic energy of its particles increases further, and they move even faster. At a certain temperature, called the boiling point, particles gain enough energy to overcome the attractive forces completely and escape from the surface and the bulk of the liquid, changing into gas (vapour).
The boiling point of a liquid is the temperature at which it changes from liquid to gaseous state throughout the bulk of the liquid at atmospheric pressure. For example, the boiling point of water is 100°C (373.15 K).

During boiling, the temperature of the substance remains constant at its boiling point until all the liquid has converted into gas. The heat energy supplied during this phase change is used to overcome the forces of attraction between particles and is called the latent heat of vaporisation.
Gas to Liquid (Condensation or Liquefaction): When a gas is cooled, the kinetic energy of its particles decreases, and they move slower. The attractive forces between particles become more dominant, causing them to come closer together and form a liquid. Condensation occurs at the boiling point of the substance at a given pressure.
Liquid to Solid (Freezing or Solidification): When a liquid is cooled, the kinetic energy of its particles decreases, and they move slower. The attractive forces become strong enough to hold the particles in fixed positions, forming a solid. Freezing occurs at the melting point of the substance at a given pressure.
Solid to Gas (Sublimation): Some substances can change directly from a solid state to a gaseous state without passing through the liquid state upon heating. This process is called sublimation. The reverse process, from gas directly to solid, is called deposition or desublimation. Examples: Camphor, naphthalene balls, dry ice (solid carbon dioxide), ammonium chloride.

Effect Of Change Of Pressure

Changing the pressure on a substance can also cause it to change its state. Pressure primarily affects the distances between particles, especially in gases.

Gas to Liquid (Liquefaction by Pressure): By applying pressure on a gas (at a temperature below its critical temperature), the particles can be brought closer together. If simultaneously cooled (to reduce kinetic energy), the attractive forces become sufficient to cause liquefaction (condensation). For example, natural gas is liquefied (LNG) by applying high pressure and cooling, so it can be transported more efficiently.
Solid Carbon Dioxide (Dry Ice): Solid carbon dioxide (dry ice) sublimes directly to gas at atmospheric pressure. However, if the pressure is increased to above 5.11 atmospheres, solid carbon dioxide melts into liquid carbon dioxide before vaporising when heated. This shows the effect of pressure on phase transitions.
Pressure and Melting/Boiling Points: Changing pressure also affects the melting and boiling points of substances.
- The boiling point of a liquid increases with increasing pressure. This is why food cooks faster in a pressure cooker (higher pressure, higher boiling point of water). Conversely, boiling point decreases with decreasing pressure (e.g., at high altitudes, water boils at a lower temperature).
- The melting point of most solids increases with increasing pressure (as higher pressure resists expansion upon melting). However, for substances like ice, the melting point decreases with increasing pressure (as ice contracts upon melting). This anomaly in ice explains phenomena like skating.

Thus, temperature and pressure are the two key factors that determine the state of matter. By controlling these conditions, we can induce phase changes.

Evaporation

Evaporation is a process by which a liquid changes into a gas or vapour at any temperature below its boiling point. Unlike boiling, which occurs throughout the bulk of the liquid at a specific temperature, evaporation is a surface phenomenon.

Mechanism of Evaporation

Particles in a liquid are in continuous random motion and possess kinetic energy. The kinetic energy is not the same for all particles; it follows a distribution. Some particles near the surface have higher kinetic energy than the average. If these high-energy particles at the surface gain enough energy to overcome the attractive forces of the other liquid particles, they escape from the liquid surface into the gaseous phase. This process continues as long as there are liquid particles at the surface with sufficient energy to escape.

Factors Affecting Evaporation

The rate of evaporation depends on several factors:

Surface Area: The rate of evaporation increases with increasing surface area. This is because evaporation is a surface phenomenon; a larger surface area means more particles are exposed to the surroundings and have the opportunity to escape. (e.g., Wet clothes dry faster when spread out).
Temperature: The rate of evaporation increases with increasing temperature. At higher temperatures, more particles have sufficient kinetic energy to overcome the attractive forces and escape from the liquid surface.
Humidity: Humidity is the amount of water vapour present in the air. The rate of evaporation decreases with increasing humidity. If the air already contains a large amount of water vapour, it cannot hold much more water vapour from the liquid, thus slowing down evaporation.
Wind Speed: The rate of evaporation increases with increasing wind speed. Wind carries away the water vapour particles that escape from the liquid surface, reducing the amount of water vapour in the surrounding air. This prevents the air from becoming saturated with vapour and allows for faster evaporation.
Nature of the Liquid: Liquids with weaker interparticle forces evaporate faster than liquids with stronger interparticle forces. For example, alcohol (spirit) evaporates faster than water because the forces between alcohol molecules are weaker than those between water molecules.

How Does Evaporation Cause Cooling?

Evaporation is a cooling process. When a liquid evaporates, the high-energy particles escape from the liquid surface. As the most energetic particles leave, the average kinetic energy of the remaining liquid particles decreases. Since the temperature of a substance is related to the average kinetic energy of its particles, a decrease in average kinetic energy results in a decrease in the temperature of the remaining liquid.

The liquid absorbs heat energy from its surroundings to compensate for the energy lost by the escaping high-energy particles and to maintain the evaporation process. This absorption of heat from the surroundings makes the surroundings feel cooler.

Examples of Cooling by Evaporation:

Wearing Cotton Clothes in Summer: Cotton is a good absorber of water. In summer, we sweat. Cotton clothes absorb sweat, and as the sweat evaporates from the clothing surface, it takes latent heat of vaporisation from our body, making us feel cool.
Water Kept in Earthen Pots (Matkas): Earthen pots are porous. Water seeps through the pores to the outer surface. This water evaporates from the outer surface, taking heat from the remaining water inside the pot. This keeps the water inside the earthen pot cool.
Sprinkling Water on Rooftops in Summer: Sprinkling water on hot rooftops or open grounds in summer helps to cool the area. As the water evaporates, it absorbs heat from the surface, causing cooling.
Evaporation of Spirit/Acetone: When you put some spirit or acetone on your palm, you feel a cooling sensation. This is because these liquids evaporate very quickly from your palm, absorbing heat from your skin.

Evaporation is a continuous process that helps regulate temperature in nature and is utilised in various cooling applications.