Physical Nature Of Matter

Early scientific thought debated whether matter was a continuous substance or made of discrete particles. Evidence from simple experiments supports the particulate nature of matter.

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Matter Is Made Up Of Particles

To understand if matter is continuous like a block or made of particles like sand, we can perform a simple activity: Dissolving a substance like salt or sugar in water. When salt dissolves in water, it seems to disappear, and the water level doesn't rise significantly. This observation is explained by the idea that the tiny particles of salt or sugar separate and fit into the empty spaces existing between the particles of water. If matter were continuous, dissolving one substance in another would simply add its volume to the first, leading to a noticeable increase in the overall volume.

This intermixing indicates that matter is not a solid, unbroken mass but is composed of discrete, tiny particles.

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How Small Are These Particles Of Matter?

Particles of matter are incredibly small, often beyond the scope of normal visualization. Their minute size can be illustrated by dilution experiments. Consider dissolving just a few crystals of potassium permanganate, which is intensely coloured, in a large volume of water (e.g., 100 mL). Then, take a small amount (e.g., 10 mL) of this coloured solution and dilute it again in another 90 mL of clear water. Repeating this process several times (5-8 dilutions) shows that the colour, although fading, is still detectable even in highly diluted solutions.

This means that the initial few crystals must have contained millions or even billions of individual particles that dispersed and continued to divide into smaller units through the dilution process. A similar effect is observed using substances with strong smells, like Dettol; the scent can be detected even after significant dilution. These activities demonstrate that the particles constituting matter are exceedingly small – they are microscopic and particulate.

Characteristics Of Particles Of Matter

Particles of matter exhibit certain fundamental characteristics that dictate the physical behaviour and properties of different substances and their states.

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Particles Of Matter Have Space Between Them

As concluded from the activity of dissolving salt or sugar in water, the particles of the solute (salt or sugar) are able to fit into the spaces present between the particles of the solvent (water). This proves that the particles of matter are not packed together completely tightly; there are interparticle spaces. When we make beverages like tea or coffee, or a solution like lemonade (nimbu paani), the particles of different ingredients mingle by occupying the spaces between one another. The presence and extent of these spaces vary significantly depending on the state of matter (solid, liquid, or gas).

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Particles Of Matter Are Continuously Moving

Particles of matter are in constant, random motion. This movement is a manifestation of their kinetic energy. The particles vibrate, move from place to place, or both, depending on the state.

Evidence for this continuous motion comes from:

• The spreading of smells: The aroma from an unlit incense stick is only noticeable nearby, but when lit, its particles gain energy, move faster, mix with air particles, and quickly spread the smell throughout the room.
• Mixing of liquids: When a drop of ink is added to water, its colour gradually spreads throughout the water, even without stirring. Honey also mixes but much slower than ink due to its higher viscosity and stronger intermolecular forces. This spontaneous mixing is due to the movement of both ink and water particles.
• Effect of temperature on mixing: Dropping a crystal of copper sulphate or potassium permanganate into hot water and cold water shows that the colour spreads much faster in hot water. This is because the particles in hot water (and the crystal) have more kinetic energy and thus move and diffuse faster at higher temperatures.

The spontaneous intermixing of particles of two different types of matter due to the random motion of particles is called diffusion. Diffusion occurs in solids, liquids, and gases, but its rate is highest in gases and lowest in solids. Heating increases the rate of diffusion as particles gain more kinetic energy.

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Particles Of Matter Attract Each Other

Particles of matter exert forces of attraction on one another. These forces hold the particles together. The strength of this attractive force varies significantly between different substances and different states of matter.

We can observe the effect of these forces by trying to break apart different materials:

• An iron nail is very difficult to break or deform because the attractive forces between iron particles are very strong.
• A piece of chalk is relatively easier to break, indicating weaker attractive forces between chalk particles compared to iron.
• A rubber band can be stretched and broken with sufficient force, suggesting attractive forces are present but can be overcome.

Another example is the surface of water. When you try to cut the surface of water with your finger, the water tends to remain together. This cohesive property is due to the attractive forces between water molecules.

These attractive forces are often referred to as intermolecular forces. The strength of these forces is a key factor determining the state of matter and many of its properties. They are strongest in solids, intermediate in liquids, and weakest in gases.

States Of Matter

Matter exists in different physical states primarily due to the differences in the arrangement, movement, and forces of attraction between its constituent particles. The three most common states are solid, liquid, and gas.

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The Solid State

Solids are characterized by having a definite shape, distinct boundaries, and a fixed volume. Examples include a pen, book, needle, iron nail, or wooden stick. When placed in different containers or subjected to moderate forces, they maintain their form and volume.

• Solids have negligible compressibility; applying force does not significantly reduce their volume because particles are already closely packed.
• They possess rigidity, meaning they resist changes in shape. While they may break under excessive force, it's difficult to deform them.
• Particles in solids are held together by very strong forces of attraction and are arranged in ordered, fixed positions. They can only vibrate about these mean positions.

Even substances like rubber bands, sugar/salt crystals, and sponges, which might seem exceptional, fit the definition of solids when examined closely. A rubber band is solid because it regains its original shape after stretching, unless the force is excessive. Individual sugar or salt crystals retain their shape regardless of the container. A sponge is compressible because it has trapped air pockets, not because the solid material itself is easily compressed.

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The Liquid State

Liquids have a fixed volume but no definite shape; they take the shape of the container they fill. Examples include water, oil, milk, and juice. If spilled, they spread out, confirming the lack of a fixed shape. Measuring a set amount (e.g., 50 mL) and pouring it into various containers shows that the volume remains constant, while the shape conforms to the container's shape.

• Liquids are fluid, meaning they can flow and are not rigid.
• Liquids have low compressibility compared to gases, but are generally slightly more compressible than solids.
• Particles in liquids are held by weaker attractive forces than in solids. They are close together but can move past each other, allowing the liquid to flow.

Solids, liquids, and gases can dissolve (diffuse) into liquids. The dissolution of oxygen and carbon dioxide from the air in water is vital for aquatic life. The rate of diffusion in liquids is higher than in solids due to the greater spaces between particles and their ability to move freely.

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The Gaseous State

Gases have neither a definite shape nor a definite volume. They expand to fill the entire volume of any container they are placed in. Examples include air, the gas in balloons, or compressed gases in cylinders (like LPG or oxygen).

• Gases are highly compressible. This property is used to store large quantities of gas in small cylinders (e.g., LPG, CNG). Experiments show that a piston can easily compress the air in a syringe, whereas compressing water or chalk (liquid and solid) is very difficult.
• Gases are also highly fluid.
• Particles in gases are far apart, move randomly at high speeds, and have very weak attractive forces between them.
• Due to their high speed and large interparticle distances, gases diffuse very rapidly into other gases. The rapid spreading of the smell of hot food or perfume demonstrates this high rate of diffusion.

The constant, random motion of gas particles causes them to collide with each other and the walls of the container. The force exerted by the gas particles per unit area on the container walls is what we perceive as gas pressure.

Can Matter Change Its State?

Yes, matter can transition from one physical state to another. The most common example is water, which exists as a solid (ice), a liquid (water), and a gas (water vapour). These changes of state occur when conditions like temperature or pressure are altered, affecting the energy and arrangement of the particles.

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Effect Of Change Of Temperature

Changing the temperature is a common way to induce a change of state.

• Solid to Liquid (Melting or Fusion): When a solid is heated, the thermal energy absorbed increases the kinetic energy of its particles. They vibrate more strongly about their fixed positions. At a specific temperature, known as the melting point, the particles gain enough energy to overcome the strong forces of attraction holding them in fixed lattice positions. They break free and start moving more randomly, transitioning the substance into a liquid state. The melting point is the minimum temperature at which a solid melts at atmospheric pressure. For ice, the melting point is approximately 273.15 K (or $0^\circ\text{C}$). The process is called melting or fusion.

During melting, even as heat is continuously supplied, the temperature of the substance remains constant until all the solid has melted. This supplied heat energy is used to change the state by overcoming the interparticle forces, rather than increasing the temperature. This energy is called latent heat of fusion. It is the amount of heat required to convert 1 kg of a solid to a liquid at its melting point and atmospheric pressure. Consequently, particles in water at $0^\circ\text{C}$ have more energy than particles in ice at the same temperature because they have absorbed the latent heat of fusion.

• Liquid to Gas (Boiling or Vaporisation): When a liquid is heated, the particles gain more kinetic energy and move faster. At a certain temperature, the boiling point, particles throughout the bulk of the liquid gain sufficient energy to overcome the attractive forces and escape into the gaseous (vapour) state. Boiling is a bulk phenomenon. The boiling point is the temperature at which a liquid starts boiling at atmospheric pressure. For water, the boiling point is 373 K (or $100^\circ\text{C}$).

Similar to melting, the temperature remains constant during boiling as heat is absorbed. This energy, the latent heat of vaporisation, is used to convert the liquid into a gas by overcoming the interparticle forces. It is the amount of heat required to convert 1 kg of a liquid to a gas at its boiling point and atmospheric pressure. Steam (water vapour) at $100^\circ\text{C}$ has more energy than liquid water at $100^\circ\text{C}$ due to the latent heat of vaporisation absorbed.

Besides these transitions, some substances can change directly from a solid to a gas without becoming a liquid. This is called sublimation (e.g., camphor, naphthalene, dry ice). The reverse process, changing directly from a gas to a solid, is called deposition.

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Effect Of Change Of Pressure

Pressure primarily affects the state of gases. Since gas particles are far apart, applying pressure can force them closer together. Reducing the space between gas particles can lead to liquefaction.

By applying pressure and simultaneously reducing the temperature, gases can be converted into liquids. For example, LPG is stored as a liquid under pressure in cylinders.

A notable example of pressure-induced state change is solid carbon dioxide ($\text{CO}_2$), known as dry ice. It is stored under high pressure. When the pressure is reduced to normal atmospheric pressure (1 atm), solid $\text{CO}_2$ changes directly into gaseous $\text{CO}_2$ through sublimation, without forming a liquid. This is why it's called "dry ice" – it doesn't melt into a liquid.

Therefore, the state of a substance (solid, liquid, or gas) is determined by a combination of both temperature and pressure conditions.

Evaporation

Not all transitions from liquid to gas require the liquid to reach its boiling point. The phenomenon of a liquid changing into vapour at any temperature below its boiling point is called evaporation.

Evaporation is a surface phenomenon. Particles at the surface of the liquid, having higher kinetic energy than those in the bulk, can overcome the attractive forces of their neighbours and escape into the surrounding air as vapour. This continuous escape of energetic particles from the surface leads to the gradual conversion of the liquid into gas.

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Factors Affecting Evaporation

The rate at which a liquid evaporates is influenced by several environmental factors:

• Surface Area: A larger surface area exposed to the atmosphere allows more particles at the surface to escape. Spreading out wet clothes increases the surface area, thus speeding up drying (evaporation).
• Temperature: Increasing the temperature provides more kinetic energy to the liquid particles. More particles gain enough energy to overcome the intermolecular forces and escape the surface, increasing the evaporation rate. Wet clothes dry faster on a hot day.
• Humidity: Humidity is the amount of water vapour already present in the air. If the air is already rich in water vapour (high humidity), it cannot accommodate much more, so the rate of evaporation decreases. Clothes dry slower on a humid day.
• Wind Speed: When wind blows, it carries away the water vapour particles that have just evaporated from the surface of the liquid. This reduces the concentration of water vapour in the surrounding air, allowing more liquid particles to escape and increasing the rate of evaporation. Clothes dry faster on a windy day.

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How Does Evaporation Cause Cooling?

Evaporation is always accompanied by a cooling effect on the surface or surrounding from which it occurs. When the high-energy particles escape from the liquid surface during evaporation, the average kinetic energy of the remaining liquid particles decreases. To maintain evaporation, these remaining particles (or the escaping particles) absorb energy from the immediate surroundings (like the liquid itself or the surface it's on).

This absorption of energy (equal to the latent heat of vaporisation) from the surroundings cools the surroundings down.

Examples of cooling by evaporation:

• Putting acetone, petrol, or perfume on your palm causes a cooling sensation because these volatile liquids evaporate quickly, absorbing heat from your skin.
• Water sprinkled on hot ground or roofs evaporates, taking heat from the surface and cooling it down due to water's high latent heat of vaporisation.
• Sweating is the body's natural cooling mechanism. As sweat evaporates from the skin surface, it absorbs heat from the body, thus cooling us down. Wearing cotton clothes helps in summer because cotton is a good absorber of sweat, facilitating its evaporation and cooling.
• Water kept in an earthen pot remains cool in summer because water continuously seeps through the porous walls to the outer surface and evaporates. This evaporation draws heat from the water inside the pot, cooling it.

It is important to distinguish evaporation from condensation. The water droplets seen on the outer surface of a glass containing ice-cold water are formed by the condensation of water vapour from the warm air surrounding the cold glass. The water vapour loses energy to the cold surface and changes back into liquid water.

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