Matter States and Changes

Matter In Our Surroundings (States & Changes)

Everything around us is made up of matter. Matter is defined as anything that occupies space and has mass. The matter around us exists in three distinct states: solid, liquid, and gas.

These states of matter are due to variations in the characteristics of the particles that make up the matter. The particles are constantly in motion, and the way they interact and arrange themselves determines the state of the matter. Let's explore the properties of these three states.

The Solid State

Matter in the solid state has a definite shape and a definite volume.

Properties of Solids:

1. Definite Shape and Volume: Solids have a fixed shape and volume that do not change unless an external force is applied.
2. Rigidity: Solids are generally rigid and resist deformation.
3. Incompressibility: Solids are highly incompressible, meaning their volume does not significantly decrease when pressure is applied.
4. High Density: Particles are closely packed, resulting in high density.
5. Intermolecular Forces: Particles are held together by strong intermolecular forces, which keep them in fixed positions.
6. Particle Movement: Particles in a solid are not free to move but vibrate about their mean positions.
7. Diffusion: Diffusion in solids is very slow compared to liquids and gases.

Examples: Ice, stone, wood, a pen, a book.

Even though some substances like rubber band (can be stretched), sugar/salt (take the shape of the container), and sponge (can be compressed) might seem to violate these properties, they are still considered solids based on closer examination of their particle structure or specific conditions:

• A rubber band changes shape under force but regains its original shape when the force is removed. If excessive force is applied, it breaks. It is still a solid.
• Sugar and salt crystals are individual solid particles. They take the shape of the container not because they flow like liquids, but because they are small individual solids stacking up. Each crystal retains its own shape.
• A sponge has small pores filled with air. When compressed, the air is expelled. The solid material of the sponge itself is not compressed significantly.

The Liquid State

Matter in the liquid state has a definite volume but no definite shape. It takes the shape of the container it is placed in.

Properties of Liquids:

1. Definite Volume: Liquids occupy a fixed volume.
2. No Definite Shape: Liquids take the shape of the container they are poured into.
3. Fluidity: Liquids can flow, hence they are called fluids. They are not rigid.
4. Low Compressibility: Liquids are much less compressible than gases but more compressible than solids (though often considered practically incompressible for many purposes).
5. Moderate Density: Density is generally lower than solids (except for some cases like ice being less dense than water) and higher than gases.
6. Intermolecular Forces: Intermolecular forces are weaker than in solids, allowing particles to slide past each other.
7. Particle Movement: Particles can move and slide over each other.
8. Diffusion: Diffusion in liquids is faster than in solids. Substances from solids, liquids, and gases can diffuse into liquids. Example: Aquatic animals breathe oxygen dissolved in water.

Examples: Water, milk, juice, oil.

The Gaseous State

Matter in the gaseous state has neither a definite shape nor a definite volume. It occupies the entire volume of the container.

Properties of Gases:

1. No Definite Shape or Volume: Gases have no fixed shape or volume. They fill the entire container.
2. Fluidity: Gases can flow, hence they are also considered fluids.
3. High Compressibility: Gases are highly compressible. This property is used in cylinders storing compressed gases (e.g., LPG cylinders for cooking, CNG for vehicles, oxygen cylinders for hospitals).
4. Low Density: Particles are far apart, resulting in very low density compared to solids and liquids.
5. Very Weak Intermolecular Forces: Intermolecular forces between particles are very weak.
6. Particle Movement: Particles move randomly and at high speeds in all directions. They collide with each other and with the walls of the container.
7. Diffusion: Diffusion in gases is very fast due to the high speed and large spaces between particles. Different gases mix readily.

Examples: Air, oxygen, hydrogen, nitrogen, steam.

The pressure of a gas is the force exerted by the gas particles per unit area on the walls of the container, caused by the collisions of the fast-moving particles.

Can Matter Change Its State?

Yes, matter can change its state from solid to liquid, liquid to gas, or vice versa, and also directly from solid to gas or gas to solid. These changes occur when the conditions, particularly temperature and pressure, are altered. These transitions are called phase changes or phase transitions.

The physical state of matter can be changed by:

1. Changing the temperature.
2. Changing the pressure.

Effect Of Change Of Temperature

When a solid is heated, its particles gain kinetic energy and start vibrating more vigorously. At a certain temperature, the particles have enough energy to overcome the intermolecular forces holding them in fixed positions, and they start moving more freely. This is the point where the solid melts and turns into a liquid.

• Melting (Fusion): The process by which a solid changes into a liquid on heating.
• Melting Point: The fixed temperature at which a solid melts to become a liquid at atmospheric pressure. The melting point of ice is 0$^\circ$C (or 273.15 K). The melting point is an indication of the strength of the forces of attraction between the particles of a solid.

During melting, the temperature of the substance remains constant at the melting point, even though heat is being supplied. This heat energy is used to overcome the forces of attraction between the particles and is called the latent heat of fusion.

Latent heat of fusion: The amount of heat energy required to change 1 kg of a solid into liquid at its melting point without any rise in temperature.

When a liquid is heated, its particles gain kinetic energy and move even faster. At a certain temperature, the particles have enough energy to overcome the intermolecular forces and escape into the gaseous state.

• Boiling (Vaporisation): The process by which a liquid changes into a gas (vapour) rapidly at a fixed temperature throughout the bulk of the liquid.
• Boiling Point: The fixed temperature at which a liquid boils to become a gas at atmospheric pressure. The boiling point of water is 100$^\circ$C (or 373.15 K).

Similar to melting, the temperature remains constant at the boiling point while the liquid boils. The heat energy supplied during boiling is the latent heat of vaporisation.

Latent heat of vaporisation: The amount of heat energy required to change 1 kg of a liquid into gas at its boiling point without any rise in temperature.

Other temperature-related phase changes:

• Freezing (Solidification): The process by which a liquid changes into a solid on cooling. It occurs at the melting point of the substance.
• Condensation: The process by which a gas (vapour) changes into a liquid on cooling.
• Sublimation: The process by which a solid changes directly into a gas (vapour) without passing through the liquid state, and vice versa. Examples: Camphor, naphthalene, dry ice (solid CO$_2$).
• Deposition: The process by which a gas changes directly into a solid without passing through the liquid state. (Often considered the reverse of sublimation). Example: Formation of frost.

Effect Of Change Of Pressure

Pressure also plays a significant role in changing the state of matter, particularly for gases and in some cases for solids and liquids.

• Liquefaction of Gases: Gases can be liquefied by applying pressure and reducing temperature. When pressure is increased, the particles of a gas come closer together. If the temperature is simultaneously lowered (to reduce the kinetic energy of the particles), the intermolecular forces become strong enough to hold the particles in the liquid state.
• Critical Temperature and Pressure: For every gas, there is a specific temperature called the critical temperature above which it cannot be liquefied by pressure alone, no matter how high the pressure is. The pressure required to liquefy the gas at its critical temperature is called the critical pressure.
• Effect on Boiling Point: The boiling point of a liquid increases with an increase in pressure and decreases with a decrease in pressure. Example: Water boils at 100$^\circ$C at standard atmospheric pressure (approx. 1 atm). At higher altitudes (where pressure is lower), water boils at a lower temperature (e.g., around 70$^\circ$C in Shimla). Conversely, in a pressure cooker, the pressure is increased, raising the boiling point of water and allowing food to cook faster at a higher temperature.
• Effect on Melting Point: The effect of pressure on the melting point of solids is generally small but can be significant in some cases. For most substances, the melting point increases with increasing pressure. However, for substances that expand on freezing (like ice), the melting point decreases with increasing pressure. This is why ice skates glide on a thin layer of water formed under the pressure of the blades.

Solid carbon dioxide (dry ice) is an example where pressure is key. It is stored under high pressure. When the pressure is reduced to 1 atmosphere, solid CO$_2$ changes directly to gas (sublimes) without melting.

Evaporation

Evaporation is the process where a liquid changes into a gas or vapour at any temperature below its boiling point. It is a surface phenomenon.

Unlike boiling, which occurs throughout the bulk of the liquid at a fixed temperature, evaporation occurs only at the surface of the liquid and can happen at various temperatures.

Particles at the surface of a liquid with higher kinetic energy can overcome the forces of attraction of other particles and escape into the gaseous state.

Examples of evaporation in daily life: Wet clothes dry in the sun or under a fan, puddles disappear after rain, water evaporates from bodies of water like rivers and oceans, sweat evaporates from our skin.

Factors Affecting Evaporation

The rate of evaporation is influenced by several factors:

1. Surface Area: The rate of evaporation increases with an increase in surface area. A larger surface area means more liquid particles are exposed to the surroundings, increasing the chance of particles escaping into the vapour state. Example: Clothes dry faster when spread out compared to being folded.
2. Temperature: The rate of evaporation increases with an increase in temperature. At higher temperatures, more particles have enough kinetic energy to overcome the intermolecular forces and escape from the liquid surface. Example: Wet clothes dry faster on a hot sunny day.
3. Humidity: Humidity is the amount of water vapour present in the air. The rate of evaporation decreases with an increase in humidity. If the air already contains a lot of water vapour, it cannot hold much more, thus slowing down the process of evaporation from a liquid surface. Example: Clothes take longer to dry on a humid day.
4. Wind Speed: The rate of evaporation increases with an increase in wind speed. Wind carries away the water vapour from the surface of the liquid, decreasing the concentration of water vapour in the surroundings and allowing more liquid particles to escape. Example: Clothes dry faster on a windy day.

How Does Evaporation Cause Cooling?

Evaporation is a cooling process. When a liquid evaporates, the particles with higher kinetic energy (i.e., the 'hottest' particles) leave the liquid surface and convert into vapour.

This means that 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 leads to a decrease in temperature.

In simpler terms, to evaporate, particles take energy (latent heat of vaporisation) from the liquid itself and the surroundings. This absorption of energy from the surroundings makes the surroundings feel cooler.

Examples of cooling by evaporation:

• Sweating: When we sweat, the sweat evaporates from our skin surface. The heat energy required for evaporation is taken from our body, which helps to cool us down.
• Water kept in an earthen pot (matka): Earthen pots have small pores in their walls. Water seeps through these pores to the outer surface and evaporates. The heat required for this evaporation is taken from the remaining water inside the pot, making the water cooler. This is a common practice in many Indian homes during summer.
• Sprinkling water on rooftops: In hot summer evenings, people sprinkle water on the rooftop or courtyard. The water evaporates, taking heat from the surface, thus cooling the area.
• Putting acetone on palm: When you put a little acetone (nail polish remover) on your palm, it evaporates quickly. The energy needed for evaporation is taken from your palm, making your palm feel cold.

So, the phenomenon of evaporation leads to cooling.