Nature of matter

In a solid, the force of attraction between particles is strong, so the particles are packed very close together and are generally arranged in an orderly manner. They only vibrate about fixed positions. Option C is incorrect because free movement of particles is more characteristic of liquids or gases. Exam tip: Identify a solid by its fixed shape and fixed volume.

The force of attraction between particles is weaker in a liquid than in a solid. Therefore, liquid particles can move with greater freedom while remaining close together, which allows liquids to flow. Option C is more suitable for solids because their particles generally only vibrate about fixed positions. Exam tip: particles in solids mainly vibrate, whereas particles in liquids can slide past one another and change positions.

Gas particles have large spaces between them, so they are far apart and randomly arranged. They move freely in all directions and occupy the entire available container. Option D describes liquid particles, which are close together but can slide past one another. Exam tip: particles are fixed in solids, close together in liquids, and far apart in gases.

Both liquids and gases can flow easily; therefore, they are called fluids. Their particles can move relative to one another. In solids, particles mainly vibrate about fixed positions, so solids generally do not flow. Saying only liquids or only gases is incomplete. Exam tip: A substance that can flow is classified as a fluid.

Both liquids and gases can flow and take the shape of their container, so they are called fluids. Solids generally retain a fixed shape and do not flow. Exam tip: A substance that can flow is classified as a fluid.

Wood is a solid because it has a definite shape and a definite volume. Its particles are relatively closely packed, so it retains its shape. Oxygen gas, steam, and air are gases; they have neither a definite shape nor a definite volume. Exam tip: Definite shape and definite volume are key properties of a solid.

Oil is a liquid because it flows and takes the shape of container. Its volume remains nearly fixed.

Carbon dioxide is an example of a gas. Gases have neither a fixed shape nor a fixed volume, so they spread out to occupy the available space in a container. Salt and ice are solids, and glass is also generally a solid. Exam tip: identify a state of matter by checking whether it has a fixed shape and volume.

In a solid, particles are closely packed and the force of attraction between them is strong. Therefore, they only vibrate about fixed positions and cannot move freely past one another, giving a solid a definite shape. Options B and D are characteristics more closely associated with gases. Exam tip: A solid has both a definite shape and a definite volume.

Liquid particles are close together, but unlike solid particles, they are not locked in fixed positions. They can slide past one another, so a liquid flows and takes the shape of its container. Option A describes particles in a solid. Exam tip: A liquid has a definite volume but no definite shape.

The force of attraction between gas particles is very weak, and there is a large space between them. They move continuously and randomly in all directions, so they spread through the available space and fill the entire container. Option D is incorrect because a gas has no fixed shape of its own. Exam tip: A gas has neither fixed shape nor fixed volume; it takes the shape and volume of its container.

Melting is the change of state in which a solid becomes a liquid on heating. For example, ice changing into water is melting. Freezing is the reverse process, in which a liquid changes into a solid. Exam tip: Remember the direction—solid → liquid = melting.

The change of a liquid into a gas is called vaporization. It may occur at the surface at any temperature, whereas boiling occurs throughout the liquid at its boiling point. Melting is the change from solid to liquid, freezing is liquid to solid, and deposition is gas directly to solid. Exam tip: identify the initial and final states before choosing the term for a change of state.

Condensation is the change of state in which a gas loses heat and becomes a liquid. For example, water vapour forms water droplets on cooling. Melting is the change of a solid into a liquid, so it is not a gas-to-liquid change. Exam tip: remember that gas → liquid is condensation, whereas liquid → gas is vaporisation.

Freezing is the change of state in which a liquid loses heat and becomes a solid. For example, water forms ice on cooling. Condensation is the change from gas to liquid, so it is not correct here. Exam tip: Remember the direction of change—liquid → solid is freezing.

Sublimation is the change in which a solid changes directly into a gas without passing through the liquid state. Therefore, sublimation is correct. In melting, a solid becomes a liquid, whereas in condensation, a gas becomes a liquid. Exam tip: Remember solid → gas as sublimation and gas → solid as deposition.

Under ordinary conditions, camphor changes directly from the solid state to vapour without becoming a liquid. This direct change from solid to gas is called sublimation, so option A is correct. In melting, a solid first becomes a liquid, while evaporation is the conversion of a liquid into vapour from its surface. Exam tip: If a solid changes directly into vapour, choose sublimation.

Naphthalene balls gradually change directly from the solid state into vapour without becoming liquid, so their size decreases over time. The direct conversion of a solid into a gas is called sublimation. Vaporization refers mainly to the conversion of a liquid into vapour, so it is not correct here. Exam tip: Camphor, iodine and ammonium chloride are common examples of substances that sublime.

In evaporation, molecules at the free surface of a liquid gain sufficient kinetic energy and escape as vapour. Therefore, it occurs only at the liquid surface. In contrast, boiling occurs throughout the liquid. Exam tip: associate “surface” with evaporation and “throughout the liquid” with boiling.

Boiling takes place throughout a liquid at its boiling point, producing vapour bubbles. In contrast, evaporation occurs only from the surface and can take place slowly at any temperature. Option A describes evaporation, not boiling. Exam tip: Vapour bubbles throughout the liquid indicate boiling.

Evaporation is the escape of some liquid molecules from the surface into the gas state. At any temperature, molecules have different kinetic energies. When temperature increases, the average kinetic energy increases, so a larger number of surface molecules can overcome the attractive forces holding them in the liquid and escape.

As a result, the rate of evaporation increases when temperature rises, provided other important conditions such as exposed surface area, humidity, and wind remain comparable. This is why wet clothes generally dry faster in warm conditions than in cold conditions. Option A is correct. Heating does not make evaporation zero or necessarily keep it unchanged; it supplies more energetic molecules for surface escape.

Wind continuously removes water vapour from near the surface of the cloth. This keeps the vapour concentration above the surface low, allowing more water molecules to escape; hence, the rate of evaporation increases. The mass of water decreases as the cloth dries, and wind does not change the chemical composition or boiling point of water. Exam tip: Wind, larger surface area, and higher temperature increase the rate of evaporation.

In sunlight, the temperature of the cloth and the water in it increases. At a higher temperature, water molecules have greater kinetic energy, so more molecules escape from the surface as vapour and evaporation becomes faster. Lower air pressure is not the main reason here. Exam tip: Higher temperature, larger surface area, and faster wind increase evaporation.

When water vapour in the surrounding air touches the cold bottle surface, it cools and changes into tiny droplets of liquid water. This process is called condensation. The droplets are not caused by leakage from the drink; they come from water vapour already present in the air. Exam tip: The conversion of water vapour into liquid on a cold surface is condensation.

An earthen pot has tiny pores. A small amount of water reaches the outer surface through these pores and evaporates. Evaporation requires heat, which is taken from the water and the pot, so the remaining water becomes cool. The pot does not completely block outside heat; evaporation is the main cause of cooling. Exam tip: Remember: porous pot + evaporation = cooling.