Nature of matter

Particles are usually farther apart in a gas than in a liquid. When a gas changes into a liquid, the particles lose energy and move closer together. This change is called condensation. It happens when the gas is cooled or otherwise loses enough energy for attractive forces to bring its particles nearer. Thus the distance between particles decreases during gas-to-liquid change.

Option C describes condensation and is therefore correct. In liquid becoming gas, particles spread farther apart. Ice melting changes a solid into a liquid and generally increases particle freedom and separation, while camphor sublimation changes a solid directly into a gas and produces much greater separation. These changes do not represent the required decrease.

Liquid particles remain close so volume is nearly fixed. Exam tip: remember liquid shape and volume separately.

The direct answer is option A: particles move freely. A gas has no fixed shape because its particles are far apart, have weak attraction, and move continuously in all directions. When gas is placed in a vessel, the particles spread throughout the available space, so the gas takes the shape of that vessel. Option A correctly states the essential reason: free movement. Option B is wrong because fixed particles describe a solid more nearly, not a gas. Option C is wrong because gas particles do have mass, even though the particles may be very small and widely separated. Option D is wrong because a gas is not a solid; solid and gas are different states of matter. The exact idea is not that the gas has no shape at all, but that it has no own fixed shape and adopts the container's shape. Memory cue: gas particles are free and far apart, so gas fills the container.

Solid particles only vibrate about their positions. Exam tip: fixed positions are the main reason for solid shape.

Gas particles have large intermolecular spaces and move freely. Therefore, gases are highly compressible, diffuse quickly, and occupy the entire volume of their container. A gas has no fixed shape; it takes the shape of its container. Exam tip: underline negative words such as “not” before selecting an option.

A liquid takes the container shape but need not fill it completely. Exam tip: remember the difference between liquid and gas.

A solid normally has a definite shape and a definite volume because its particles are closely packed and held in fairly fixed positions. The particles can vibrate, but they do not move freely throughout the available space. Consequently, a solid keeps its own shape instead of spreading to match the shape of a container. Its fixed volume also means that it does not occupy every part of a larger empty container.

Option D is correct because filling the whole container is a characteristic of a gas, not a solid. Gas particles move freely and spread in all directions, so a gas expands to occupy the entire container. Fixed shape, fixed volume and closely placed particles are all consistent with the solid state.

Particles in a solid are closely packed and experience relatively strong attractive forces. Therefore, a solid has a definite shape and can retain it after removal from a container. A liquid has a definite volume but takes the shape of its container. Exam tip: a definite shape is a key property of solids.

Particles in a liquid are close together but can slide past one another. Therefore, a liquid has a definite volume but no definite shape, and it can flow out when the container is tilted or overturned. A gas also spreads, but it fills the available container; the word “flow” here points to a liquid. Exam tip: a substance with definite volume but no definite shape is a liquid.

In the gaseous state, particles have large spaces between them and move freely and rapidly in all directions. Therefore, a gas spreads quickly and occupies the available space when released from a closed container. A liquid can flow, but its particles are closer together, so it does not spread as rapidly as a gas. Exam tip: rapid diffusion and filling the available space are key properties of gases.

The governing concept is compressibility, which depends mainly on the amount of empty space between particles. Gas particles are far apart, so applied pressure can reduce the gaps between them and noticeably decrease the gas volume. Liquid particles are already close together because intermolecular attraction keeps them in a condensed state. There is comparatively little empty space left to remove, so even considerable pressure produces only a small volume change. Therefore option A is correct. Liquids certainly contain particles and have mass, so options B and D are false. They are not always crystals; a liquid does not possess a fixed crystal lattice, making option C incorrect as well.

In a solid, particles are very close together and the force of attraction between them is strong. Therefore, they only vibrate about fixed positions, making a solid difficult to deform. Option D is more suitable for gases, whose particles move relatively freely. Exam tip: relate the rigidity and fixed shape of solids to strong intermolecular attraction.

Gas particles have large intermolecular spaces, so increasing pressure can reduce the volume of a gas noticeably. Solids and liquids are nearly incompressible, so pressure has a much smaller visible effect on them. Exam tip: the most compressible state of matter is the gaseous state.

The upper surface of water in a glass is in contact with air and is clearly visible; this is called the free surface of a liquid. A gas spreads throughout its container and therefore has no definite free surface, while an iron nail is a solid. Exam tip: a liquid has a definite volume and, in an open container, a free upper surface.

Shape and volume are the most useful properties for identifying a state of matter. A solid has definite shape and volume, a liquid has definite volume but no definite shape, and a gas has neither definite shape nor definite volume. Mass is present in all states, so it is not the best basis for distinguishing them. Exam tip: first compare whether shape and volume are definite or indefinite.

In a physical change the chemical identity does not change. Exam tip: treat change of state as physical change.

In a chemical change, the chemical composition of a substance changes and one or more new substances are formed. Therefore, formation of a new substance is its main identification. A change only in shape or state, such as melting of ice, is usually a physical change because no new substance is produced. Exam tip: rusting, burning, and curdling of milk are common examples of chemical changes.

Air is a homogeneous mixture of gases such as nitrogen, oxygen, argon, and carbon dioxide, and its composition can vary. Distilled water and sodium chloride are pure substances. Exam tip: variable composition indicates a mixture.

The governing concept is the distinction between physical and chemical changes. In a physical change, the form or state may change but the substance remains chemically the same. In burning, wood reacts with oxygen in the air and produces new substances, including ash, carbon dioxide, water vapour, and other gases. The original wood cannot be recovered simply by reversing the process, and heat and light are released during the reaction. These are clear signs of a chemical change. Therefore option B is correct. Melting and condensation are physical state changes and do not describe combustion; option A is therefore wrong, while C and D are incomplete and irrelevant.

The governing concept is that a physical change alters the appearance, shape, or state of a substance without producing a new substance. When paper is folded, its shape and perhaps its creases change, but the paper remains chemically the same material with the same composition. It can generally be unfolded, although a crease may remain. No new substance, gas, or reaction product is formed. Therefore option A is correct. Chemical change would involve a new composition, such as burning paper to form ash and gases. Condensation is gas-to-liquid change, and sublimation is direct solid-to-gas change; neither describes folding.

Sugar dissolves without forming a new substance and can be recovered. Exam tip: dissolving is often physical.

Burning forms new substances so it is a chemical change. Exam tip: write combustion as chemical change.

The governing concept is the distinction between physical and chemical changes. When liquid water becomes vapour, only its physical state changes from liquid to gas. The substance remains water and its chemical composition is still H₂O before and after vaporisation. Heating increases the molecules’ kinetic energy and separates them more widely, but it does not create a new substance. Cooling the vapour can reverse the process by producing liquid water again, which further supports its classification as a physical change. Therefore option B is correct. Rusting and combustion are chemical changes because they produce new substances. Option A is too broad and is specifically wrong here because the chemical identity of water remains unchanged.

The governing concept is that a chemical change forms one or more new substances with properties different from those of the original material. When magnesium burns in oxygen, it undergoes the reaction 2Mg + O₂ → 2MgO. Magnesium oxide is a new compound, so the chemical identity of the starting magnesium changes. The brilliant white light and white solid residue are observable evidence, although the formation of magnesium oxide is the decisive reason for classification. Therefore option B is correct. The process is not merely compression, because no squeezing explains the new compound, and it is not only melting, because melting changes state without changing composition. Option A is therefore also incorrect.

Cooking changes composition and properties. Exam tip: cooking food is often considered chemical change.