Nature of matter

A pure substance has a uniform, definite composition and characteristic properties throughout the sample. A pure element contains one kind of atom, while a pure compound contains elements combined in a fixed ratio. For example, pure water has hydrogen and oxygen in a definite chemical ratio, so its composition does not change from one portion to another.

Therefore option A is correct. A material with different composition in different parts is a mixture, not a pure substance. A pure substance may be a solid, liquid, or gas, so option C is too narrow. Colour alone is only one possible observable property and cannot define purity, making option D incorrect. The essential idea is fixed composition, not physical state or colour.

A mixture is a physical combination of two or more substances. Its components retain their individual properties, and their proportions may vary, so option B is correct. A fixed ratio of components is a characteristic of a compound, not a mixture. Exam tip: Look for the terms ‘physical combination’ and ‘variable composition’ when identifying a mixture.

Pure copper is an element, so it contains only copper atoms and has a fixed composition. Salt water, air, and soil contain more than one substance, so they are mixtures rather than pure substances. Exam tip: An element or compound with fixed composition is a pure substance.

Air is a physical mixture of several gases, including nitrogen, oxygen, argon, carbon dioxide, and water vapour. The proportions of its components may vary with place and conditions, so it is a mixture. In contrast, pure oxygen and pure gold are elements, while pure water is a compound. Exam tip: A substance containing more than one component in variable proportions is a mixture.

An element contains one type of atoms. It cannot be broken into simpler substances by ordinary chemical methods.

Oxygen is an element because it consists of only one kind of atom. Water is a compound, as hydrogen and oxygen are chemically combined in it. Salt solution and milk are mixtures; milk is specifically a colloid. Exam tip: A substance containing only one type of atom is an element.

A compound is a pure substance formed when atoms of two or more different elements combine chemically in a definite proportion by mass. Chemical combination produces a new substance with properties different from those of its constituent elements; for example, sodium chloride has properties unlike sodium and chlorine. A compound can be represented by a fixed chemical formula, such as H2O or CO2, and it cannot be separated into its elements by ordinary physical methods. Option C is therefore correct. Option A describes only part of an element, while B and D do not give a scientific definition of a compound.

Water is a compound because hydrogen and oxygen are chemically combined in a fixed proportion. Its formula is \(H_2O\). Iron and oxygen are elements, whereas air is a mixture of different gases. Exam tip: A pure substance formed by the chemical combination of two or more elements in a fixed composition is a compound.

In a mixture substances are physically mixed. In a compound elements are chemically combined in fixed ratio.

In a homogeneous mixture, the composition and properties are uniform in every part, so the individual components are not usually visible separately. Salt water or sugar solution is an example. Option B describes a heterogeneous mixture, whose composition may differ from one part to another. Exam tip: the phrase “uniform throughout” indicates a homogeneous mixture.

In a heterogeneous mixture, the components are not distributed uniformly, so different parts can have different compositions. For example, in a mixture of sand and water, sand may settle at the bottom while water remains above. Option A describes a homogeneous mixture, not a heterogeneous one. Exam tip: A mixture in which components or layers can be distinguished is generally heterogeneous.

Salt dissolves and distributes uniformly throughout water, so its components cannot be distinguished separately. Therefore, salt water is a homogeneous mixture, or a solution. In a heterogeneous mixture, the composition is not uniform throughout, as in sand and water. Exam tip: A mixture that appears uniform in every part is generally homogeneous.

Sand does not dissolve in water, and its particles are not uniformly distributed throughout the mixture; they may settle at the bottom on standing. Therefore, sand and water form a heterogeneous mixture. A suspension is a type of heterogeneous mixture, but the question asks for the broader classification. Exam tip: If components are visibly separate or form a sediment, the mixture is generally heterogeneous.

Sugar dissolves completely in water to form a single-phase mixture with uniform composition throughout. Therefore, sugar solution is a homogeneous mixture. In colloids and suspensions, particles are relatively larger and are not dissolved uniformly like those in a true solution. Exam tip: A true solution is generally a homogeneous mixture.

Milk contains water, tiny fat droplets, proteins, lactose, and mineral salts, so it is classified as a mixture. More specifically, milk is a colloidal mixture, not a pure substance. A pure compound has a fixed composition, whereas the proportions of components in milk can vary. Exam tip: If a material contains several components in variable proportions, identify it as a mixture.

Common table sugar is sucrose, a pure compound with the chemical formula C12H22O11. Its carbon, hydrogen, and oxygen atoms are chemically bonded in a fixed ratio, giving pure sucrose a definite composition and characteristic properties. Therefore option C is correct. It is not a mixture, because its constituent elements are chemically combined rather than merely physically blended, and sucrose cannot be separated into those elements by simple physical methods. It is not an element because it contains atoms of three different elements. Energy is not a classification of matter. The answer refers to pure sucrose; commercial products may contain small amounts of additional substances.

Iron is an element because it is a pure substance made of only one kind of atom, namely Fe atoms. A compound contains two or more elements chemically combined in a fixed ratio, whereas mixtures and solutions are not pure substances. Exam tip: A substance containing only one type of atom is an element.

Heat is a form of energy, not matter. Matter has mass and occupies space; water, iron and air have these properties. Although air is not visible, it has mass and occupies space. Exam tip: Anything that has mass and occupies space is classified as matter.

A pure substance has a fixed composition because it contains only one kind of constituent particles. Elements and compounds are pure substances. The composition of a mixture can change with the amounts of its components; colloids and suspensions are also types of mixtures. Exam tip: When you see “fixed composition”, choose pure substance.

The direct answer is C: components of a mixture can generally be separated by physical methods. A mixture contains two or more substances that are combined without forming a new chemical substance. Because the original substances retain their own properties, differences such as particle size, boiling point, solubility, density or magnetism can be used for separation. For example, filtration separates an insoluble solid from a liquid, evaporation recovers a dissolved solid, and distillation separates liquids with different boiling points. Option C is correct. A is wrong because nuclear reactions are not the normal way to separate mixtures and change atomic nuclei. B is wrong because mixtures can often be separated. D is wrong because making new elements is neither necessary nor a method of ordinary mixture separation. A useful distinction is that physical separation does not normally change chemical identities, whereas a chemical reaction forms new substances. Exam cue: mixture means separation by physical properties.

In a compound elements are joined by chemical bonds. Therefore chemical method is needed for separation.

Pure water is a compound in which hydrogen and oxygen are always present in a fixed proportion. Its formula is \(H_2O\), and the mass ratio of hydrogen to oxygen is \(1:8\). A variable composition is a property of mixtures, not of pure compounds. Exam tip: A substance with fixed composition is generally a compound.

The ratio of components in a mixture need not be fixed. Therefore composition of air can vary with place.

An element is a pure substance made of only one type of atom; for example, iron contains only Fe atoms. Components of a mixture can be separated by physical methods, but an element cannot be separated in this way. An element may be solid, liquid, or gaseous, so it is not always a liquid. Exam tip: “one type of atom” is the key identifying feature of an element.

The governing definition of matter is anything that has mass and occupies space. At an introductory level, matter is described by physical states—solid, liquid, and gas—and by composition, such as element, compound, or mixture. Option D combines both essential ideas: matter has measurable mass, occupies volume, and can be organized according to state and composition. Option A is false because light is electromagnetic radiation rather than ordinary matter; option B contradicts the definition; and option C is false because solids and liquids are also matter. Thus D is the only complete and scientifically accurate summary.