Nature of matter

Iron filings are magnetic, whereas sulfur powder is not attracted by a magnet. Therefore, when a magnet is brought near the mixture, the iron filings stick to it and sulfur remains behind. Distillation is used mainly to separate liquids or components of a solution, so it is not suitable here. Exam tip: Use magnetic separation when one component of a mixture is magnetic.

Chromatography is a separation method based on different movement of components. Separating ink colors is a common example.

Fractional distillation is based on boiling point differences. Crude oil mixture is separated into fractions.

The governing concept is the distinction between a pure substance and a mixture by composition and separability. A pure substance contains one kind of matter and has a definite, uniform composition; a compound such as water has elements in a fixed ratio. A mixture contains two or more substances physically combined, so the proportion of its components may vary from sample to sample. Therefore option D is correct. Option A wrongly assigns variable composition to pure substances, B confuses formation with classification, and C is incorrect because every pure substance has a definite composition.

Air contains gases such as nitrogen, oxygen, carbon dioxide, and water vapour that are physically mixed, and their proportions can vary. Therefore, air is a mixture. In a compound, elements are chemically combined in a fixed ratio, which is not true for air. Exam tip: Variable composition is a key sign of a mixture.

The direct answer is B: air is not a compound because its gases are not chemically combined in a fixed ratio. A compound is a pure substance formed when elements combine chemically in a definite proportion, giving it a fixed composition and characteristic properties. Air is mainly a physical mixture of gases such as nitrogen, oxygen, argon, and carbon dioxide. Their amounts can vary with location, pollution, altitude, humidity, and other conditions, and each gas largely retains its own properties. Option A is wrong because air is not always solid; it is normally a gas. Option B is correct because variable composition and absence of a single fixed chemical formula show that air is a mixture, not a compound. Option C is wrong because air has mass and can exert pressure. Option D is wrong because invisibility does not decide whether something is a compound; many compounds are invisible gases. Exam cue: fixed ratio and chemical bonding mean compound; variable composition and physical mixing mean mixture.

Pure gold consists of only one kind of atom—gold atoms—so it is an element. A mixture contains two or more substances, whereas a compound has elements chemically combined. Exam tip: A pure substance made of only one type of atom is an element.

The governing concept is the classification of matter according to composition. Sodium chloride, NaCl, is formed when sodium and chlorine combine chemically in a fixed 1:1 ratio of atoms. Its properties differ from those of both elemental sodium and elemental chlorine, and it can be decomposed only by a chemical process, not by simple physical separation. Therefore option D, compound, is correct. An element contains only one kind of atom, a mixture has variable composition and retains the components’ identities, and a colloid is a type of heterogeneous mixture. NaCl is not any of these alternatives.

Elements and compounds are pure substances because each has a fixed composition. An element contains only one kind of atom, whereas a compound contains elements chemically combined in a fixed ratio. A mixture has variable composition, so it is not the correct option. Exam tip: fixed composition is a key feature of a pure substance.

The components of a mixture are not chemically combined in a fixed ratio. Therefore, the nature of the components and their relative amounts can affect properties such as colour, density, and boiling point. A single atom alone does not determine the properties of a mixture. Exam tip: the composition of a mixture is variable, whereas a compound has a fixed composition.

When a compound forms a new substance forms. Therefore its properties can differ from constituent elements.

Water is formed by the chemical combination of hydrogen and oxygen, so it is a compound. Its elements are combined in a fixed ratio, and the properties of water differ from those of hydrogen and oxygen gases. In a mixture, components are combined without chemical bonding, so option A is not correct. Exam tip: A new substance formed through a chemical reaction is generally a compound.

A pure substance generally melts at a fixed temperature or over a very narrow temperature range, so its melting point is sharp and definite. In contrast, a mixture may melt over a wider range because of impurities or different components, and its melting point may also be lower. Option B is incorrect because melting point is not always zero. Exam tip: A sharp melting point is an important indication of purity.

Fixed melting point or boiling point can be useful to identify purity. This is a physical property.

The direct answer is C: salt and water form a physical mixture. In a mixture, substances are combined physically rather than forming one new substance in a fixed chemical ratio. Salt water can be separated because water and dissolved salt retain separable properties. Step by step, evaporation removes water as vapour and leaves salt behind; distillation boils and condenses water so that water is collected separately while salt remains in the flask. Option A is wrong because saying they are chemically combined would suggest formation of a compound with a fixed composition, which is not the normal description of salt water. Option B is wrong because whether substances are elements does not explain the separation; salt itself is a compound and water is also a compound, yet they can make a mixture. Option C is correct because physical mixtures can usually be separated by physical methods. Option D is meaningless and wrong because both salt and water are matter. Memory cue: if components can be separated by filtration, evaporation, distillation, or another physical method, suspect a mixture.

In a compound components are chemically bonded. Therefore physical separation generally does not work.

In a mixture components are physically mixed. Therefore methods like filtration and distillation work.

A suspension is a heterogeneous mixture containing relatively large particles. When left undisturbed, these particles can settle due to gravity, so option B is correct. In contrast, particles in a true solution are very small and do not settle on standing. Exam tip: settling of particles on standing is a key property of a suspension.

In a true solution, solute particles are extremely small, generally less than 1 nm in size. Hence, they cannot be seen with the naked eye and the solution appears transparent. The Tyndall effect is a property of colloids, not true solutions. Exam tip: particles of a true solution do not settle, are not visible, and do not show the Tyndall effect.

In a suspension, the dispersed particles are relatively large and remain distributed in the medium only temporarily. When left undisturbed, gravity can cause them to settle at the bottom; therefore, option A is correct. Colloidal particles generally do not settle in this way, whereas suspension particles do. Exam tip: sedimentation is a key property of suspensions.

Colloidal particles are very small and undergo continuous collisions with molecules of the dispersion medium. This Brownian motion helps them remain dispersed instead of settling under gravity. Small size alone is not the complete reason; continuous random motion also contributes to colloidal stability. Exam tip: Associate the non-settling nature of colloids with Brownian motion.

The components of a mixture are not chemically combined in a fixed ratio. In an alloy, the amounts of its constituents, such as metals or a metal and a non-metal, can be varied according to the required properties; for example, brass may contain copper and zinc in different proportions. Hence, an alloy is a mixture. A fixed composition is a property of a compound, not an alloy. Exam tip: Variable composition is a key feature of mixtures.

Sugar dissolves completely in water to form a single-phase mixture with uniform composition throughout; therefore, an aqueous sugar solution is homogeneous. In sand and water and wet soil, the components remain distinguishable, while oil and water form separate layers; these are heterogeneous mixtures. Exam tip: a homogeneous mixture has the same composition in every part of the sample.

Chemical classification is based on composition. Remember elements compounds mixtures solutions colloids and suspensions with examples.

Pure substances have fixed composition and mixtures can have variable composition. Classification becomes easy through examples.