Some Basic Concepts of Chemistry: Matter, Pure Substances & Mixtures: 18 MCQs with answers

C · Anything that occupies space and has mass

Matter is defined as anything that occupies space and has mass. Visibility, conductivity and fixed composition are not part of the definition.

B · Physics, biology and earth sciences depend on chemical knowledge

Chemistry sits at the centre because other sciences — physics, biology, applied sciences, earth and space sciences — all rely on chemical knowledge.

B · A pure substance

A pure substance has a definite chemical composition and therefore definite properties, the same regardless of origin.

A · Copper

Copper contains only one kind of atom, so it is an element. Water and glucose are compounds; salt water is a mixture.

A · Elements and compounds

Pure substances divide into elements and compounds. Homogeneous/heterogeneous are types of mixtures; metals/nonmetals are types of elements.

A · True

A pure substance has a definite chemical composition and fixed properties regardless of its origin, whereas a mixture has no definite composition and no definite properties.

A · A homogeneous mixture

Salt water is uniform throughout, so it is a homogeneous mixture. The salt and water are not chemically combined.

C · Heterogeneous mixture

A heterogeneous mixture is visibly non-uniform, with different parts having different compositions — exactly what sand and iron filings show.

D · Its ingredients are not chemically combined

Physical separation works on mixtures because the ingredients are not chemically joined. Compounds need chemical methods because their elements are bonded.

B · Water is a compound whose properties differ from its elements

A compound's properties differ from those of its constituent elements because the elements are chemically combined in a fixed ratio, not merely blended.

B · Incorrect — milk is a mixture, not a pure substance

Milk is a mixture. Uniform appearance does not make something a pure substance; the chemical test is fixed composition, which milk does not have.

B · They show properties of both metals and nonmetals

Metalloids are elements that show some properties of metals and some of nonmetals, which is why they form a third branch of elements.

C · Glucose

Glucose is a compound — a pure substance of two or more elements chemically combined in a fixed ratio. Gold is an element; air and tap water are mixtures.

A · Both A and R are true, and R correctly explains A

A compound is a pure substance precisely because it has a definite chemical composition — the reason directly explains the assertion.

C · It is a pure substance

Fixed properties regardless of origin indicate a pure substance. It could be an element or a compound — the evidence does not yet decide which.

A · I and III only

Mixtures have no definite composition and hence no definite properties, so II is false. Their ingredients are not chemically combined, so physical separation works — I and III are correct.

B · Homogeneous mixture

The correct classification is a homogeneous mixture because it is uniform throughout, but the varying proportions indicate it is not a pure substance or a compound.

C · The sample must have a fixed composition to be classified as pure.

The flaw in the student's reasoning is that a pure substance must have a fixed composition. The presence of dissolved salts means the sample is a homogeneous mixture, not a pure substance.