Q.Which are mixtures and pure substances from the following ? i. sea water ii. gasoline iii. skin iv. a rusty nail v. a page of the textbook. vi. diamond

A pure substance containing only one type of atom is classified as an element; this is a statement about chemical composition, not physical state.

Pure substances are classified into elements and compounds based on their composition:

• An element is a form of matter that cannot be broken down into simpler substances by chemical means and consists of only one kind of atom (e.g. Na, Cu, O2, S8 — all atoms of the same type, though they may combine as molecules of that one element).
• A compound is formed when two or more different elements combine chemically in a fixed ratio (e.g. H2O, NaCl).
• 'Solid' and 'liquid' are physical states of matter and say nothing about how many kinds of atoms are present — a solid or liquid can be an element (solid Fe, liquid Br2) or a compound (solid NaCl, liquid H2O).

Since the question specifically describes 'only one type of atoms', this is the defining property of an element.

Graphite consists only of carbon atoms bonded together (it is an allotrope of the element carbon), so it is indeed an element, and this is precisely because it satisfies the definition given in the reason.

Assertion (A): Graphite is a form (allotrope) of carbon, containing only carbon atoms — no other element is combined in it. So graphite is correctly classed as an element (specifically, the element carbon in one of its structural forms).

Reason (R): An element is defined as the purest form of a substance made of only one kind of atom. This is the standard, correct definition, and it is exactly why graphite — being made of only carbon atoms — qualifies as an element.

A mixture is a physical (not chemical) blend of two or more substances in any proportion; each substance keeps its own identity and can be separated by physical means.

Unlike a compound, where elements combine chemically in a fixed ratio to form a new substance, in a mixture the components are only physically mingled — no new bonds form, and the ratio of components can vary. Examples: air (a mixture of N2, O2, CO2, etc.), salt water, and sand + iron filings. Mixtures can be homogeneous (uniform composition, like sugar solution) or heterogeneous (non-uniform, like sand and water).