Nature of matter

Tea contains water and dissolved substances so it is a mixture. Exam tip: beverages are often mixtures.

Milk contains water, tiny fat droplets, proteins and other substances, so it is a mixture—more specifically, a colloidal mixture. In a compound, constituents are chemically combined in a fixed ratio, whereas the components of milk remain physically mixed. Exam tip: remember milk as a mixture, not a pure substance.

Components of mixtures can generally be separated by physical methods. Exam tip: identify mixtures through separation methods.

A compound is formed when two or more elements combine chemically in a fixed ratio, so it has a fixed composition. Its components cannot be separated by simple physical methods; chemical change may be required. In contrast, the components of a mixture may be present in any ratio. Exam tip: a fixed ratio is a key feature of a compound.

Air is a mixture so it does not have one fixed formula. Exam tip: be careful with formulas of mixtures.

Hydrogen is an element, so it consists of only hydrogen atoms. Water is a compound because it contains both hydrogen and oxygen, whereas salt water and brass are mixtures. Exam tip: a pure substance made of only one kind of atom is an element.

Water is made of hydrogen and oxygen in a fixed ratio. Exam tip: fixed ratio indicates compound.

Wood contains many organic substances so it is a mixture. Exam tip: natural materials are often mixtures.

Paper may contain cellulose and other substances. Exam tip: think before treating daily objects as pure substances.

A mixture having uniform composition and properties throughout is called a homogeneous mixture. For example, an aqueous salt solution is generally homogeneous. Suspensions and colloids do not have completely uniform particle distribution, so they are heterogeneous. Exam tip: phrases such as “same throughout” or “uniform composition” indicate a homogeneous mixture.

Visible non-uniformity indicates a heterogeneous mixture. Exam tip: visible difference can guide the answer.

Iron is an element and hence a pure substance. Air is a homogeneous mixture of gases such as nitrogen, oxygen, and carbon dioxide. Therefore, iron followed by air is the correct order of pure substance and mixture. Option B contains the same materials but in the reverse order. Exam tip: check the required order as well as the classification of each material.

Lemonade contains substances such as water, lemon juice, sugar and sometimes salt mixed physically. Its components retain their properties and can be present in varying proportions, so it is a mixture. In a compound, elements are chemically combined in a fixed ratio, which is not the case in lemonade. Exam tip: a substance with variable composition is generally a mixture.

The governing concept is the chemical classification of matter, which is separate from its physical state. Oxygen gas exists mainly as O₂ molecules. Each molecule contains two atoms, but both atoms are of the same element, oxygen. A substance made of only one kind of atom is an element, so oxygen is correctly classified as an element even though it occurs as a diatomic molecule. It is not a compound because a compound must contain atoms of at least two different elements chemically combined in a fixed ratio, as in H₂O. It is not a mixture because a mixture contains more than one substance with variable composition. An alloy is generally a solid mixture involving metals. Therefore option A is correct.

Carbon dioxide, CO₂, is formed by the chemical combination of carbon and oxygen in a fixed proportion, so it is a compound. In a mixture, components are not chemically combined in a fixed ratio; therefore, option C is incorrect. Exam tip: a pure substance made of two or more elements chemically combined in a fixed ratio is a compound.

Sodium chloride (
NaCl
) is a compound formed by the chemical combination of sodium and chlorine. Its elements are present in a fixed ratio, and its properties differ from those of the constituent elements. In a homogeneous mixture, the composition can vary, but it cannot vary in NaCl. Exam tip: a substance with a fixed chemical formula and fixed composition is a compound.

Soil has many components such as minerals and organic matter. Exam tip: identify mixed natural materials.

A pure substance has a definite chemical composition and characteristic properties. This means that a sample taken from one part of the substance has the same types and relative amounts of particles as a sample taken from another part, as long as the substance has not been contaminated. Elements and compounds are examples of pure substances.

A mixture does not have one fixed composition in this sense; the proportions of its components can vary from sample to sample. A heterogeneous mixture is also non-uniform, so it cannot satisfy the stated condition. Therefore option A, pure substance, is correct. The description does not mean merely a clean-looking material, and it is not specifically a substance made only of dust.

Elements can be classified as metals non-metals and metalloids. Exam tip: this is based on chemical nature.

The governing concept is the classification of elements into metals and non-metals using their characteristic properties. Copper is a metal. It generally has metallic lustre, conducts heat and electricity efficiently, and is malleable and ductile, meaning it can be beaten into sheets and drawn into wires. These properties arise from the structure of metallic bonding, in which electrons can move through the material. Sulfur and oxygen are non-metals, while carbon is also classified as a non-metal even though some of its allotropes have unusual properties. The question asks which listed element belongs to the metallic class, not which one is useful or solid. Therefore option C, copper, is the only correct answer.

Oxygen is a non-metal. It generally gains electrons to form an anion and does not show metallic properties such as lustre, malleability, and electrical conductivity. Iron, silver, and sodium are metals; sodium is an especially reactive alkali metal. Exam tip: most elements on the right side of the periodic table are non-metals.

Silicon is a metalloid because it shows metal-like and non-metal-like properties. Exam tip: remember silicon and boron.

Metals generally have a shiny surface, a property called metallic lustre. Most metals also conduct electricity, so being a poor conductor is not a common metallic property. Brittleness is more commonly associated with non-metals. Exam tip: lustre, malleability and electrical conductivity are key properties of metals.

Metals generally contain mobile electrons in their structure. These electrons can move through the material when a potential difference is applied, so metals usually conduct electric current well. The particles in a metal can also transfer vibrational energy efficiently from one part to another, which gives metals good thermal conductivity.

Therefore, the standard general property asked here is that metals conduct both heat and electricity. Conductivity of sound is not the defining answer in this context, and colour and smell are not forms of energy conduction. Some exceptions exist for particular materials or conditions, but the broad school-level rule remains valid. Hence, option A is correct.

The governing concept is the usual physical behavior of non-metals. Most non-metals lack the freely moving electrons that allow metals to conduct electricity efficiently, so they are generally poor electrical conductors. Option C states this common property. Option A describes the behavior of most metals, not non-metals. Option B is also mainly associated with metals, since non-metals are often dull, although there are exceptions. Option D is incorrect because non-metals are not metals and may be solids, liquids, or gases at room conditions. Graphite, an allotrope of carbon, is a well-known exception that conducts electricity.