Nature of matter

Temperature can change the state of matter, and melting point and boiling point help us describe when important state changes occur. The melting point is the temperature at which a solid changes into a liquid under stated conditions. The boiling point is the temperature at which a liquid changes into a gas. These values show how a substance responds to heating and help identify its physical behaviour.

Option A is correct because both properties are directly connected with changes of state. Taste, name, and price do not tell us when a substance melts or boils. The word “only” in the other choices also makes them unsuitable, since those properties have no role in measuring state-change temperatures. Therefore, melting point and boiling point provide the useful scientific information.

Metals contain mobile electrons that transfer thermal energy rapidly from the hotter part of a utensil to its cooler parts. Therefore, thermal conductivity is the correct property. Brittleness describes a material's tendency to break, not its ability to transfer heat. Exam tip: For metal cooking utensils, associate the relevant property with thermal conductivity.

Copper is a very good conductor of electricity, so electric current can pass through it easily; therefore, it is used in electric wires. Thermal conductivity concerns the flow of heat, not electric current. Copper’s ductility also helps in drawing it into wires, but the main property for its electrical use is electrical conductivity. Exam tip: distinguish conductivity for carrying current from ductility for making wires.

The governing concept is electrical conductivity and insulation. Rubber has very high electrical resistance, so it is a poor conductor, or electrical insulator. When placed around a conducting wire, it reduces the chance that current will pass to a person or to another wire, thereby helping prevent electric shock and short circuits. Thus option A is correct. Metallic lustre is the shiny appearance typical of many metals and does not provide electrical safety. Combustibility would be an undesirable fire-related property, not the reason for covering wires. Ductility is the ability to be drawn into wires; rubber’s useful feature here is insulation, not metallic ductility.

The tendency of a substance to break when struck with a hammer, rather than change shape, is called brittleness. Hence, brittleness is correct. Malleable materials, such as most metals, can be hammered into sheets instead of breaking. Exam tip: breaking indicates brittleness, whereas being drawn into wires indicates ductility.

The shiny appearance of a substance is called lustre, which results from reflection of light from its surface. Therefore, option A is correct. Solubility is the ability to dissolve in a solvent, combustibility is the ability to burn, and acidity describes acidic nature; none of these describes surface shine. Exam tip: metallic lustre is a common physical property of metals.

A soluble substance can spread uniformly in a solvent. Exam tip: connect solution formation with solubility.

The governing concept is that a pure substance has characteristic, fixed physical constants under a specified pressure. A pure crystalline substance generally melts at a definite, narrow temperature, whereas impurities often lower and broaden the melting range. Therefore, option A is the scientifically useful property for checking identity and purity. A name may be incorrect or insufficient, and price has no necessary relation to composition. Packet colour is a commercial feature, not a dependable property of the material inside. In practice, melting point should be measured carefully and compared with a reliable reference; one test alone may not establish purity in every case, but it is far more meaningful than the other choices.

Physical and chemical properties are understood on different bases. Exam tip: identifying property type makes answers easier.

A pure substance has fixed composition while a mixture can have variable composition. Exam tip: remember fixed and variable.

Both elements and compounds are pure substances because each has a fixed composition. An element contains only one kind of atom, whereas a compound is formed by the chemical combination of two or more elements in a fixed ratio. Therefore, option B applies only to compounds. Exam tip: fixed composition is a key feature of a pure substance, while variable composition indicates a mixture.

An element has one kind of atoms while a compound has two or more elements. Exam tip: remember atoms and ratio.

The governing concept is the distinction between pure substances and mixtures. A pure substance has a uniform, fixed composition; it may be an element or a compound. Copper is an element, water in the intended pure form is the compound H₂O, and oxygen is an element, so every member of option A is a pure substance. Soil contains many solids, air contains several gases, and milk contains water, fats, proteins, and other components. Lemonade, smoke, sea water, and tea are also mixtures with variable compositions. Thus option A is the only complete list in which each item is pure. The classification assumes “water” means pure water, not tap water.

Air milk and soil are mixtures with different components. Exam tip: identify natural and daily materials carefully.

A homogeneous mixture has the same composition and properties throughout a sample, so one part appears like every other part. Salt solution is a familiar example when the salt is evenly dissolved. In a heterogeneous mixture, the composition is not uniform; different parts may contain different amounts or visibly different components. Sand mixed with water illustrates this difference because the particles and regions can be distinguished.

Option A is correct because it clearly states uniform composition for a homogeneous mixture and non-uniform composition for a heterogeneous mixture. A homogeneous mixture is not necessarily an element, because it may contain several substances. A heterogeneous mixture is not pure, and both types do contain components. The distinction is based on uniformity, not on whether matter has components.

The governing concept is the classification of mixtures by uniformity. In salt water, salt dissolves and its particles distribute essentially uniformly throughout the sample, so the mixture is homogeneous and appears as one phase. In sand water, sand does not dissolve; its particles remain separate, may settle, and make the composition non-uniform from one part to another. It is therefore a heterogeneous mixture. Option A correctly states this difference. Neither sample is a pure element or a single fixed compound, so options B and C are incorrect. Both are ordinarily liquid mixtures, not gases, which eliminates option D. Filtration can separate sand from water, whereas dissolved salt requires evaporation or another suitable method.

Iron is an element because it consists of only one kind of atom. Water is a compound because hydrogen and oxygen are chemically combined in a fixed proportion. In option B, the same substances are in the reverse order. Air, brass, sand, and milk are mixtures, so they do not form a correct element-compound pair. Exam tip: check both the classification of each substance and the order in which they are asked.

Direct answer: Option A, sodium chloride and air. A compound is a pure substance formed when elements combine chemically in a fixed ratio; sodium chloride, NaCl, contains sodium and chlorine chemically combined. A mixture contains two or more substances physically combined in variable amounts; air contains mainly nitrogen and oxygen along with smaller amounts of other gases. Thus the requested pair is compound first and mixture second, exactly as in A. B reverses them: air is the mixture and sodium chloride is the compound, so it does not fit the order asked. C gives copper and oxygen as elements, not a clearly identified compound-and-mixture pair; merely listing substances does not make the pair correct. D gives sulfur and carbon, also elements, not the required pair. A formula often suggests a compound, but the formula alone is not the whole rule; chemical combination and fixed composition matter. Memory cue: NaCl is a fixed compound; air is a variable mixture.

Sugar water is homogeneous and oil water is heterogeneous due to layers. Exam tip: read the order of pairs carefully.

Physical state and chemical class describe two different features of matter. Physical state tells us whether a substance is solid, liquid, or gas under stated conditions. Chemical class tells us whether it is an element, compound, or mixture, based on its composition. These two descriptions can be given together without contradiction.

Oxygen is a gas under ordinary conditions, so “gas” describes its physical state. It is made only of oxygen atoms and cannot be chemically broken into a simpler substance, so it is an element. Thus option A correctly shows both ideas separately. Soil is generally a mixture and is not always a gas. Water as a pure substance is a compound, though natural water may contain mixtures. Gold is an element, not a compound, so the other choices are incorrect.

The words liquid and compound describe two different aspects of water. Liquid is its physical state under suitable conditions: water flows and takes the shape of its container while retaining a definite volume. Compound describes its chemical composition. Pure water is made of hydrogen and oxygen chemically combined in a fixed ratio, represented by the formula \\(H_2O\\). Thus, one classification tells how the substance behaves physically, while the other tells what it is made of.

Option A is correct because it separates state from composition. Calling water a liquid does not mean it is a mixture, and calling it a compound does not mean it must be a gas. Water can be a liquid compound at room temperature, and the two descriptions are compatible rather than contradictory.

The governing concept is that matter can be classified in two different ways: by physical state and by composition. Air is called a gas because, at ordinary conditions, it has no fixed volume, fills its container, and is in the gaseous state. It is called a mixture because it contains several gases, mainly nitrogen and oxygen, along with smaller and variable amounts of argon, carbon dioxide, water vapour, and other components. These gases are not chemically combined in one fixed ratio, and air can be separated by physical methods. Hence option A gives both required reasons. Air is not a pure element, not a liquid compound, and it certainly has mass.

Direct answer: A, milk flows and contains many components. A substance can be classified in two independent ways: by physical state and by composition. Milk is a liquid because it flows and takes the shape of its container. It is also a mixture because it contains water, fats, proteins, sugars, minerals, and other substances; these components are not combined in one fixed chemical formula. Milk is commonly described as a colloidal mixture, but the school-level conclusion here is simply liquid plus mixture. Option A states both reasons. Option B is wrong because milk is not a single element; an element contains only one kind of atom. Option C is wrong because a compound has a definite composition and formula, unlike milk, whose composition can vary. Option D is wrong because milk is not a gas; it has a definite liquid form at ordinary conditions. Memory cue: classify matter by state and composition separately; milk is liquid by state and mixture by composition.

A pure compound is a substance formed when two or more different elements combine chemically in a definite, fixed proportion by mass. Its composition and properties remain uniform from sample to sample, and it can be separated into its elements only by a chemical process, not by simple physical methods. Therefore, option A gives the essential criterion: chemically joined elements in a fixed ratio. Hand-mixing produces a mixture, while visible layers indicate physical separation or immiscibility. A composition that changes from sample to sample is also a property of a variable mixture, not a pure compound. The governing idea is the law of definite proportions.

In a mixture components are physically mixed and ratio is not fixed. Exam tip: remember variable ratio.