Nature of matter

Correct answer: A. Two or more elements combined in fixed ratio

In a compound elements are chemically combined in a fixed ratio. Exam tip: focus on fixed ratio.

Correct answer: B. Compound

Pure water is a compound because hydrogen and oxygen are chemically combined in a fixed ratio. Its formula is \(H_2O\). An element contains only one type of atom, so water is not an element. Exam tip: a pure substance made of two or more elements in a fixed ratio is a compound.

Correct answer: C. Iron

Iron is an element because it consists of only one kind of atom and cannot be broken into simpler substances by chemical methods. Water and salt are compounds, whereas lemonade is a mixture. Exam tip: Pure names of metals such as iron, copper, and gold are generally elements.

Correct answer: B. In any ratio

Components in a mixture can be present in any ratio. Exam tip: connect variable composition with mixtures.

Correct answer: C. Mixture

Salt and sand do not chemically combine so a mixture forms. Exam tip: separate physical mixing from chemical combination.

Correct answer: A. Uniform composition throughout

A homogeneous mixture has the same composition and properties throughout a sample, at least at the scale being considered. Its components are distributed uniformly, so one portion has the same relative composition as another portion. A salt solution is a common example: after the salt dissolves completely, the dissolved salt and water are evenly distributed.

Therefore, option A, uniform composition throughout, is correct. Components of a homogeneous mixture are not usually visible separately, so the option saying they are easily seen describes a heterogeneous mixture more closely. A homogeneous mixture need not be solid; it may be solid, liquid, or gas. Uniformity, not colour or physical state, is the defining feature.

Correct answer: B. It has different compositions in different parts.

A heterogeneous mixture has a non-uniform composition, so different portions of the sample may contain different amounts or kinds of its components. For example, in sand mixed with water, the sand and water can be distinguished and a sample taken from one region may not have the same composition as a sample from another. Therefore option B is correct. A completely uniform composition describes a homogeneous mixture, not a heterogeneous one. A mixture may contain several elements or compounds, so it is not limited to one element. Also, mixtures do not have a fixed chemical formula because their component proportions can vary; a fixed formula is characteristic of a pure compound.

Correct answer: B. Homogeneous mixture

Salt dissolves uniformly in water so the solution is homogeneous. Exam tip: solutions are generally homogeneous.

Correct answer: B. Heterogeneous mixture

Oil and water form separate layers so the mixture is heterogeneous. Exam tip: layer formation indicates non-uniformity.

Correct answer: A. Homogeneous mixture

Sugar dissolves and distributes uniformly throughout water, so no separate components are visible. Therefore, sugar solution is a homogeneous mixture. In a suspension, particles may be visible and can settle on standing; this does not happen in a true sugar solution. Exam tip: a mixture with uniform composition in every part is homogeneous.

Correct answer: B. Heterogeneous

Sand does not dissolve uniformly in water so the mixture is heterogeneous. Exam tip: identify insoluble solids.

Correct answer: C. Alloy

Brass is an alloy made mainly by mixing copper and zinc. An alloy is a homogeneous mixture of metals, or of a metal with another element, so brass is not a compound. A pure metal contains only one metallic element, whereas brass contains both copper and zinc. Exam tip: remember brass as copper + zinc and bronze as copper + tin.

Correct answer: A. Alloy

Bronze is a metallic alloy, usually made by combining copper and tin; therefore, alloy is the correct category. It is not a single element because it contains more than one metal, and it is neither a pure non-metal nor a gaseous compound. Exam tip: remember bronze = copper + tin.

Correct answer: B. Homogeneous mixture

An alloy is a mixture of two or more elements, usually including metals. Its components are generally distributed uniformly throughout the material, so it is considered a homogeneous mixture. Unlike a compound, an alloy does not have a fixed composition. Exam tip: classify brass and bronze as mixtures, not compounds.

Correct answer: D. Soil

Soil contains different particles and substances. Exam tip: non-uniform materials are heterogeneous.

Correct answer: B. Elements and compounds

The governing concept is the classification of matter according to composition. A pure substance has a fixed, uniform composition and consists of only one kind of chemical substance. Pure substances are divided into elements and compounds. An element contains only one type of atom and cannot be broken into simpler substances by ordinary chemical methods; a compound contains two or more elements chemically combined in a fixed ratio and can be decomposed chemically. Therefore option B is correct. Solid and gas in option A are physical states, not composition classes. Homogeneous and heterogeneous usually describe mixtures, while acids and metals are not complementary classes of all pure substances. Mixtures are excluded because their composition can vary.

Correct answer: C. Homogeneous and heterogeneous

Mixtures can be homogeneous or heterogeneous. Exam tip: classify mixtures by uniformity of composition.

Correct answer: B. Element

A substance made of one kind of atoms is called an element. Exam tip: focus on the type of atoms.

Correct answer: C. Compound

The governing concept is the distinction between chemical combination and physical mixing. When two or more elements combine chemically in a definite proportion, their atoms form a new pure substance with properties different from those of the original elements. Such a substance is called a compound, so option C is correct. For example, hydrogen and oxygen combine chemically to form water, whose properties differ from both gases. A mixture in option A contains substances physically combined in variable proportions and can often be separated by physical methods. A solution in option D is a type of homogeneous mixture, not necessarily a chemically formed substance. Metal in option B describes a broad class of elements, not the product of chemical combination of two elements.

Correct answer: C. Mixture

The governing concept is classification by composition. Sea water contains water together with dissolved salts and other substances, and the amounts of these components can vary from place to place. Because its composition is not fixed and its components are not chemically combined into one new pure substance, sea water is considered a mixture. More specifically, the dissolved portion is generally described as a homogeneous solution when the sample is uniform. Therefore option C is correct. It is not an element, because it contains more than one kind of substance. It is not a pure compound, because the salt and water can be separated physically and occur in variable proportions. Option D is unrelated because sea water is not a metal.

Correct answer: A. Pure substance

Distilled water has very few impurities and is treated as pure water. Exam tip: consider distilled water as a pure substance.

Correct answer: C. Heterogeneous mixture

The direct answer is C: a heterogeneous mixture. Blood contains plasma, red blood cells, white blood cells, platelets, dissolved substances, and other components. These components are not one single pure substance and are not chemically bonded in one fixed formula. Blood is therefore a mixture. In school classification, whole blood is generally treated as heterogeneous because its cellular components are dispersed in plasma and can be separated or observed as different parts; its composition is not uniform at the microscopic level. Option A is wrong because an element contains only one kind of atom, whereas blood has many substances. Option B is wrong because a compound has elements chemically combined in a fixed ratio, which blood does not have as a whole. Option D is wrong because blood is a liquid containing several components, not a pure gas. Memory cue: blood is not one formula; many components together make it a mixture.

Correct answer: D. Air

The governing concept is the classification of matter into elements, compounds, and mixtures. An element contains only one kind of atom and cannot be separated into simpler substances by ordinary chemical methods. Sodium, sulfur, and carbon each consist of atoms of one chemical element, so they are elements even though their physical forms differ. Air, however, is a homogeneous mixture of several gases, mainly nitrogen and oxygen, along with smaller amounts of other gases. Its composition can vary, and its components can be separated by physical methods. Therefore, option D is correct; the other three are genuine elements.

Correct answer: D. Gold

Gold is an element because it contains only gold atoms. In contrast, water contains hydrogen and oxygen, carbon dioxide contains carbon and oxygen, and sodium chloride contains sodium and chlorine chemically combined; therefore, they are compounds. Exam tip: a substance made of only one type of atom is an element.

Correct answer: A. Uniformly distributed

A solution is a homogeneous mixture with uniformly distributed solute. Exam tip: remember uniformity of solution.