In our daily life we use many substances like salt, sugar, milk, air and water. Are all these substances pure? In everyday language, we call anything pure if it is clean and has no contamination. But in chemistry, the word "pure" has a very specific meaning. A pure substance is one that consists of a single kind of matter - that is, only one kind of particle - throughout. For example, pure water contains only water molecules and nothing else, whereas milk contains water, fat, proteins and many other substances, so it is not a pure substance in the chemical sense.
The classification of matter into pure substances and mixtures helps us understand its composition and properties. We also need to learn how to separate the components of mixtures, because most natural substances exist as mixtures. In this chapter we study the difference between pure substances and mixtures, the types of mixtures, solutions and their properties, and the various methods used to separate the components of a mixture.
Understanding whether a substance is pure or a mixture is important not only in chemistry laboratories but also in everyday situations such as purifying drinking water, separating salt from sea water and extracting pure metals from their ores.
Pure substance: A pure substance is a form of matter which has a fixed composition and definite properties. It is made up of only one kind of particle. Pure substances are of two types - elements and compounds.
Mixture: A mixture is a substance which contains two or more pure substances mixed together in any proportion. The components of a mixture retain their individual properties. Air, sea water, soil, milk and most of the food we eat are mixtures.
Mixtures can be separated into their components by physical methods, whereas pure substances cannot be broken down into simpler substances by physical methods. A mixture can be homogeneous or heterogeneous.
Homogeneous mixture: A mixture in which the components are uniformly mixed such that we cannot see the different components separately is called a homogeneous mixture. A solution of sugar in water is a homogeneous mixture because sugar and water cannot be seen separately. Air is a homogeneous mixture of gases.
Heterogeneous mixture: A mixture in which the components are not uniformly mixed and can be seen separately is called a heterogeneous mixture. A mixture of sand and iron filings, a mixture of oil and water, and a fruit salad are heterogeneous mixtures.
A mixture of two or more phases is heterogeneous. For example, a mixture of salt, iron filings, sulphur and sand has four distinct visible components, so it is a heterogeneous mixture.
Element: An element is a pure substance which cannot be split into simpler substances by chemical means. It is made up of only one kind of atom. Examples are hydrogen, oxygen, iron, gold, copper and carbon. There are more than 100 known elements.
Elements can be classified as metals, non-metals and metalloids. Metals such as iron, copper and gold are lustrous, malleable, ductile and good conductors of heat and electricity. Non-metals such as sulphur, carbon and oxygen are not lustrous and are poor conductors. Metalloids like silicon and germanium have properties of both metals and non-metals.
Compound: A compound is a pure substance made up of two or more elements combined chemically in a fixed proportion by mass. The properties of a compound are entirely different from the properties of its constituent elements. For example, water (H2O) is a compound of hydrogen and oxygen. Hydrogen is a combustible gas and oxygen supports combustion, but water is used to extinguish fire. Sodium (a reactive metal) and chlorine (a poisonous gas) combine to form sodium chloride (common salt), which we eat.
A compound is always homogeneous. Its constituents cannot be separated by physical methods but can be separated by chemical methods.
A mixture differs from a compound in several ways. In a mixture, the components are present in any proportion, whereas in a compound the elements combine in a fixed proportion by mass. The components of a mixture retain their individual properties, while a compound has properties different from its constituents. A mixture can be homogeneous or heterogeneous, but a compound is always homogeneous. Mixtures can be separated by physical methods, while compounds can be separated only by chemical methods.
For example, a mixture of iron filings and sulphur can be separated using a magnet, and the iron filings retain their magnetic property. But when iron and sulphur are heated together they form iron sulphide, a compound which is not attracted by a magnet and whose properties differ from both iron and sulphur.
A solution is a homogeneous mixture of two or more substances. In a solution, the substance that is dissolved is called the solute and the substance in which the solute is dissolved is called the solvent. For example, in a sugar solution, sugar is the solute and water is the solvent.
Solutions can have solutes and solvents in any physical state. A solution of common salt in water has a solid solute and a liquid solvent. In carbonated drinks, the gas carbon dioxide is dissolved in liquid water, so it is a gas-in-liquid solution. Air is a solution of gases. Alloys like brass (copper and zinc) are solid solutions.
Properties of a solution: A solution is a homogeneous mixture. The particles of a solution are too small to be seen with the naked eye and do not scatter a beam of light passing through it. The particles do not settle down when the solution is left undisturbed, and a solution cannot be separated by filtration. Solutions are stable, and the concentration of a solution can be expressed in terms of mass by mass percentage, mass by volume percentage, or volume by volume percentage.
Saturated solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution. The maximum amount of solute that can be dissolved in a given amount of solvent at a given temperature is called its solubility.
Unsaturated solution: A solution in which more solute can be dissolved at a given temperature is called an unsaturated solution.
Concentrated and dilute solutions: A solution containing a relatively large amount of solute is called a concentrated solution, while one containing a small amount of solute is called a dilute solution.
The concentration of a solution is the amount of solute present in a given amount of solution or solvent. It is often expressed as mass by mass percentage, which is calculated as (mass of solute / mass of solution) x 100. For example, a 20% solution of salt in water means 20 g of salt is dissolved in 80 g of water to give 100 g of solution.
Suspension: A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. For example, chalk powder in water, wheat flour in water, and muddy river water are suspensions. The particles of a suspension are visible to the naked eye, are larger than 100 nm, scatter a beam of light passing through the suspension, settle down when left undisturbed, and can be separated by filtration.
Colloidal solution (Colloid): A colloid is a heterogeneous mixture in which the particle size is intermediate between a true solution and a suspension, ranging from 1 nm to 100 nm. The particles of a colloid are not visible to the naked eye but can be seen under a powerful microscope. Colloids scatter a beam of light passing through them; this scattering of light is called the Tyndall effect. Milk, smoke, ink, cheese, butter and clouds are examples of colloids.
The components of a colloid are the dispersed phase and the dispersion medium. The dispersed phase is the substance whose particles are dispersed, and the dispersion medium is the substance in which the particles are dispersed. In a colloid, the dispersed phase particles do not settle down because they are constantly hit by the particles of the dispersion medium.
Different methods are used to separate the components of a mixture based on the difference in their physical properties.
Filtration: This method is used to separate insoluble solid particles from a liquid. A filter paper or a porous membrane is used. For example, chalk powder can be separated from water by filtration.
Evaporation: This method is used to separate a solid that has dissolved in a liquid. The liquid evaporates on heating, leaving behind the solid. Common salt is obtained from sea water by evaporation.
Crystallisation: Crystallisation is the process of obtaining pure crystals of a solid from its saturated solution by cooling the solution slowly. Impurities remain in the mother liquor. It is used to purify salt, sugar and alum.
Sublimation: Sublimation is used to separate a mixture of a sublimable solid (like ammonium chloride, camphor or iodine) and a non-sublimable solid (like sand or salt). On heating, the sublimable substance changes directly into vapour and condenses back into solid on a cool surface.
Centrifugation: Centrifugation is used to separate the components of a mixture where the solid particles are heavier than the liquid, such as cream from milk or butter from curd. The mixture is spun rapidly so that the heavier particles settle down.
Decantation: Decantation is the process of separating a liquid from the settled solid by carefully pouring the liquid out. A mixture of oil and water can be separated using a separating funnel, where the denser liquid settles below and is drained out.
Distillation: Distillation is used to separate a mixture of two miscible liquids that have a large difference in their boiling points. The mixture is heated; the liquid with the lower boiling point vaporises first, is condensed and collected, leaving behind the other liquid. Fractional distillation is used when the boiling points differ by a small amount and is used to separate the components of crude oil and gases from air.
Chromatography: Chromatography is used to separate and identify the coloured components in a mixture. It works on the principle of difference in the adsorption of different components on a stationary medium. It is used to separate dyes in ink and pigments in flowers.
A change in which no new substance is formed and the composition of the substance remains the same is called a physical change. Melting of ice, boiling of water and stretching of a rubber band are physical changes. Physical changes are generally reversible, and no new substance is formed.
A change in which one or more new substances with different properties are formed is called a chemical change. Burning of a candle, rusting of iron, curdling of milk and digestion of food are chemical changes. Chemical changes are irreversible and are accompanied by changes in energy, colour, odour, or the evolution of a gas.
When we burn a magnesium ribbon in air, it forms a white powder of magnesium oxide. This is a chemical change because a new substance is formed. The composition and properties of the original substances change.
| Type | Particle Size | Visible? | Tyndall Effect | Settle on Standing? | Separation |
|---|---|---|---|---|---|
| True Solution | Less than 1 nm | No | No | No | Cannot be filtered |
| Colloid | 1 nm to 100 nm | No (naked eye) | Yes | No | Not by filtration |
| Suspension | Larger than 100 nm | Yes | Yes | Yes | By filtration |
| Mixture | Compound |
|---|---|
| Components in any proportion | Fixed proportion by mass |
| Components retain individual properties | New properties different from constituents |
| Can be homogeneous or heterogeneous | Always homogeneous |
| Separated by physical methods | Separated by chemical methods |
In this chapter we learned the chemical meaning of a pure substance and classified matter into pure substances (elements and compounds) and mixtures. We understood the differences between homogeneous and heterogeneous mixtures, and studied solutions, suspensions and colloids in detail, including the Tyndall effect and the concept of concentration. We then examined a range of separation techniques - filtration, evaporation, crystallisation, sublimation, centrifugation, decantation, distillation and chromatography - and when each is used. Finally, we distinguished physical changes from chemical changes. This knowledge of the composition and classification of matter is essential not only for chemistry but also for everyday processes like purifying water, obtaining salt and separating useful materials from mixtures.