Comprehensive theory, key formulas, diagrams, and memory aids for Is Matter Around Us Pure.
When you buy milk, ghee, or butter from the market, you often see the word "Pure" written on the packaging. For a common person, "pure" means having no adulteration. However, for a scientist, almost all of these things are actually mixtures of different substances and hence not pure. For example, milk is a mixture of water, fat, and proteins.
In this chapter, we will explore the scientific definition of purity and understand how matter is classified into pure substances and mixtures.
For a scientist, a pure substance consists of a single type of particles. All the constituent particles of that substance are the same in their chemical nature. Pure substances can be further classified into two types: Elements and Compounds.
An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements are normally divided into: * Metals: They have a lustre (shine), conduct heat and electricity, are ductile (can be drawn into wires), and malleable (can be hammered into sheets). Examples: Gold, Silver, Copper, Iron, Sodium. * Non-metals: They display a variety of colours, are poor conductors of heat and electricity, and are not lustrous, sonorous, or malleable. Examples: Hydrogen, Oxygen, Iodine, Carbon. * Metalloids: Elements having intermediate properties between those of metals and non-metals. Examples: Boron, Silicon, Germanium.
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion. For example, water ($H_2O$) is a compound made of hydrogen and oxygen. The properties of a compound are entirely different from those of its constituent elements. (Hydrogen is a combustible gas, oxygen supports combustion, but water is used to extinguish fires!). The constituents of a compound cannot be separated by physical methods; they require chemical or electrochemical reactions.
A mixture contains more than one pure substance mixed in any proportion. The substances in a mixture retain their individual chemical properties. Mixtures can be separated into their pure components by physical methods.
Depending upon the nature of the components that form a mixture, we have two types:
When we mix a solid into a liquid, the resulting mixture can behave in three different ways depending on the size of the solid particles.
A solution is a homogeneous mixture of two or more substances. * Components: It has a solvent (the component present in a larger amount that dissolves the other) and a solute (the component present in a lesser amount that is dissolved). * Properties: The particles are extremely small (less than 1 nanometer). Because they are so small, they do not scatter a beam of light passing through the solution (so the path of light is not visible). They do not settle down when left undisturbed; a solution is stable.
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. * Properties: The particles can be seen by the naked eye. They are large enough to scatter a beam of light passing through them, making its path visible (Tyndall effect). If left undisturbed, the particles settle down (it is unstable). Once they settle, the suspension breaks and no longer scatters light. Example: Chalk powder in water, muddy water.
A colloid is a heterogeneous mixture, but because the particles are relatively small, it appears to be homogeneous. * Properties: The particle size is between that of a true solution and a suspension. The particles cannot be seen with the naked eye, but they are large enough to scatter a beam of light (this scattering of light is called the Tyndall effect). They do not settle down when left undisturbed; a colloid is quite stable. * Examples: Milk, fog, smoke, shaving cream. * Components: The solute-like component is the dispersed phase, and the medium in which it is suspended is the dispersing medium.
Most natural substances are not chemically pure. Different physical methods are used to separate the individual components of a mixture.
Understanding the difference between pure substances (elements and compounds) and mixtures is fundamental to chemistry. While compounds have a fixed chemical formula and unique properties, mixtures are physical blends that retain the properties of their constituents. Depending on particle size, mixtures behave as true solutions, suspensions, or colloids. By exploiting the different physical properties of the components (like boiling point, density, or solubility), we can employ various separation techniques to purify matter.