Penerangan
Pure and Impure Substances
In science, matter is classified into pure substances and impure substances, which are commonly known as mixtures. A pure substance consists of only one type of constituent particle, which can be either individual atoms or identical molecules. Because of this uniform composition, a pure substance has a fixed set of physical and chemical properties, such as a sharp melting point and a specific boiling point under standard conditions. Examples of pure substances include elemental gold, pure water, and carbon dioxide gas.
In contrast, an impure substance or mixture contains two or more different elements or compounds that are physically combined rather than chemically bonded. Because the components are not chemically joined, the proportions of the substances in a mixture can vary infinitely, and each component retains its original chemical properties. The components of a mixture do not possess a fixed melting or boiling point; instead, mixtures melt or boil over a temperature range. Matter can be thoroughly understood by examining how these particles are organized, allowing scientists to separate mixtures using physical techniques while chemical methods are required to break down pure compounds.
Elements and Compounds
An element is the simplest chemical form of matter that cannot be broken down into simpler substances by ordinary chemical or physical methods. Elements are made of only one kind of atom. Atoms are the basic building blocks of matter, and while some elements exist as single independent atoms, such as helium and neon, many exist as molecules containing two or more identical atoms chemically bound together, such as oxygen gas or hydrogen gas. Every element is represented by a unique chemical symbol derived from its English or Latin name.
A compound is a pure substance formed when two or more different elements combine chemically in a fixed proportion by mass. When elements react to form a compound, their atoms form chemical bonds, resulting in a completely new substance with properties entirely different from those of its constituent elements. For example, hydrogen is an explosive flammable gas and oxygen supports combustion, but when they combine in a fixed mass ratio of 1:8 to form water, the resulting compound is a liquid that extinguishes fire. Compounds are represented by chemical formulas that indicate the types of atoms and their exact numerical ratios, such as sodium chloride or water.
Homogeneous and Heterogeneous Mixtures
Mixtures are broadly categorized into two main types based on the uniformity of their composition throughout the sample. A homogeneous mixture has a uniform composition and appearance throughout every part of the system. The constituent particles are distributed evenly at the molecular or atomic level, meaning any sample taken from the mixture will have identical properties and proportions. Examples include a solution of table salt dissolved in water, clean air, and steel.
A heterogeneous mixture lacks uniformity, consisting of two or more distinct phases or regions that have different properties and compositions. The individual components can often be seen with the naked eye or under a microscope, and their distribution is non-uniform. Examples include a mixture of sand and iron filings, oil mixed with water, and chalk powder suspended in water. The boundary between the phases in a heterogeneous mixture makes it possible to physically distinguish the separate components without sophisticated equipment.
Solutions, Suspensions, and Colloids
Liquid and fluid mixtures are further classified into three categories based on particle size, stability, and optical properties: solutions, suspensions, and colloids. A solution is a homogeneous mixture where solute particles are dissolved completely within a solvent. The dissolved particles are smaller than one nanometre in diameter, meaning they cannot be seen with a microscope, will never settle out over time, and pass straight through filter paper. Light passes through a true solution without scattering, an effect known as the absence of the Tyndall effect.
A suspension is a heterogeneous mixture in which visible solute-sized particles are dispersed throughout a fluid without dissolving. The particle size in a suspension is greater than one hundred nanometres. These large particles will eventually settle out under gravity if the mixture is left undisturbed, and they can be completely retained by filter paper. Suspensions often scatter light, making them appear cloudy or opaque.
A colloid is a mixture that appears homogeneous to the naked eye but is actually heterogeneous at the microscopic level. The dispersed particles range in size from one to one hundred nanometres, which is large enough to scatter light beams—exhibiting a pronounced Tyndall effect—but small enough not to settle out under gravity. Colloidal particles pass through ordinary filter paper. Examples of colloids include milk, fog, and smoke, where the dispersed phase can be solid, liquid, or gas distributed within a continuous dispersion medium.
Alloys as Mixtures
An alloy is a solid homogeneous mixture of two or more metals, or a metal and a non-metal, prepared by melting the components together and allowing the mixture to solidify. Although alloys are solid solutions, they are classified as mixtures because their constituent elements retain their individual chemical properties and their proportions can be varied within certain limits. Pure metals often lack the structural strength, hardness, or corrosion resistance required for industrial applications. By introducing different atoms into the metallic crystal lattice, alloys disrupt the orderly layers of metal atoms, preventing them from sliding past one another easily. This structural modification increases the overall hardness and tensile strength of the material.
Brass is a well-known alloy made from copper and zinc, combining high malleability with resistance to corrosion. Bronze is an alloy of copper and tin used historically for tools and statues. Steel, an alloy of iron with small amounts of carbon and other transition metals, provides immense structural integrity for construction. Dental amalgams are liquid-solid alloy mixtures involving mercury. Because alloys do not have a fixed chemical formula, their exact composition is tailored to achieve specific mechanical and thermal properties.
Physical and Chemical Changes
Matter undergoes constant transformation, which is classified into physical and chemical changes. A physical change is a transformation in which no new chemical substance is formed. The chemical identity of the particles remains completely unaltered, and only physical properties such as state, shape, size, or temperature are modified. Most physical changes are easily reversible by simple physical means. Examples include melting ice, dissolving sugar in water, magnetising an iron nail, and breaking a glass rod. At the particle level, the spacing or arrangement of particles changes, but the molecules themselves remain identical.
A chemical change, also referred to as a chemical reaction, results in the formation of one or more entirely new substances with different chemical properties and compositions. During a chemical change, existing chemical bonds within reactant particles are broken, atoms are rearranged, and new chemical bonds are formed to create products. Chemical changes are generally permanent and difficult to reverse. Indicators of a chemical change include a permanent colour change, evolution of a gas, release or absorption of heat and light energy, or formation of a precipitate. Examples include the rusting of iron, burning magnesium ribbon, digestion of food, and souring of milk.
