Elements, Compounds & Mixtures
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
Pure Substances and Mixtures All matter can be classified as either pure substances (elements or compounds) or mixtures. Understanding the differences between these is fundamental to chemistry. Pure Substance A substance that contains only one type of element or compound, with no other substances mixed in.
It has a fixed composition and specific properties. Elements Elements are the simplest type of pure substance and cannot be broken down into simpler substances. Element A substance made of only one type of atom.
Elements cannot be broken down into simpler substances by chemical means. All atoms in an element have the same atomic number (same number of protons) There are about 118 known elements About 90 elements occur naturally on Earth Elements are listed in the Periodic Table Each element has a unique chemical symbol (e.g., H for hydrogen, O for oxygen) Examples of Elements Metals: Iron (Fe), Copper (Cu), Gold (Au), Sodium (Na) Non-metals: Oxygen (O₂), Carbon (C), Sulfur (S), Nitrogen (N₂) Metalloids: Silicon (Si), Germanium (Ge) Noble gases: Helium (He), Neon (Ne), Argon (Ar) Important Some elements exist as molecules in their natural state (e.g., O₂, N₂, Cl₂), but they are still elements because they contain only one type of atom.
Compounds Compounds are pure substances formed when two or more different elements chemically combine. Compound A substance made of two or more different elements chemically bonded together in fixed proportions.
Compounds have different properties from the elements they contain. Atoms of different elements are chemically bonded together The ratio of atoms is fixed (e.g., water is always H₂O - 2 hydrogen to 1 oxygen) Can only be separated into elements by chemical reactions, not physical methods Properties are completely different from the elements that form them Represented by chemical formulas (e.g., H₂O, CO₂, NaCl) Examples of Compounds Water (H₂O): Hydrogen and oxygen chemically combined Carbon dioxide (CO₂): Carbon and oxygen chemically combined Sodium chloride (NaCl): Sodium and chlorine chemically combined (table salt) Methane (CH₄): Carbon and hydrogen chemically combined Ammonia (NH₃): Nitrogen and hydrogen chemically combined Key Concept Compounds have completely different properties from their constituent elements.
For example, sodium (a reactive metal) and chlorine (a poisonous gas) combine to form sodium chloride (harmless table salt). Mixtures Mixtures contain two or more substances that are not chemically bonded together.
Mixture Two or more substances (elements or compounds) physically combined but not chemically bonded. The components retain their individual properties and can be separated by physical methods. Components are not chemically bonded together Can be separated by physical methods (filtration, distillation, chromatography, etc.) Components keep their own properties Composition can vary (no fixed ratio) No chemical reaction occurs when forming a mixture Types of Mixtures Homogeneous Mixtures (Solutions) Uniform composition throughout - all parts look the same.
Salt dissolved in water Air (mixture of gases) Sugar dissolved in tea Alloys like brass (copper and zinc) Heterogeneous Mixtures Non-uniform composition - different parts can be distinguished. Sand and water Oil and water Salad Rocks and minerals Comparing Elements, Compounds, and Mixtures Property Element Compound Mixture Composition One type of atom only Two or more elements chemically bonded Two or more substances physically combined Bonding Atoms may be bonded to each other (e.g., O₂) Chemical bonds between different atoms No chemical bonds between components Separation Cannot be separated (it's pure) Can only be separated by chemical reactions Can be separated by physical methods Ratio of components N/A (only one type) Fixed ratio Variable ratio Properties Unique to that element Different from constituent elements Components keep their own properties Examples O₂, Fe, C, He H₂O, CO₂, NaCl Air, seawater, soil Methods of Separating Mixtures Because mixtures are not chemically bonded, they can be separated using physical methods: 1.
Filtration Separates insoluble solids from liquids. Example: Separating sand from water The liquid passes through filter paper; solid particles are trapped 2. Evaporation/Crystallization Separates a dissolved solid from a solution.
Example: Obtaining salt from salt water Heat the solution until water evaporates, leaving solid crystals 3. Distillation Separates liquids with different boiling points. Example: Separating water from salt water (to get pure water) Heat the mixture, collect and condense the vapor 4.
Chromatography Separates dissolved substances based on how they move through a medium. Example: Separating dyes in ink Different substances travel at different rates 5. Magnetic Separation Separates magnetic materials from non-magnetic materials.
Example: Separating iron filings from sulfur Use a magnet to attract magnetic substances Exam Tip Remember: mi…
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