Element, Compound or Mixture

• Definition of a Compound: A compound is a pure substance formed when two or more different elements are chemically bonded together in fixed proportions. Unlike mixtures, the elements in a compound lose their original properties and form a new substance with unique chemical and physical characteristics.

• Fixed Composition: Compounds have a definite chemical formula, such as $H_2O$ or $CO_2$, meaning the ratio of atoms is always the same. This fixed ratio is maintained by strong chemical bonds that cannot be broken by physical processes like filtration or evaporation.

• Separation of Compounds: To separate a compound back into its constituent elements, a chemical reaction or energy input (like electrolysis) is required. Physical sorting is impossible because the particles are bonded at the molecular level.

Comparison Table

• Purity Indicators: Pure substances (elements and compounds) have sharp, specific melting and boiling points. Mixtures, being impure, typically melt and boil over a range of temperatures due to the different boiling points of their components.

• Interpreting Particle Diagrams: Always look for variety in the particles. If every particle (atom or molecule) is identical, the substance is pure. If there are two or more different types of particles not touching/bonded, it is a mixture.

• Melting Point Data: If an exam question provides a temperature range (e.g., "melts from $105$ to $112$ degrees Celsius"), immediately identify the substance as impure or a mixture. A single value (e.g., "boils at exactly $100$ degrees Celsius") indicates a pure substance.

• Vocabulary Precision: Avoid the term "pure" in a conversational sense. In chemistry, "pure" means a single chemical species. Tap water is a mixture, even if it is safe to drink, because it contains dissolved minerals.

• The 'Natural' Misconception: Students often think 'pure' means 'natural' or 'unprocessed'. In science, a highly processed chemical like distilled water is 'pure', while naturally occurring air is a complex 'mixture' of nitrogen, oxygen, and other gases.

• Physical vs Chemical Change: A common error is thinking that dissolving a substance creates a compound. Dissolving (like sugar in tea) is a physical change forming a mixture; no new chemical bonds are created, and the sugar can be recovered by evaporation.

• Mixtures of Compounds: Remember that a mixture isn't just different elements. It can be a mixture of two compounds (like oil and water) or an element and a compound (like oxygen dissolved in water).