Matter is everything around us that has mass and occupies space. Scientists classify matter based on its composition — what it is made of. The three fundamental categories are elements, compounds, and mixtures.
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Key Concepts and Definitions
Element: A pure substance that cannot be broken down into simpler substances by chemical means. There are 118 known elements. Examples: oxygen (O), iron (Fe), gold (Au), carbon (C).
Compound: A pure substance formed when two or more elements combine chemically in a fixed ratio. The properties of a compound are entirely different from its constituent elements. Examples: water (H2O), common salt (NaCl), carbon dioxide (CO2).
Mixture: A combination of two or more substances that are NOT chemically combined. The components retain their individual properties and can be separated by physical methods. Mixtures can be homogeneous (uniform throughout, e.g. saltwater) or heterogeneous (non-uniform, e.g. soil).
- Key Differences:
- Elements and compounds are pure substances; mixtures are impure.
- Compounds have a fixed composition; mixtures have variable composition.
- Compounds can only be separated by chemical methods; mixtures by physical methods.
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Worked Examples
Is air an element, compound, or mixture?
Air contains nitrogen (~78%), oxygen (~21%), argon, carbon dioxide, and other gases in variable amounts. Since the composition varies and components retain individual properties, air is a mixture (specifically a homogeneous mixture).
Is water an element, compound, or mixture?
Water is made of hydrogen and oxygen combined in a fixed ratio of 2:1 (H2O). It has properties completely different from hydrogen and oxygen. Therefore water is a compound.
Classify table salt, sand, and a salt-sand mix.
Table salt (NaCl) — compound. Sand (SiO2) — compound. A salt-sand mix — heterogeneous mixture (the two retain their properties and can be separated by sieving or dissolving in water).
Identify the type of mixture — sugar dissolved in water vs. oil and water shaken together.
Sugar in water dissolves uniformly — homogeneous mixture (solution). Oil and water form visible layers — heterogeneous mixture.
Why cannot a compound be separated by physical methods?
In a compound, elements are chemically bonded. Breaking chemical bonds requires a chemical reaction (e.g., electrolysis separates water into H2 and O2), not a simple physical process like filtering or heating.
Iron filings and sulfur powder are mixed. Is this a compound or a mixture?
Before heating, the iron and sulfur are simply mixed — a heterogeneous mixture. A magnet can pull out iron filings. When heated strongly, they react to form iron sulfide (FeS), a new compound with properties different from both iron and sulfur.
Give three examples each of elements, compounds, and mixtures from daily life.
Elements: oxygen (in air), copper (coins/wires), silicon (chips). Compounds: water (H2O), chalk (CaCO3), sugar (C12H22O11). Mixtures: seawater, milk, brass (copper + zinc alloy).
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Key Formulas / Facts
- Total known elements: 118 (as per IUPAC)
- Homogeneous mixture = uniform composition throughout (also called a solution)
- Heterogeneous mixture = visible distinct phases or components
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Common mistakes
> Students often confuse alloys (like brass, bronze, steel) with compounds. Alloys are mixtures of metals because their composition is variable and components retain properties. Also, do not assume that if something looks uniform it must be a compound — a saltwater solution looks uniform but is a mixture.
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Summary
Matter is classified as elements (single type of atom, cannot be broken further), compounds (two or more elements chemically combined in fixed ratio, new properties), or mixtures (two or more substances physically combined, variable composition, separable by physical means). Mixtures are further divided into homogeneous and heterogeneous.