Classification of Matter: Mixtures, Substances, Elements, and Compounds

Matter and Classification

As chemists, we study matter and classify it to talk about it easily. We split matter into two broad categories based on whether it can be simplified by physical means: mixtures and pure substances.

Mixtures

Mixtures are physical combinations of two or more pure substances. They can be separated by physical means to yield pure substances.

Pure Substances

Pure substances cannot be simplified by physical means. They are divided into elements and compounds.

Elements

Elements are the simplest substances; the atom is the smallest unit with the properties of the element. Elements are represented by elemental symbols, abbreviations of one or more letters, with exactly one capital letter in the symbol. The periodic table lists all elements; capitalization matters (e.g., Co vs CO).
Some symbols come from Latin/Greek roots: Fe for iron (ferrum), Ag for silver, Au for gold. Examples: C, S, I are straightforward single-letter symbols. Potassium is K, Calcium is Ca, Thorium is Th. Lead is Pb. Phosphorus is P (note: phosphate is not an element; it is a polyatomic ion).

Compounds and Formulas

A compound is a chemical combination of two or more elements in a fixed ratio, and it can be chemically separated into elements. Examples: $CO<em>2$ (carbon dioxide), $CO$ (carbon monoxide), $C</em>2H<em>6O$ (ethanol), $CH</em>4O$ or $CH<em>3OH$ (methanol). The identity of a compound is determined by which elements it contains and their ratio (e.g., $H</em>2O$ has 2 H for 1 O; $CO_2$ has 1 C for 2 O). Formulas encode element identity and their ratio; sometimes they also convey connectivity (structure), e.g., methanol shows how hydrogens are attached to carbon and oxygen.

Mixtures: Homogeneous vs Heterogeneous

Mixtures are also subdivided by uniformity:

• A homogeneous mixture has a uniform composition throughout (described mathematically). Examples: $ext{salt in water}$, $ext{wine}$, $ext{Kool-Aid}$.
• A heterogeneous mixture is not uniform throughout. Examples: salad dressing, trail mix, oil and water.

Quick Practice: Element vs Compound and Separation

Use the periodic table to decide if a substance is an element or not. Be careful with spelling and symbols:

• Arsenic – As (element)
• Carbon monoxide – CO (compound)
• Glucose – a compound (C$6$H${12}$O$_6$)
• Calcium – Ca (element)
• Phosphate – not an element (polyatomic ion; contains phosphorus as the element P)
• Phosphorus – P (element)
• Thorium – Th (element)
• Potassium – K (element)
• Iron – Fe (element)
• Lead – Pb (element)
• Cobalt – Co (element)

Connectivity and Separation Concepts

A description example: when a liquid passes through a filter and crystals form on the filter, the process is a physical separation, implying the substance started as a mixture.

Summary

Matter is classified by physical separability: mixtures vs pure substances (elements vs compounds). Elements have simple symbols and cannot be chemically simplified; compounds are defined by the elements present and their fixed ratios, often shown by formulas that may also reveal connectivity. Mixtures can be homogeneous or heterogeneous, with homogeneous mixtures being uniform throughout and easier to describe mathematically.