iso-

Quick Definition

Meaning: Equal, same, or uniform

Origin: Greek (isos = equal)

Pronunciation: EYE-soh

Why It Matters

The prefix iso- is deceptively simple: it means 'equal' or 'same.' But in chemistry, it carries a specific meaning that trips up students constantly. In organic chemistry, iso- refers to structural isomers — molecules with the same molecular formula but different arrangements. Isobutane is not 'equal butane'; it's a branched isomer of butane. This is different from the general concept of isotopes, which are atoms of the same element with different neutron counts. The key insight: iso- signals a comparison. Something is the same as something else in one respect, but different in another. In physical chemistry, isothermal means constant temperature, isobaric means constant pressure, and isochoric means constant volume. These are conditions of thermodynamic processes, and the 'iso' tells you which variable is being held constant. A frequent exam error is confusing isothermal with adiabatic: isothermal means temperature doesn't change, while adiabatic means no heat is exchanged. Temperature can change in an adiabatic process. The prefix is your first clue about what stays the same.

Examples

• Isotope
• Isomer
• Isothermal
• Isobaric

Related Terms