unstable

unstable — adjective

1. not firmly fixed, positioned, or balanced, so that something or someone is likel

1.adjectiveB1
Definition

not firmly fixed, positioned, or balanced, so that something or someone is likely to move, fall, or break.

Example

The old wooden chair wobbled because one of its legs was unstable.

collocation: unstable + leg / structure / surface

Niran placed the vase on an unstable shelf, and it crashed to the floor.

Common collocations
  • politically unstable
  • structurally unstable
  • dangerously unstable
  • unstable foundation
  • unstable government
  • unstable condition
Synonyms
  • unsteady

    less formal, often used for movement or gait rather than fixed position

  • wobbly

    informal; describes something that moves from side to side when touched

  • precarious

    more formal; suggests danger of collapsing or falling at any moment

  • rickety

    informal; describes old, poorly made structures likely to break

Antonyms
  • stable

    the direct opposite in both physical and abstract contexts

  • steady

    emphasises consistent position or movement without shaking

Usage Note

Frequently used for physical objects (furniture, buildings, ground surfaces) and extended to abstract things such as political systems or economic conditions.

Common Mistakes

The table is unstable because it is old.
The table wobbles because one of its legs is unstable.
'unstable' describes a lack of firmness or balance, not just age.

2. describes a person whose emotions, behaviour, or state of mind changes suddenly

2.adjectiveB2
Definition

describes a person whose emotions, behaviour, or state of mind changes suddenly and extremely, often because of a mental health condition.

Example

After losing her job, Noa became emotionally unstable and struggled to sleep.

collocation: emotionally / mentally unstable

The court ordered a psychological evaluation to determine whether the defendant was mentally unstable.

Common collocations
  • emotionally unstable
  • mentally unstable
  • psychologically unstable
  • unstable behaviour
  • unstable mood
  • unstable personality
Synonyms
  • volatile

    suggests sudden, explosive mood swings; slightly more formal

  • unbalanced

    less clinical; suggests a lack of mental equilibrium

  • erratic

    focuses on unpredictable behaviour rather than internal state

Antonyms
  • stable

    the opposite in mental/emotional contexts

  • well-adjusted

    positive informal term for healthy mental state

Usage Note

Often used in clinical, legal, or journalistic contexts. Avoid using this term casually or as an insult in everyday speech because it carries a strong negative judgement about a person's mental health.

Common Mistakes

My friend was unstable because she cried at a movie.
My friend was emotionally unstable after the accident and could not control her tears.
'unstable' describes a serious pattern of mood changes, not a temporary emotional reaction.
He is so unstable today.' (vague)
He has been emotionally unstable lately and needs professional support.
be specific about the context.

3. describes a chemical substance or compound that readily changes its composition,

3.adjectiveC1
Definition

describes a chemical substance or compound that readily changes its composition, decomposes, or reacts with other substances, often in a dangerous or unpredictable way.

Example

The lab assistant stored the unstable compound in a special container to prevent an explosion.

collocation: unstable + compound / chemical / substance

Jin warned the trainees that the mixture becomes unstable if heated too quickly.

Common collocations
  • chemically unstable
  • highly unstable
  • unstable compound
  • unstable element
  • unstable isotope
  • unstable mixture
Synonyms
  • reactive

    emphasises a tendency to react with other substances

  • volatile

    specifically suggests rapid evaporation or dangerous reaction at room temperature

  • decomposable

    technical; focuses on breaking down into simpler components

Antonyms
  • stable

    the direct opposite; a stable substance does not easily react or decompose

  • inert

    technical; refers to substances that do not react with other chemicals

Usage Note

Mostly used in scientific or technical writing. In everyday language, sense 1 or 2 is preferred. The term does NOT describe emotional states when used in a chemistry context.

Common Mistakes

That chemical is unstable so it will explode immediately.
That chemical is unstable and may decompose if not stored properly.
'unstable' does not always mean explosive; it means the substance is prone to chemical change.

Word family

Synonym discussion

Sense: adjective/1

All five words describe something that is not firmly in place, but they differ in degree and tone. 'Unstable' is the broadest term, covering physical objects, political systems, and emotional states. 'Unsteady' focuses on movement or balance, especially in a person's gait or hands — a waiter with an unsteady tray. 'Wobbly' is informal and suggests side-to-side motion, like a wobbly table or a child's first steps. 'Precarious' is more formal and implies danger: a precarious stack of dishes could topple at any moment. 'Rickety' applies to old, badly built structures that look ready to collapse, such as a rickety fence or a rickety staircase. For a general description of a chair that needs fixing, say 'unstable' or 'wobbly'; for a dangerous cliff-edge situation, choose 'precarious'.

unstable ↗unsteady ↗wobblyprecarious ↗rickety

Common collocations

Nouns

Adverbs

Etymology

LatinOld FrenchMiddle EnglishEnglish

'Unstable' comes from the prefix 'un-' (meaning 'not') attached to the adjective 'stable'. 'Stable' entered English through Old French 'estable', from Latin 'stabilis' meaning 'firm, steady', which itself comes from the Latin verb 'stare' ('to stand'). So the word's literal sense is 'not able to stand firm'.