vapour pressure
C1technical
Definition
Meaning
The pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.
A quantitative measure of a substance's tendency to evaporate or escape from its condensed state; a key property in fields like meteorology, chemistry, and engineering, influencing processes such as boiling, distillation, and atmospheric humidity.
Linguistics
Semantic Notes
This is a compound noun, predominantly used in scientific and technical contexts. It represents an abstract physical property. While 'vapor pressure' (American) is often a mass noun, it can be counted when referring to the pressures of different substances (e.g., 'the vapour pressures of various solvents').
Dialectal Variation
British vs American Usage
Differences
Spelling: British English uses 'vapour', American English uses 'vapor'. No difference in pronunciation beyond general accent.
Connotations
Identical scientific meaning. In everyday speech, it's rarely used; any usage likely signals a technical or educational context.
Frequency
Used with essentially identical frequency in relevant scientific and engineering contexts in both variants.
Vocabulary
Collocations
Grammar
Valency Patterns
The vapour pressure of [SUBSTANCE] is [VALUE].[SUBSTANCE] has a high/low vapour pressure.At [TEMPERATURE], the vapour pressure reaches [VALUE].Vocabulary
Synonyms
Neutral
Weak
Vocabulary
Antonyms
Usage
Context Usage
Business
Rare, except in specific industries like chemical manufacturing, pharmaceuticals, or fuel production where product specifications might reference vapour pressure (e.g., 'The fuel's vapour pressure must meet winter blend standards.').
Academic
Core concept in chemistry, physics, chemical engineering, meteorology, and environmental science lectures, textbooks, and research papers.
Everyday
Extremely rare. Might appear in simplified explanations of weather (humidity, dew point) or why a puddle dries.
Technical
Fundamental term in process design, safety data sheets (flammability), thermodynamics, HVAC (heating, ventilation, and air conditioning), and materials science.
Examples
By Part of Speech
verb
British English
- The mixture is designed to vapour-pressure equilibrate rapidly.
- We need to vapour-pressure test the new solvent.
American English
- The formula is engineered to vapor-pressure equilibrate quickly.
- We must vapor-pressure test the new compound.
adjective
British English
- The vapour-pressure reading was anomalous.
- Refer to the vapour-pressure curve on the chart.
American English
- The vapor-pressure reading was abnormal.
- Check the vapor-pressure curve on the graph.
Examples
By CEFR Level
- Water has a vapour pressure. On a hot day, this pressure is higher.
- Alcohol disappears faster than water because it has a higher vapour pressure.
- The weather forecast mentioned high humidity, which is related to the vapour pressure of water in the air.
- Safety guidelines require chemicals with high vapour pressure to be stored in well-ventilated areas.
- The Clausius-Clapeyron equation describes the relationship between temperature and the vapour pressure of a substance.
- Engineers lowered the process temperature to reduce the volatile solvent's vapour pressure and minimise fugitive emissions.
Learning
Memory Aids
Mnemonic
Think of a sealed fizzy drink bottle. The pressure you feel when you shake it is like the vapour pressure from the carbon dioxide wanting to escape the liquid.
Conceptual Metaphor
ESCAPE FORCE / FUGITIVE TENDENCY (The substance's molecules are conceptualised as prisoners trying to escape from the liquid or solid prison, and the vapour pressure is the force of their attempt.)
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Avoid direct calques like 'давление пара' without understanding it refers to the intrinsic property of the substance, not just any pressure from steam. The Russian term is accurate but the conceptual linkage to evaporation might be weaker for learners.
Common Mistakes
- Confusing 'vapour pressure' with atmospheric or gas pressure in general. Using it as a verb (e.g., 'It vapours pressure'). Incorrect pluralisation (e.g., 'vapours pressures').
Practice
Quiz
What does a high vapour pressure indicate about a substance at a given temperature?
FAQ
Frequently Asked Questions
No. Atmospheric pressure is the pressure exerted by the weight of the atmosphere. Vapour pressure is the specific pressure exerted by the vapour of a particular substance in a closed system.
The liquid boils. Bubbles of vapour can form within the liquid because the substance's internal pressure can overcome the external atmospheric pressure.
No, for a pure substance at a constant temperature, vapour pressure depends only on the temperature and the identity of the substance, not on the volume of liquid or vapour present.
A fuel's vapour pressure affects its volatility, which influences engine starting in cold weather (higher pressure is better) but also contributes to evaporative emissions and vapour lock in fuel lines (higher pressure can be problematic).