In our daily lives, we witness evaporation often — water drying on clothes, puddles vanishing after rain, or perfume disappearing into the air. But behind these familiar sights lies a fascinating scientific concept called vapour pressure. Understanding vapour pressure gives us insight into how liquids behave and transform, especially under the influence of temperature.
Let’s explore this invisible yet powerful phenomenon in detail.
What is Vapour Pressure?
Vapour pressure is the pressure exerted by the vapour (gas) of a liquid when it is in equilibrium with its liquid phase at a given temperature in a closed system.
Let’s break that down:
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Imagine a closed container with some liquid (like water) in it.
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Molecules at the surface of the liquid escape into the air as vapour.
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Some of these vapour molecules return to the liquid by condensation.
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When the rate of evaporation equals the rate of condensation, the system reaches equilibrium.
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The pressure exerted by the vapour at this point is called vapour pressure.
So, vapour pressure tells us how easily a liquid can evaporate at a given temperature.
Why is Vapour Pressure Important?
Vapour pressure is crucial in various areas of science, industry, and daily life:
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Boiling Point Determination
A liquid boils when its vapour pressure equals the surrounding atmospheric pressure. At sea level, water boils at 100°C because its vapour pressure matches the atmospheric pressure (1 atm). At higher altitudes, where atmospheric pressure is lower, water boils at lower temperatures. -
Weather and Evaporation
The evaporation rate of water bodies, soil, and plants depends on vapour pressure. Meteorologists consider it while predicting humidity and rainfall. -
Perfume and Paints
Substances with high vapour pressure evaporate quickly — that’s why perfumes spread their scent easily. Similarly, fast-drying paints and inks rely on solvents with high vapour pressure. -
Refrigeration
Coolants used in refrigerators and air conditioners are selected based on their vapour pressure characteristics. -
Engineering and Safety
Fuels like petrol have high vapour pressures, making them volatile. Understanding this helps in safe storage and transport.
Factors Affecting Vapour Pressure
Several factors influence vapour pressure. Let’s look at the main ones:
1. Temperature
Temperature is the most important factor. As temperature increases, more liquid molecules gain energy to escape into the gas phase.
🔥 Higher temperature = Higher vapour pressure
That’s why hot water evaporates faster than cold water.
2. Nature of the Liquid
Different liquids have different intermolecular forces:
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Strong forces (like hydrogen bonding in water) result in low vapour pressure.
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Weak forces (like in acetone or alcohol) result in high vapour pressure.
So, volatile liquids (those that evaporate easily) have high vapour pressure.
3. Presence of Solutes
Adding non-volatile solutes (like salt or sugar) lowers the vapour pressure of a liquid. This is because fewer water molecules are available to escape as vapour.
This concept is the basis of:
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Boiling point elevation
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Freezing point depression
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Osmosis
Vapour Pressure and Boiling
Boiling happens when vapour pressure equals external pressure. That’s why:
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At high altitudes (low external pressure), water boils below 100°C.
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In a pressure cooker (high pressure), water boils above 100°C.
Understanding this helps in cooking, pressure control, and even in manufacturing processes.
Vapour Pressure in Real Life
Let’s look at some relatable examples:
☁️ Drying Clothes
Clothes dry faster in warm, dry air because higher temperature and lower humidity increase the vapour pressure gradient between wet clothes and air.
🚗 Fuel Handling
Petrol has a high vapour pressure, so it evaporates quickly. That’s why petrol tanks are sealed tightly to avoid loss and reduce fire risk.
💅 Nail Polish Remover
Acetone, a common ingredient, has very high vapour pressure. That’s why it evaporates quickly and cools the skin.
Measurement of Vapour Pressure
Vapour pressure can be measured using devices like:
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Manometers
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Isoteniscopes
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Dynamic methods (like boiling point method)
These measurements help chemists and engineers in material design, safety studies, and chemical processing.
Summary
| Property | High Vapour Pressure | Low Vapour Pressure |
|---|---|---|
| Volatility | High | Low |
| Boiling Point | Low | High |
| Intermolecular Forces | Weak | Strong |
| Examples | Acetone, Ether, Alcohol | Water, Glycerin, Oil |
Conclusion
Though we often ignore it, vapour pressure silently governs many physical phenomena around us. From boiling water to drying clothes, from fragrance spreading in a room to fuel storage safety — vapour pressure is everywhere.
By understanding this concept, we gain deeper insights into the hidden forces of nature and how science applies to even the most ordinary activities in our life.
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