Definition formula factors affecting vapour pressure of liquids
Liquid State Vapour Pressure is essential in chemistry and helps students understand various practical and theoretical applications related to this topic.
What is Liquid State Vapour Pressure in Chemistry?
A liquid state vapour pressure refers to the pressure exerted by the vapour present above a liquid in a closed container when the liquid and vapour are in equilibrium at a specific temperature.
This concept appears in chapters related to physical chemistry, states of matter, and boiling point, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The concept of liquid state vapour pressure does not have a single molecular formula, as it is a property seen in all liquids. It depends on the specific liquid’s molecules and their tendency to escape to the vapour phase at a certain temperature. Generally, the more volatile the substance, the higher its vapour pressure.
Preparation and Synthesis Methods
Liquid state vapour pressure is not made or synthesized, but is measured by observing how a liquid behaves in a closed container over time. At a certain temperature, molecules from the surface of the liquid escape into the air (evaporation). When a dynamic equilibrium is reached—where the rate of evaporation equals condensation—the vapour pressure stabilizes. This is typical for liquids like water, ethanol, or acetone.
Physical Properties of Liquid State Vapour Pressure
Some key points about the physical properties of liquid state vapour pressure:
- Different liquids have different vapour pressures at the same temperature.
- Vapour pressure increases with temperature.
- Liquids with weaker intermolecular forces have higher vapour pressure.
- Vapour pressure at boiling point equals atmospheric pressure.
Chemical Properties and Reactions
- Liquid state vapour pressure is a physical property, not a substance, so it is not involved in direct chemical reactions.
- However, it is closely connected with the tendency of a substance to vaporize and participate in physical changes like evaporation and boiling.
- Factors such as impurities in the liquid, or the formation of solutions, can modify the vapour pressure due to colligative properties.
Frequent Related Errors
- Confusing vapour pressure with evaporation rate or boiling point only.
- Mixing up vapour pressure of a pure liquid with its value in a solution.
- Forgetting that temperature is the single biggest factor affecting vapour pressure.
- Assuming all liquids have the same vapour pressure at the same temperature.
- Not realizing that opening the container prevents equilibrium vapour pressure from being reached.
Uses of Liquid State Vapour Pressure in Real Life
Vapour pressure plays a vital role in many real-life situations:
- Helps explain why petrol evaporates quickly and why water doesn’t.
- Predicts when and how liquids boil at different altitudes.
- Guides safe storage of volatile chemicals and fuels.
- Determines shelf-life and formulation of perfumes and medicines.
- Explains drying of puddles, evaporation of sweat, and condensation on cold surfaces.
Relation with Other Chemistry Concepts
Liquid state vapour pressure is closely related to topics such as intermolecular forces, physical properties of liquids, Raoult's law, and Clausius-Clapeyron equations. Understanding these helps you see how solutions form, how the boiling point is impacted, and how vaporization occurs.
Step-by-Step Reaction Example
Let’s see how to calculate the change in vapour pressure in a solution using Raoult’s Law:
1. Find the mole fraction (xA) of the solvent.2. Multiply xA by the pure solvent's vapour pressure (P0A).
3. If two components, add partial vapour pressures: Ptotal = P0AxA + P0BxB.
4. Use the calculated value to compare with atmospheric pressure for boiling point.
5. Final Answer: This tells us the total vapour pressure above the solution.
Lab or Experimental Tips
Remember: Vapour pressure is always measured in a closed container to ensure equilibrium. Vedantu educators often use visuals with covered beakers to reinforce this in live classes.
Heating the liquid gently shows how vapour pressure rises with temperature. Always label the axes of your vapour pressure vs. temperature graph for clarity.
Try This Yourself
- Explain why acetone evaporates faster than water at the same temperature.
- Draw and label a graph of vapour pressure vs. temperature for water.
- Name a daily life example where controlling vapour pressure is essential for safety.
Final Wrap-Up
We explored liquid state vapour pressure—its definition, relation with temperature, physical meaning, and real-life impact. For stepwise explanations, solved numericals, and in-depth revision, explore the full range of live classes and notes available with Vedantu.
Mastering this core concept makes topics like boiling point, evaporation, and solution chemistry much easier to understand.
FAQs on Liquid State Vapour Pressure in Chemistry
1. What is liquid state vapour pressure?
Liquid state vapour pressure is the pressure exerted by the vapour of a liquid when it is in dynamic equilibrium with its liquid phase at a given temperature.
- It arises because molecules escape from the liquid surface into the vapour phase.
- At equilibrium, the rate of evaporation equals the rate of condensation.
- Vapour pressure depends only on the nature of the liquid and temperature, not on the amount of liquid present.
2. What is meant by equilibrium vapour pressure?
Equilibrium vapour pressure is the constant pressure exerted by a vapour when the rates of evaporation and condensation of a liquid are equal.
- This state is called dynamic equilibrium.
- It is established in a closed container.
- The vapour pressure remains constant as long as temperature is constant.
3. How does temperature affect the vapour pressure of a liquid?
The vapour pressure of a liquid increases with increase in temperature.
- At higher temperature, more molecules have sufficient kinetic energy to escape into the vapour phase.
- This increases the number of vapour molecules and hence the pressure.
- The relationship is described by the Clausius–Clapeyron equation: ln P = −(ΔHvap/RT) + C.
4. What is the difference between vapour pressure and boiling point?
Vapour pressure is the pressure exerted by a vapour in equilibrium with its liquid, while the boiling point is the temperature at which vapour pressure equals external pressure.
- Vapour pressure is measured at a fixed temperature.
- Boiling occurs when vapour pressure = atmospheric pressure.
- At 1 atm, the temperature is called the normal boiling point.
5. What factors affect the vapour pressure of a liquid?
The main factors affecting vapour pressure are temperature and intermolecular forces.
- Temperature: Higher temperature increases vapour pressure.
- Intermolecular forces: Stronger forces (like hydrogen bonding) lower vapour pressure.
- Nature of liquid: Volatile liquids have higher vapour pressures.
6. Why do liquids with strong intermolecular forces have low vapour pressure?
Liquids with strong intermolecular forces have low vapour pressure because fewer molecules can escape into the vapour phase.
- Strong attractions (e.g., hydrogen bonding in H2O) hold molecules tightly together.
- More energy is required for molecules to overcome these forces.
- As a result, fewer molecules enter the vapour phase at a given temperature.
7. What is the formula for Raoult’s law in vapour pressure?
Raoult’s law states that the partial vapour pressure of a component in an ideal solution is PA = XAPA0.
- PA = partial vapour pressure of component A
- XA = mole fraction of A in solution
- PA0 = vapour pressure of pure A
8. What is vapour pressure lowering in solutions?
Vapour pressure lowering is the decrease in the vapour pressure of a solvent when a non-volatile solute is added.
- Solute particles reduce the number of solvent molecules escaping to vapour phase.
- According to Raoult’s law: ΔP = PA0 − PA.
- It is a colligative property, depending only on the number of solute particles.
9. How is vapour pressure related to volatility?
Volatility is directly proportional to vapour pressure, meaning liquids with high vapour pressure are more volatile.
- Volatile liquids evaporate easily at room temperature.
- They usually have weak intermolecular forces.
- Examples include diethyl ether and acetone.
10. Can you give an example of vapour pressure and boiling in water?
At 100°C, the vapour pressure of water equals 1 atm, which is why water boils at 100°C under normal atmospheric pressure.
- Below 100°C, vapour pressure of H2O(l) is less than 1 atm.
- At high altitudes, atmospheric pressure is lower, so water boils below 100°C.
- Boiling occurs when vapour pressure = external pressure.
