Factors Affecting Vapour Pressure in 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 Understanding Vapour Pressure of a Liquid: Concepts & Examples
1. What is vapour pressure of a liquid?
Vapour pressure of a liquid is the equilibrium pressure exerted by its vapour above the liquid surface at a specific temperature. This pressure is reached when the rate of evaporation equals the rate of condensation, creating a dynamic equilibrium.
2. How does temperature affect the vapour pressure of a liquid?
Temperature directly influences vapour pressure. As temperature increases, vapour pressure rises because:
- More liquid molecules gain energy and escape into the vapour phase
- Evaporation rate increases while condensation remains constant, until a new equilibrium is set
- This relationship explains why liquids boil at higher temperatures
3. What is Raoult's law of vapour pressure?
Raoult's law states that in an ideal solution, the partial vapour pressure of each component equals the product of the vapour pressure of the pure component and its mole fraction in the mixture. This law is expressed as:
- Psolution = PA0 xA + PB0 xB
- PA0 and PB0 are vapour pressures of pure components
- xA and xB are mole fractions
4. What factors affect the vapour pressure of a liquid?
Several factors influence vapour pressure of a liquid:
- Temperature: Higher temperature increases vapour pressure.
- Intermolecular forces: Stronger forces (like hydrogen bonding) lower vapour pressure.
- Nature of liquid: Volatile liquids possess higher vapour pressures.
5. What units are used to express vapour pressure?
Vapour pressure is commonly measured in the following units:
- Millimeters of mercury (mmHg)
- Atmospheres (atm)
- Pascals (Pa)
6. How is vapour pressure measured in the laboratory?
Vapour pressure can be measured using one of these common methods:
- Manometric method: Uses a manometer to compare liquid’s vapour pressure with atmospheric pressure in a closed system.
- Dynamic method: Circulates an inert gas through the liquid and measures the pressure of the released vapour.
- Static method: Observes pressure stabilization inside a sealed container at constant temperature.
7. How are boiling point and vapour pressure related?
Boiling point is the temperature at which the vapour pressure of a liquid equals the surrounding atmospheric pressure. At this point:
- The liquid rapidly changes to vapour throughout the bulk
- Lower vapour pressure liquids have higher boiling points
8. Why do volatile liquids have higher vapour pressures?
Volatile liquids have higher vapour pressures because:
- They have weaker intermolecular forces
- Molecules escape to the vapour phase more easily
- This results in greater equilibrium vapour above the liquid surface
9. What is equilibrium vapour pressure?
Equilibrium vapour pressure is the pressure exerted by the vapour when a liquid and its vapour coexist in a closed container and the rates of evaporation and condensation become equal. This represents dynamic equilibrium.
10. Can different liquids have the same vapour pressure at different temperatures?
Yes, different liquids can have the same vapour pressure but at different temperatures. This is because each liquid’s intermolecular forces determine how much energy (temperature) is needed for its molecules to enter the vapour phase.
11. Why does water boil at 100°C at sea level?
Water boils at 100°C at sea level because at this temperature, its vapour pressure equals atmospheric pressure (1 atm). This allows bubbles of vapour to form throughout the liquid, resulting in boiling.
12. Can a liquid have zero vapour pressure?
No, all liquids exert some vapour pressure above absolute zero. Only when a substance either solidifies or is in a perfect vacuum (with all molecules removed) does the vapour pressure become zero.
