Liquid State Vapour Pressure Surface Tension Viscosity
Chapter: States of Matter | Topic: Liquid State Vapour Pressure Surface Tension Viscosity
LIQUID STATE VAPOUR PRESSURE SURFACE TENSION VISCOSITY
Topic: Liquid State Vapour Pressure Surface Tension Viscosity
Multiple Choice Questions
Q1. Vapour pressure of a liquid depends on:
- The volume of the liquid
- The surface area of the liquid
- The temperature and nature of the liquid
- The shape of the container
Q2. The surface tension of a liquid is due to:
- Adhesive forces between liquid and container
- Cohesive forces between liquid molecules at the surface
- Gravitational forces
- Vapour pressure
Q3. Which of the following liquids will have the highest viscosity?
- Water
- Ethanol
- Glycerol
- Acetone
Q4. The boiling point of a liquid is the temperature at which:
- Its vapour pressure equals atmospheric pressure
- Its vapour pressure is maximum
- Its vapour pressure is zero
- Its vapour pressure is half of atmospheric pressure
Q5. Surface tension of a liquid decreases with:
- Decrease in temperature
- Increase in temperature
- Addition of impurities
- Both (b) and (c)
Q6. Viscosity of a liquid is related to:
- The ease with which molecules can flow past each other
- The vapour pressure of the liquid
- The surface area of the liquid
- The density of the liquid
Short Answer Questions
Q7. Define vapour pressure. Why does a liquid have a vapour pressure even at room temperature?
Q8. Explain the effect of temperature on the vapour pressure of a liquid. Draw a graph showing the variation of vapour pressure with temperature.
Q9. Define surface tension. Why do small droplets of liquid assume a spherical shape?
Q10. Define viscosity. Why does honey flow more slowly than water? Explain in terms of intermolecular forces.
Q11. Explain why the boiling point of a liquid increases at higher altitudes when cooking in a pressure cooker becomes necessary.
Q12. What is the effect of temperature on surface tension and viscosity? Explain the molecular basis for these effects.
Long Answer Questions
Q13. (a) Define vapour pressure and explain how it arises at the molecular level.
(b) Explain the effect of:
(i) Temperature on vapour pressure
(ii) Nature of liquid on vapour pressure
(iii) Addition of a non-volatile solute on vapour pressure
(c) The vapour pressure of water at 25°C is 23.8 mmHg. Explain what this value means at the molecular level.
Q14. (a) Define surface tension and explain its origin in terms of intermolecular forces.
(b) Explain the following phenomena using surface tension:
(i) Capillary rise
(ii) Formation of spherical droplets
(iii) A needle floating on water
(iv) Detergents helping in cleaning
(c) Why does hot water clean better than cold water? Explain in terms of surface tension and viscosity.
Q15. (a) Define viscosity and coefficient of viscosity. What are the units of coefficient of viscosity?
(b) Explain the factors affecting viscosity:
(i) Temperature
(ii) Molecular size and shape
(iii) Intermolecular forces
(c) Explain why:
(i) Glass is considered a supercooled liquid
(ii) Lubricating oils become less effective at high temperatures
(iii) Blood is more viscous than water
Numerical / Application-Based Problems
Q16. The vapour pressures of water at different temperatures are given below:
| Temperature (°C) | Vapour Pressure (mmHg) |
|---|---|
| 0 | 4.6 |
| 20 | 17.5 |
| 40 | 55.3 |
| 60 | 149.4 |
| 80 | 355.1 |
| 100 | 760.0 |
(a) Plot a graph of vapour pressure vs temperature.
(b) From the graph, estimate the temperature at which the vapour pressure of water is 100 mmHg.
(c) Calculate the ratio of vapour pressures at 100°C to that at 0°C. What does this large ratio tell you about the effect of temperature on vapour pressure?
(d) At what temperature will water boil on a mountain where the atmospheric pressure is 500 mmHg?
Q17. The following data gives the surface tension and viscosity of some liquids at 20°C:
| Liquid | Surface Tension (× 10⁻³ N/m) | Viscosity (× 10⁻³ Pa·s) |
|---|---|---|
| Water | 72.8 | 1.00 |
| Ethanol | 22.3 | 1.20 |
| Mercury | 465.0 | 1.55 |
| Glycerol | 63.0 | 1410.0 |
(a) Explain why mercury has such a high surface tension compared to other liquids.
(b) Explain why glycerol has an extremely high viscosity despite having a surface tension similar to water.
(c) Calculate the ratio of viscosity of glycerol to that of water. What does this tell you about the relative ease of flow?
(d) A steel needle of density 7.8 g/cm³ and radius 0.5 mm is placed gently on water. Given that the surface tension of water is 72.8 × 10⁻³ N/m, will the needle float or sink? Show your calculation. (Assume the needle is cylindrical and 5 cm long.)
Q18. In India, the properties of liquids play crucial roles in daily life and industry:
(a) During the monsoon season, clothes take longer to dry in Mumbai than in Delhi. Explain this using the concept of vapour pressure and relative humidity.
(b) A farmer in Rajasthan uses a traditional khus (vetiver) curtain to cool his room. Water is sprinkled on the curtain and air is drawn through it. Explain how this works using evaporation, vapour pressure, and the cooling effect.
(c) In the textile industry, surfactants are added to water during dyeing to reduce surface tension. Explain why reducing surface tension helps the dye penetrate fabric fibres more effectively.
(d) Engine oil becomes thinner (less viscous) in summer and thicker (more viscous) in winter. A mechanic recommends using 10W-40 multigrade oil for Indian conditions. Explain what the numbers mean and why multigrade oil is better than single-grade oil for vehicles operating across India's wide temperature range (from 0°C in the Himalayas to 50°C in Rajasthan).