Class 11 Chemistry – Chapter 5: States of Matter (FBISE)
This section provides complete, exam-oriented notes for Class 11 Chemistry Chapter 5 – States of Matter following the Federal Board (FBISE) syllabus. The chapter is designed to help students understand the behavior and properties of matter in different states clearly and effectively.
Key topics covered include properties of solids, liquids, and gases, crystalline solids, glacier ice caps, liquid crystals, vapor pressure, evaporation, surface tension, boiling point, and other essential concepts for understanding the states of matter.
Students can also access video lectures, solved numericals, MCQs, test series, and live classes for this chapter through our official YouTube channel and stay updated through our WhatsApp channel.
5.2 Physical Properties of Liquids
Liquids possess unique physical properties such as evaporation, vapour pressure, and boiling point, which are governed by the kinetic energy of molecules and intermolecular forces.
5.2.1 Evaporation
- Definition: The spontaneous change of a liquid into its vapour at the surface of the liquid at any given temperature.
- Process: Molecules in a liquid are in constant motion, but their energy is not equally distributed. High-energy molecules move faster and can escape the attractive forces of neighboring molecules to leave the surface as vapour.
- Cooling Effect: Evaporation causes cooling because when high-energy molecules leave, the average kinetic energy of the remaining molecules decreases, causing the temperature of the liquid to fall.
5.2.2 Vapour Pressure
- Definition: The pressure exerted by the vapour of a liquid in equilibrium with the liquid at a given temperature.
- Dynamic Equilibrium: In a closed system, the rate of evaporation eventually equals the rate of condensation ($Liquid \rightleftharpoons Vapours$).
- Factors Affecting Vapour Pressure:
- Nature of Liquid: Liquids with strong intermolecular forces (like water with hydrogen bonding) have lower vapour pressure compared to those with weak forces (like alcohol).
- Temperature: Increasing temperature increases the kinetic energy of molecules, allowing more to escape and thus increasing vapour pressure.
5.2.3 Boiling Point
- Definition: The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure or any external pressure.
- External Pressure: Boiling point changes with external pressure. For example, water boils at 100°C at sea level (760 mmHg) but boils at 98°C in Murree Hills where external pressure is lower.
Relevant Questions and Answers
Q1. Why does high air humidity make it feel hotter?
Answer: Our body cools itself through the evaporation of sweat. When humidity is high, the air already contains a high concentration of water vapour, making the evaporation process more difficult. Because we cannot cool down efficiently, we feel hotter.
Q2. Compare the vapour pressure of Glycerol and Isopentane at 20°C.
Answer: At 20°C, Glycerol has a very low vapour pressure of 0.00016 torr due to strong intermolecular forces, whereas Isopentane has a very high vapour pressure of 580 torr because of its weaker intermolecular forces.
Q3. How does a pressure cooker help in cooking food faster?
Answer: In a pressure cooker, water vapour is trapped in a closed container, which artificially increases the external pressure above the liquid. This raises the boiling temperature of the water, allowing the food to cook at a higher temperature and thus more quickly.
Q4. What is the vapour pressure of water at 10°C and 90°C?
Answer: According to the data:
- At 10°C, the vapour pressure of water is 9.209 torr.
- At 90°C, the vapour pressure of water is 527.8 torr.
Liquid State Properties: Notes
1. Boiling Point ( & )
- Definition: The temperature at which the vapor pressure of a liquid becomes equal to the external or atmospheric pressure.
- Mechanism: Heating increases the kinetic energy of molecules, which in turn increases vapor pressure. Once it matches external pressure, boiling begins.
- Molar Heat of Vaporization: The amount of heat required to convert one mole of liquid into vapors at its boiling point (e.g., for water, it is ).
- Atmospheric Pressure Influence:
- High external pressure = High boiling point (more heat needed to reach that pressure).
- Low external pressure = Low boiling point.
2. Viscosity ( )
- Definition: The resistance of a liquid to flow. It is caused by internal friction between layers of molecules.
- Factors Affecting Viscosity:
- Molecular Size/Shape: Small, compact molecules (water, methanol) have low viscosity; large, irregular molecules (honey, glycerine) have high viscosity.
- Intermolecular Forces: Stronger forces (like hydrogen bonding) result in higher viscosity.
- Temperature: Viscosity decreases as temperature increases because kinetic energy reduces the strength of intermolecular forces.
- Units: SI unit is Pascal Second (
). Non-SI unit is Poise (
).
3. Surface Tension ( )
- Definition: The property of a liquid surface to behave like a stretched membrane. Quantitatively, it is the force in dynes acting at right angles to a unit length of liquid surface.
- Cause: Molecules inside the liquid are pulled in all directions (balanced), but molecules on the surface are only pulled inward and sideways, creating an unbalanced downward force.
- Key Observations:
- Liquids take the shape of a sphere (smallest surface area per unit volume) to reach a stable, low-energy state.
- Stronger intermolecular forces = Higher surface tension (e.g., Water > Alcohol).
- Surface tension decreases as temperature increases.
- Units: SI units are .
Relevant Questions and Answers
Q1: Why do liquid droplets tend to be spherical in shape?
A: Molecules on the surface of a liquid are less stable because of unbalanced inward forces. A liquid is most stable when it has the smallest possible surface area. Since a sphere has the least surface area per unit volume compared to any other shape, small droplets naturally form spheres.
Q2: How does external pressure affect the boiling point of a liquid?
A: The boiling point is directly proportional to external pressure. When external pressure is high, more heat is required to increase the vapor pressure to match it, resulting in a higher boiling point. Conversely, at lower external pressure, the liquid boils at a lower temperature.
Q3: Why does the viscosity of honey decrease when it is heated?
A: Increasing the temperature increases the kinetic energy of the molecules. This extra energy helps overcome the strong intermolecular forces and reduces the internal friction (tangling) between large molecules, allowing them to slide past each other more easily.
Q4: Compare the surface tension of water and benzene. Which is higher and why?
A: Water has a higher surface tension than benzene. This is due to the presence of strong hydrogen bonding between water molecules, whereas benzene has weaker intermolecular forces. Stronger forces lead to a greater inward pull on surface molecules.
Q5: Define the SI unit of viscosity and its relationship with Poise.
A: The SI unit of viscosity is the Pascal Second ( ), which is equivalent to . The relationship is: .
Liquid State Properties and Applications: Study Notes
1. Hydrogen Bonding in Water ($$5.2.8$$)
- Surface Tension: Water has high surface tension ($$7.275 \times 10^{-2} \text{ Nm}^{-1}$$) due to strong hydrogen bonding, which is a powerful intermolecular attractive force.
- Vapor Pressure: Strong intermolecular forces in water make evaporation slow, resulting in low vapor pressure.
- Heat of Vaporization: Water has a high heat of vaporization compared to other liquids because significant energy is needed to overcome hydrogen bonds to vaporize the liquid.
- Boiling Point: High heat of vaporization and low vapor pressure result in a high boiling point ($$B.P.$$), as more heat is required to make vapor pressure equal to external pressure.
2. Uses of Liquid Crystals ($$5.3.4$$)
- Electronic Displays: Used in digital watches, laptops, calculators, and television screens.
- Medical Applications: Cholesteric liquid crystals are applied to the skin to locate veins, arteries, infections, and tumors because these areas are warmer than surrounding tissue.
- Scientific Research:
- Nematic liquid crystals are used as solvents in Magnetic Resonance ($$NMR$$) to obtain resolved spectra and study dipole-dipole fine structures.
- Used in chromatographic separation and as anisotropic fluids for $$UV$$, $$IR$$, and visible spectroscopy.
- Industrial Uses: Polyester liquid crystals are used as fire-resistant coatings for optical cables and multifibres.
3. Glaciers and Ice Caps ($$5.3.6$$)
- Heat of Fusion: The study of polar ice sheets relies on understanding the heat of fusion, which governs the melting and freezing of ice.
- Climate Predictions: Scientists use heat of fusion data to predict the extent of ice melting and its impact on sea levels due to global warming.
- Historical Data: Analyzing heat absorbed during the melting of ice cores provides information about previous climatic conditions.
Relevant Questions and Answers
Q1: Why does water have a lower vapor pressure compared to liquids like Hexane ($$1.84$$) or $$CCl_4$$ ($$2.70$$)?
A: Water possesses strong intermolecular attractive forces in the form of hydrogen bonding. These forces hold the molecules together more tightly, making evaporation slower and resulting in a lower vapor pressure compared to liquids with weaker forces.
Q2: How are liquid crystals used in medical diagnostics?
A: Cholesteric liquid crystals are used to locate infections, tumors, and blood vessels. When applied to the skin, they react to temperature differences, identifying areas that are warmer than the surrounding healthy tissue.
Q3: What role does the "heat of fusion" play in environmental science?
A: The heat of fusion is crucial for evaluating the susceptibility of polar ice sheets to temperature fluctuations. It allows researchers to calculate the quantity of water released during melting and helps in making predictions regarding sea-level rise caused by global warming.
Q4: Why is the boiling point of water considered high?
A: Due to strong hydrogen bonding, water has a low vapor pressure. Consequently, a large amount of heat is required to increase this vapor pressure until it equals the external atmospheric pressure, leading to a high boiling point.
Q5: What are the industrial applications of liquid crystal polymers?
A: Liquid crystal polymers, such as polyesters, are developed for their fire-resistant properties. they are used as protective coatings for optical cables and multifibres in various industries.
Solid and Liquid State Properties: Detailed Notes
1. Hydrogen Bonding and Water Properties ( 5.2.8 )
- Surface Tension: Water has a high surface tension of 7.275×10 −2 Nm −1 because hydrogen bonding acts as a strong intermolecular attractive force.
- Vapor Pressure: Strong forces of attraction make evaporation slow, resulting in water having a low vapor pressure.
- Heat of Vaporization: Water has a high heat of vaporization compared to other liquids because more heat is needed to overcome strong hydrogen bonds to vaporize the liquid.
- Boiling Point ( B.P. ): High heat of vaporization results in a high boiling point; more heat is required to produce vapor pressure equal to external pressure.
2. Uses of Liquid Crystals ( 5.3.4 )
- Electronics: Used in displays for digital watches, laptops, calculators, and television screens.
- Medicine: Cholesteric liquid crystals are applied to the skin to locate veins, arteries, infections, and tumors, which are warmer than other tissues.
- Scientific Analysis:
- Nematic liquid crystals are used as solvents in magnetic resonance to obtain resolved NMR spectra and study dipole-dipole fine structures.
- Used in chromatographic separation and as anisotropic fluids for UV , IR , and visible spectroscopy.
- Industrial: Polyester liquid crystals are used as fire-resistant coatings for optical cables and multifibre.
3. Glaciers and Ice Caps ( 5.3.6 )
- Heat of Fusion: Crucial for understanding the melting and freezing behavior of polar ice sheets.
- Environmental Predictions: Scientists use heat of fusion data to predict the extent of ice melting and its impact on sea levels due to global warming.
- Climatic Information: Analyzing the heat absorbed during the melting of ice cores provides data about previous climatic conditions.
4. Classification and Properties of Solids ( 5.5.4 )
- Crystalline Solids: Have definite regular, three-dimensional geometric shapes (e.g., NaCl , diamond, ice) with a recurring pattern of atoms or molecules.
- Amorphous Solids: Particles do not have a regular orderly arrangement (e.g., glass, plastic); they have no sharp melting points and soften over a temperature range.
- Key Properties of Crystals:
- Melting Point: Have sharp melting points where particles gain enough kinetic energy to leave fixed positions simultaneously.
- Cleavage Planes: Crystals break along specific planes called cleavage planes when pressure is applied.
- Habit of a Crystal: The shape in which a crystal usually grows (e.g., cubic NaCl ). This can change if conditions change, such as adding 10\%$$urea to$$NaCl to get octahedral crystals.
Relevant Questions and Answers
Q1: Why does water have a higher surface tension than Benzene?
A: Water has a surface tension of 7.275 \times 10^{-2} \text{ Nm}^{-1}$$while Benzene is only$$2.888 \times 10^{-2} \text{ Nm}^{-1} . This is because water possesses strong hydrogen bonding, whereas benzene has weaker intermolecular forces.
Q2: How can crystalline solids be converted into amorphous solids?
A: Crystalline solids can be converted into amorphous solids by melting them and then cooling the molten mass rapidly. This prevents the particles from having enough time to arrange themselves into an orderly pattern.
Q3: What are the diagnostic applications of cholesteric liquid crystals?
A: Cholesteric liquid crystals are used to locate veins, arteries, infections, and tumors because these areas are warmer than surrounding healthy tissues, causing the crystals to react to the temperature difference.
Q4: What is the "habit of a crystal" and how can it be modified?
A: The habit of a crystal is the shape in which it usually grows under normal conditions. It can be modified by changing the growth conditions; for example, adding 10% urea to a sodium chloride solution results in octahedral crystals instead of the usual cubic ones.
Q5: Why is the heat of fusion important in the study of global warming?
A: It is a crucial element in evaluating the susceptibility of polar ice sheets to temperature fluctuations. It helps scientists predict the quantity of water released during ice melting and the subsequent impact on sea levels.