2.2 Properties Of Water Answer Key

2.2 Properties of Water – Answer Key

Water is often called the “universal solvent” and for good reason: its unique physical and chemical properties make life possible on Earth. That's why in many science curricula, Section 2. 2 – Properties of Water is a cornerstone topic that ties together concepts from chemistry, physics, biology, and environmental science. This answer key provides a comprehensive, step‑by‑step explanation of the key properties, the underlying molecular reasons, and the real‑world implications that students need to master for exams, lab reports, and everyday understanding.

Introduction

The properties of water include its polarity, high specific heat, high heat of vaporisation, surface tension, cohesion and adhesion, density anomaly, and its role as a solvent. But recognising how these microscopic interactions translate into macroscopic behaviour is essential for answering the typical questions found in a 2. Day to day, each property originates from the polar covalent O–H bonds and the resulting hydrogen‑bond network. 2‑level assessment.

1. Polarity and Hydrogen Bonding

Key point: Water molecules are polar because oxygen is more electronegative than hydrogen, creating a partial negative charge (δ‑) on the oxygen atom and partial positive charges (δ +) on the hydrogen atoms.

• Hydrogen bond definition: An attraction between the δ + hydrogen of one water molecule and the δ‑ oxygen of another.
• Typical bond energy: 15–30 kJ mol⁻¹, much weaker than covalent bonds but strong enough to dominate water’s bulk properties.

Answer tip: When asked to draw the dipole moment, show the bent molecular geometry (104.5°) with an arrow pointing from the hydrogen side toward the oxygen side, indicating the net dipole.

2. Cohesion and Adhesion

Why it matters: Cohesion explains surface tension, while adhesion is crucial for transpiration in plants and the capillary action that moves water through soil and tiny pores.

Typical exam question: Explain how cohesion and adhesion together enable the movement of water from roots to leaves.
Answer outline: Cohesive forces create a continuous column of water; adhesive forces to the xylem walls keep the column attached, while transpiration pull at the leaf tip generates a negative pressure that draws the column upward.

3. Surface Tension

• Numerical value: 72.8 mN m⁻¹ at 20 °C.
• Cause: Molecules at the surface experience a net inward hydrogen‑bond pull, minimizing surface area.
• Real‑world illustration: Water striders (Gerridae) can walk on water because their legs distribute weight over a large area, preventing surface rupture.

Sample problem: Calculate the force required to break a 2 cm long water film with surface tension 0.072 N m⁻¹.
Solution: F = γ × L = 0.072 N m⁻¹ × 0.02 m = 0.00144 N.

4. High Specific Heat (Cp)

• Value: 4.18 J g⁻¹ °C⁻¹ (or 4.18 kJ kg⁻¹ K⁻¹).
• Explanation: Large amount of energy is needed to break and reform hydrogen bonds during temperature changes.

Implication for climate: Oceans absorb vast amounts of solar energy without large temperature swings, moderating global climate and protecting coastal ecosystems.

Typical calculation: How much heat is required to raise 250 g of water from 15 °C to 35 °C?
Answer: Q = m × Cp × ΔT = 250 g × 4.18 J g⁻¹ °C⁻¹ × 20 °C = 20,900 J ≈ 21 kJ.

5. High Heat of Vaporisation

• Value: 40.7 kJ mol⁻¹ at 100 °C.
• Reason: Vaporising water requires breaking most hydrogen bonds, demanding considerable energy.

Biological relevance: Sweating and evaporative cooling rely on this property; the body loses heat efficiently as water evaporates from the skin.

Sample question: Why does the evaporation of water from a pond cool the surrounding air?
Answer: Energy is taken from the water’s kinetic pool to overcome hydrogen bonds, converting liquid to vapor. This energy is drawn from the water and adjacent air, lowering temperature.

6. Density Anomaly (Maximum Density at 4 °C)

• Observation: Water reaches its highest density at 4 °C; below this temperature, it expands.
• Molecular cause: As temperature drops, hydrogen bonds arrange water molecules into an open hexagonal lattice, increasing volume.

Environmental consequence: Ice floats, insulating liquid water beneath and protecting aquatic life during winter.

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Typical exam prompt: Explain how the density anomaly of water protects freshwater ecosystems in cold climates.
Answer outline: Ice forms a surface layer that is less dense than liquid water, remaining on top. This insulating layer reduces heat loss from the water below, preventing the entire body from freezing and allowing organisms to survive.

7. Solvent Power (Universal Solvent)

• Polarity allows dissolution of ionic compounds (e.g., NaCl) and other polar molecules (e.g., sugars, alcohols).
• Hydration shells: Water molecules surround ions, stabilising them in solution.

Practical example: In the human body, water transports nutrients, waste products, and gases because of its ability to dissolve a wide range of substances.

Typical question: Write the dissolution equation for sodium chloride in water and describe the role of water molecules.
Answer: NaCl(s) → Na⁺(aq) + Cl⁻(aq). Water molecules orient their δ + hydrogens toward Cl⁻ and δ‑ oxygens toward Na⁺, forming hydration shells that keep the ions separated.

8. pH and Auto‑ionisation

• Auto‑ionisation reaction: 2 H₂O ⇌ H₃O⁺ + OH⁻
• Equilibrium constant (Kw): 1.0 × 10⁻¹⁴ at 25 °C.
• Resulting neutral pH: 7 (since [H₃O⁺] = [OH⁻] = 1.0 × 10⁻⁷ M).

Importance for biology: Enzyme activity, metabolic pathways, and cellular homeostasis depend on maintaining a narrow pH range, which water’s buffering capacity helps to stabilise.

Sample calculation: If the temperature rises to 50 °C, Kw increases to ≈5.5 × 10⁻¹⁴. What is the new neutral pH?
Solution: [H⁺] = √Kw ≈ √5.5 × 10⁻¹⁴ ≈ 7.42 × 10⁻⁸ M → pH = −log(7.42 × 10⁻⁸) ≈ 7.13.

9. Vapor Pressure and Boiling Point

• Vapor pressure: The pressure exerted by water molecules escaping into the gas phase. At 20 °C, it is ≈2.34 kPa.
• Boiling point: The temperature at which vapor pressure equals ambient pressure (101.3 kPa at sea level).

Altitude effect: At higher elevations, lower atmospheric pressure reduces boiling temperature, which impacts cooking times and industrial processes.

Typical problem: Estimate the boiling point of water at 2 000 m altitude where atmospheric pressure ≈80 kPa.
Answer approach: Use the Clausius‑Clapeyron relation or reference boiling‑point tables; the boiling point drops to roughly 93 °C.

10. Electrical Conductivity (Pure vs. Impure Water)

• Pure water: Very low conductivity (~0.055 µS cm⁻¹) because of minimal ion concentration.
• Impure water: Conductivity increases dramatically with dissolved salts, acids, or bases.

Application: Conductivity meters are used to assess water quality; high conductivity often signals contamination.

Frequently Asked Questions (FAQ)

Q1. Why does water have a higher specific heat than most other liquids?
A: The extensive hydrogen‑bond network absorbs heat energy as bonds stretch and reform, requiring more energy per degree of temperature change.

Q2. How does surface tension enable capillary action?
A: Cohesive forces pull water molecules together, while adhesive forces to the tube walls pull the column upward. The curvature of the meniscus creates a pressure difference (Laplace pressure) that draws liquid upward.

Q3. Can water act as both an acid and a base?
A: Yes, water is amphoteric. It can donate a proton (acting as an acid) in the auto‑ionisation reaction and accept a proton (acting as a base) when reacting with strong acids or bases.

Q4. Why does ice have a lower density than liquid water?
A: In the solid state, each water molecule forms four hydrogen bonds in a regular tetrahedral lattice, creating open spaces that expand the structure and reduce density.

Q5. How does the high heat of vaporisation affect weather patterns?
A: Evaporation from oceans stores solar energy; when water condenses into clouds, the latent heat is released, powering atmospheric circulation and storm formation.

Conclusion

Understanding the properties of water—from polarity and hydrogen bonding to density anomaly and solvent capabilities—provides a foundation for countless scientific disciplines. 2 equips students not only to ace exams but also to appreciate why a simple molecule of H₂O is arguably the most remarkable substance on the planet. Mastery of Section 2.In real terms, the interplay of microscopic forces yields macroscopic phenomena that shape climate, sustain ecosystems, and drive everyday technologies. By internalising the concepts, equations, and real‑world examples presented in this answer key, learners can confidently tackle any problem related to water’s unique behavior.