Environmental Chemistry II: Water

Welcome to your comprehensive study resource for Chapter 15: Environmental Chemistry II: Water. Water is arguably the most important substance on Earth, essential for the survival of all living things. In this chapter, we explore its distribution, its unique physical and chemical properties, its behavior as a universal solvent, and the criteria that define water quality.

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1. Occurrence and Importance of Water

Water is present in enormous quantities, covering nearly 71% of the Earth’s crust. However, the availability of fresh water is surprisingly limited:

• Distribution: Approximately 97% of Earth’s water is found in oceans and seas, containing up to 3.4% dissolved salts (mainly sodium chloride), making it unfit for human use. Glaciers and ice caps hold about 2.1%, leaving only 0.6% as fresh water in lakes, rivers, and groundwater.
• Biological and Industrial Importance: Humans can survive weeks without food but only a few days without water. It serves as a lubricant in digestion, regulates body temperature through perspiration, cools automobile engines and nuclear power plants, and is indispensable for agriculture.

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2. Remarkable Physical Properties of Water

Pure water is transparent, colourless, odourless, and tasteless. It boils at 100°C and freezes at 0°C at sea level. It possesses several anomalous properties that make life on Earth possible:

• Anomalous Density: Unlike most substances, water expands when it freezes. Its density increases upon cooling until it reaches a maximum at 4°C. Below this temperature, it expands, meaning ice floats on top of water, insulating the lower layers and allowing aquatic life to survive winter.
• High Heat Capacity: Water requires a substantial amount of heat to raise its temperature. This property allows vast bodies of water (oceans, lakes) to act as giant heat reservoirs, moderating daily temperature variations and cooling industrial plants.
• High Heat of Vaporization: Evaporating a small amount of water absorbs a large amount of heat. This is crucial for regulating body temperature, as body heat is effectively dissipated through perspiration.

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3. Water as a Universal Solvent

Water is famously known as a “universal solvent” because it can dissolve a wider variety of substances than any other liquid. This remarkable ability is driven by two key properties:

• Polarity: Water molecules are highly polar, with a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom.
• Hydrogen Bonding: The strong attraction between water molecules and the ions or polar molecules of a solute allows water to pull apart and surround the solute particles. Consequently, ionic solids and polar hydrogen-bonded compounds dissolve readily in water.

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4. Composition and Electrolysis

Water is normally a poor conductor of electricity. However, when electricity is passed through acidified water in a voltameter, water splits into its constituent elements:

• The Reaction: The electrolysis of water yields hydrogen gas at the cathode and oxygen gas at the anode.
• Chemical Equation:
  2H₂O_(l) → 2H_2(g) + O_2(g)
• Volume Ratio: The splitting produces double the amount of hydrogen compared to oxygen, establishing that hydrogen and oxygen are present in water in a 2:1 ratio by volume.

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5. Soft vs. Hard Water

Water quality is defined by its physical, chemical, biological, and aesthetic characteristics. One primary chemical characteristic is hardness:

• Soft Water: Water that easily produces a rich lather with soap and does not form insoluble scum. Distilled water and most tap water fall into this category.
• Hard Water: Water that gives little lather and instead forms a white, insoluble scum when mixed with soap. This occurs as rainwater dissolves atmospheric carbon dioxide to form weak carbonic acid, which subsequently dissolves minerals from rocks as it moves through the ground.

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6. Reaction-Based Conceptual Example

Example: Formation of Weak Acid in Rainwater
Question: Explain how naturally falling rainwater becomes slightly acidic before it reaches the ground, and provide the balanced chemical equation.
Step-by-Step Solution:

• Concept: Rainwater acts as a solvent even while falling through the atmosphere.
• Mechanism: As rain drops descend, they dissolve naturally occurring carbon dioxide (CO₂) gas present in the air.
• Equation:
  CO_2(g) + H₂O_(l) → H₂CO_3(aq)
• Result: This reaction produces carbonic acid (H₂CO₃), a weak acid, which makes the rainwater slightly acidic and capable of dissolving minerals from soils and rocks upon impact.

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Essential Conceptual Review Questions

Q1: Why does ice float on the surface of lakes during winter instead of sinking?
Answer: Water exhibits strange density behavior upon cooling. While its density increases down to 4°C, it expands when it actually freezes. Because ice expands, its density decreases compared to liquid water, causing it to float on top and insulate the aquatic life below.

Q2: Why is water exceptionally good at dissolving ionic solids like sodium chloride?
Answer: Water’s exceptional solvent capability stems from its polarity and ability to form hydrogen bonds. The strongly attracted polar ends of the water molecule overpower the attractions between the solid’s ions, successfully pulling them apart and dissolving the substance.