💧 The Essential Chemistry of Water 💧

Hello Chemists! Welcome to the section on the Chemistry of the Environment. This chapter focuses entirely on water—a substance so common, yet so vital. Understanding where our water comes from, what's in it, and how we make it safe to drink is crucial, not just for your exam, but for understanding the world around you!


Don't worry if the long lists of impurities seem daunting. We will break down the key points into simple, memorable facts.


1. Testing for Water and Assessing Purity

In chemistry, we need ways to prove that a liquid is actually water, and also to check if it's pure.


A. Chemical Tests for the Presence of Water

We use substances that are anhydrous (meaning they contain no water) and change colour dramatically when they absorb water. This change is often used to test for water vapour or liquid water.


  • Anhydrous Cobalt(II) Chloride:
    When dry, this substance is blue. When water is added, it turns pink.

  • Anhydrous Copper(II) Sulfate (CuSO₄):
    When dry, this substance is a white powder. When water is added, it forms blue hydrated copper(II) sulfate (CuSO₄•5H₂O).

Quick Tip for Remembering the Copper Test: Think of the dry powder as white like bone, but when it drinks water, it becomes blue like the ocean!


B. Testing for Purity

A pure substance has fixed physical properties, such as a fixed melting point and a fixed boiling point.

  • Pure water melts sharply at 0 °C (at standard atmospheric pressure).
  • Pure water boils sharply at 100 °C (at standard atmospheric pressure).

What happens if the water is impure?
If the water contains impurities (like salts or minerals), its physical properties are affected:

  • The boiling point will be above 100 °C.
  • It will boil over a range of temperatures, not at a sharp, fixed point.

Real-World Connection: We use distilled water (very pure water) in practical chemistry because it contains fewer chemical impurities. If we used tap water, the dissolved salts in the tap water might react during our experiments and interfere with the results.


Key Takeaway for Section 1: Water can be chemically tested using color changes (blue $\rightarrow$ pink for cobalt chloride, white $\rightarrow$ blue for copper sulfate). Purity is checked by confirming its fixed melting point (0 °C) and boiling point (100 °C).


2. Natural Water: Contaminants and their Effects

Water from natural sources (rivers, lakes, reservoirs) is never pure (it's a mixture!). It contains many dissolved and suspended substances, some of which are essential, and others which are harmful.


A. Substances Found in Natural Water

Natural water may contain:

  • Dissolved gases (e.g., dissolved oxygen).
  • Metal compounds (e.g., mineral salts like calcium or magnesium ions).
  • Sewage and human/animal waste.
  • Harmful microbes (bacteria, viruses).
  • Plastics and microplastics.
  • Nitrates and Phosphates (from fertilisers and detergents).

B. Beneficial Substances (The Good Stuff!)

Some natural contaminants are necessary for life:

  • Dissolved oxygen: Essential for aquatic life (fish need this oxygen to breathe).
  • Metal compounds: Provide essential minerals for human health (like calcium).

C. Harmful Substances (The Bad Stuff!)

Many contaminants pose serious risks to human and aquatic health:

  1. Toxic Metal Compounds: Some heavy metals (like mercury or lead) are toxic and can cause severe illness if ingested, even in small amounts.
  2. Sewage and Microbes: Sewage contains harmful microbes (like bacteria) which cause serious diseases (e.g., cholera, typhoid).
  3. Plastics: Harmful to aquatic life, often leading to death or injury when animals ingest them.
  4. Nitrates and Phosphates: These chemicals, largely from agricultural runoff (fertilisers) and detergents, cause a problem called deoxygenation.

Did You Know?
The main issue with nitrates and phosphates is that they cause algae to grow too quickly (an algal bloom). When the algae die, bacteria decompose them, using up all the dissolved oxygen in the water. This lack of oxygen kills fish and other aquatic life—this process is called deoxygenation (a damaging effect on aquatic life).


Key Takeaway for Section 2: Natural water contains beneficial dissolved oxygen and minerals, but also harmful substances like toxic metals, disease-causing microbes from sewage, and nitrates/phosphates that lead to deoxygenation.


3. Treating the Domestic Water Supply (Purification)

Before natural water can be safely pumped into your house, it must go through several stages of purification. This process makes the water safe to drink (potable).


Step 1: Sedimentation

The raw water (from a reservoir or river) is first passed into large tanks. Here, the water is left still for a long time.

  • Process: Large, heavy solid particles (like mud, sand, and grit) are allowed to settle to the bottom due to gravity.
  • Purpose: To remove the largest suspended solids.

Step 2: Filtration

The water is then passed through layers of sand and gravel.

  • Process: The water passes through filters (often beds of sand or fine gravel).
  • Purpose: To remove the smaller suspended solids that did not settle during sedimentation.

Step 3: Using Carbon (Activated Charcoal)

Even after filtration, the water might have a funny smell or taste due to organic compounds.

  • Process: The water flows through large beds of activated carbon (a form of charcoal).
  • Purpose: The carbon adsorbs (attracts to its surface) the chemicals responsible for tastes and odours.

Step 4: Chlorination

This is the final, and perhaps most critical, step for safety.

  • Process: A controlled amount of chlorine gas or chlorine compounds is added to the water.
  • Purpose: Chlorine is a powerful oxidizing agent that is added to kill harmful microbes (bacteria and viruses) that could cause disease.

Quick Review: The Four Stages of Water Treatment

Sedimentation (removes large solids)
Filtration (removes smaller solids)
Carbon (removes tastes/odours)
Chlorination (kills microbes)


Key Takeaway for Section 3: Domestic water treatment involves sedimentation and filtration to remove solids, followed by using carbon to improve taste/smell, and finally chlorination to ensure the water is potable by killing disease-causing microbes.