12.1.1 Water as Solvent

This topic explains why water is an excellent solvent and how natural water sources become impure by dissolving substances from the environment.

Definition

Water as a solvent means water has the ability to dissolve many substances, making it a universal solvent.

Simple explanation: Water can dissolve more things than any other liquid — that's why it's called the universal solvent.

Main Idea

Water is very good at dissolving substances. For this reason, natural water such as rainwater and groundwater is not pure water.

• Gases from the atmosphere
• Minerals from rocks and soil
• Substances from decaying plants and animals

Simple explanation: Water dissolves things wherever it goes, so natural water is never pure.

Core Concept

How Natural Water Becomes Impure

Simple explanation: Water picks up different substances as it falls through the air and flows through the ground.

Detailed Explanation

Water as a Solvent

• Gases — oxygen (O₂), carbon dioxide (CO₂), nitrogen (N₂)
• Salts — sodium chloride (NaCl), calcium carbonate (CaCO₃)
• Minerals — from rocks and soil
• Organic substances — from decaying plants and animals
• Acids — like nitric acid (HNO₃) formed during thunderstorms

Simple explanation: Water dissolves gases, salts, minerals, and organic matter.

Rainwater Is Not Pure

• A little oxygen
• A little nitrogen
• A little carbon dioxide
• Dust particles

During thunderstorms, it also dissolves nitric acid.

Simple explanation: Rainwater is not pure — it contains dissolved gases and dust from the air.

Groundwater Is Not Pure

Groundwater dissolves minerals from rocks and soils as it moves along or beneath the Earth's surface.

Groundwater also dissolves many substances from decaying plants and animals.

Simple explanation: Groundwater picks up minerals and organic matter as it flows through the ground.

Why This Matters

• Natural water is never pure — it always contains dissolved substances.
• Some dissolved substances are beneficial (like minerals).
• Some dissolved substances are harmful (like pollutants).
• Water treatment is needed to make water safe for drinking.

Simple explanation: Water's dissolving power means we need to treat it before drinking.

Intermediate Level

Why Water Is Such a Good Solvent

• One end is slightly positive (δ+).
• The other end is slightly negative (δ−).

• Attract positive and negative ions.
• Surround solute particles.
• Pull them apart and dissolve them.

Simple explanation: Water's polarity lets it pull apart and surround many different substances.

What Rainwater Contains

Simple explanation: Rainwater contains dissolved gases, dust, and sometimes acid from thunderstorms.

What Groundwater Contains

Simple explanation: Groundwater contains minerals and organic matter from the ground.

Beneficial vs Harmful Substances in Natural Water

Simple explanation: Natural water contains both helpful and harmful substances.

Why Water Treatment Is Needed

• Water treatment removes:
• Harmful microbes
• Toxic metals
• Excess minerals
• Organic pollutants
• Treatment makes water safe for drinking.

Simple explanation: We treat water to remove harmful substances before drinking it.

Important Points

• - Water is a very good solvent — it dissolves more substances than any other liquid.
• - Natural water (rainwater, groundwater) is not pure water.
• - Rainwater dissolves oxygen, nitrogen, carbon dioxide, and dust particles from the atmosphere.
• - During thunderstorms, rainwater also dissolves nitric acid.
• - Groundwater dissolves minerals from rocks and soils.
• - Groundwater also dissolves substances from decaying plants and animals.
• - Water treatment is needed to make natural water safe for drinking.

Remember

• - Water = universal solvent.
• - Rainwater = dissolves gases (O₂, N₂, CO₂) + dust.
• - Thunderstorms → nitric acid in rainwater.
• - Groundwater = dissolves minerals from rocks/soil + organic matter.
• - Natural water = never pure.
• - Water treatment = needed for safe drinking water.

Common Mistakes

• 1. Thinking rainwater is pure. Rainwater contains dissolved gases and dust particles.
• 2. Thinking groundwater is pure. Groundwater contains dissolved minerals and organic matter.
• 3. Thinking natural water is safe to drink. Natural water may contain harmful microbes and pollutants.
• 4. Forgetting that water dissolves gases. Water dissolves oxygen, nitrogen, and carbon dioxide from the air.
• 5. Thinking only groundwater has minerals. Rainwater also contains dissolved substances.
• 6. Thinking water treatment is unnecessary. Natural water needs treatment to remove harmful substances.

Quick Revision

• Water as solvent: water is very good at dissolving substances; for this reason natural water such as rainwater and groundwater is not pure water.
• As water falls through the atmosphere, it dissolves:
• A little oxygen
• A little nitrogen
• A little carbon dioxide
• Dust particles
• During thunderstorms, it also dissolves nitric acid.
• Groundwater dissolves:
• Minerals from rocks and soils as it moves along or beneath the Earth's surface.
• Many substances from decaying plants and animals.
• Because water dissolves so many substances, natural water is never pure.
• Water treatment is needed to make water safe for drinking.

60-Second Revision

• Water = universal solvent.
• Rainwater = dissolves gases + dust.
• Groundwater = dissolves minerals + organic matter.
• Natural water = never pure.
• Warning: natural water contains dissolved substances — it is not pure water.