Dissolved Substances in Natural Water

Introduction

Water is an excellent solvent, so almost every sample of natural water carries dissolved substances. Rain picks up gases from the air, rivers collect minerals from the rocks they flow over, and the sea gathers everything the rivers bring. Tasting these three kinds of water would give very different results: rain is almost tasteless, river water may taste slightly of minerals, and seawater is far too salty to drink. This page explains where the differences come from.

Core explanation

Rainwater. Water evaporating from the sea leaves its dissolved salts behind, so the vapour that forms clouds is nearly pure. As droplets fall, they dissolve gases from the air — oxygen, nitrogen and, importantly, carbon dioxide. Carbon dioxide reacts slightly with water to form carbonic acid, so even clean rain is weakly acidic, with a pH of about 5.6. Rain may also pick up dust, sea spray near coasts, and pollutant gases such as sulfur dioxide near industrial areas. Total dissolved solids are very low — usually a few milligrams per litre.

River and ground water. When rain soaks into soil and rocks, its weak acidity helps it react with minerals. This chemical weathering releases ions such as calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), hydrogencarbonate (HCO₃⁻), sulfate (SO₄²⁻) and chloride (Cl⁻). River water typically contains tens to a few hundred milligrams of dissolved solids per litre. The exact mixture depends on the local rocks: rivers flowing over limestone carry lots of calcium and hydrogencarbonate ions, while rivers on granite carry far fewer. River water also contains dissolved oxygen, which fish need, plus organic matter from plants and soil.

Seawater. Rivers carry dissolved ions to the sea. When seawater evaporates, the water leaves but the ions stay behind. Over hundreds of millions of years, this has made the oceans salty: typical seawater contains about 35 g of dissolved salts per kilogram. The main ions are chloride and sodium, followed by sulfate, magnesium, calcium and potassium.

Water Typical dissolved solids Main dissolved substances --- --- --- Rain a few mg/L O₂, N₂, CO₂ (as carbonic acid) River about 50–500 mg/L Ca²⁺, HCO₃⁻, Mg²⁺, SO₄²⁻, O₂ Sea about 35 000 mg/L Na⁺, Cl⁻, SO₄²⁻, Mg²⁺

Why so much sodium chloride in the sea? Calcium and hydrogencarbonate ions are removed from seawater by living things that build shells and skeletons of calcium carbonate. Sodium and chloride ions are not used up in this way, so they have built up to become the dominant ions.

Step-by-step reasoning

To explain the dissolved substances in a water sample:

1. Ask where the water has been: air only, rocks and soil, or a long time in the sea. 2. Gases dissolve from the air; ions come from rocks. 3. Evaporation removes water but leaves dissolved ions behind, concentrating them. 4. Living things may remove some ions (such as calcium for shells).

Visual explanation

Draw three beakers labelled rain, river and sea. Show a handful of dots in the rain beaker, labelled as dissolved gases. Show more dots, many labelled Ca²⁺ and HCO₃⁻, in the river beaker. Crowd the sea beaker with dots, mostly Na⁺ and Cl⁻. Arrows show rain flowing into the river and the river flowing into the sea, with an evaporation arrow rising from the sea carrying only H₂O.

Real-world analogy

The sea is like a pan of soup left simmering with the lid off. You keep topping it up with lightly salted stock (river water), but steam (pure water vapour) keeps escaping. Over time the soup gets saltier and saltier, even though each cup of stock added is only slightly salty.

Real-world example

Bottled mineral water labels list the dissolved ions in milligrams per litre. A water from a limestone region might show over 100 mg/L of calcium, while one from volcanic or granite rock may show under 10 mg/L. These differences give mineral waters their distinct tastes.

Why?

Why is rain not salty, even though most of it evaporated from the sea? Evaporation separates water from its solutes: only water molecules escape into the vapour, while the ions, which have no tendency to become gases at these temperatures, stay in the sea. This is the same principle used in distillation.

Common misconception

"Rainwater is pure water." Rain has dissolved gases, especially carbon dioxide, which makes it slightly acidic, and it can carry dust and pollutants. It is much purer than river or seawater, but it is not chemically pure.

Worked example

Question: A 1.0 kg sample of seawater is evaporated to dryness and leaves 35 g of solid. What percentage of the seawater's mass was dissolved salts?

Reasoning: Percentage = (mass of salt ÷ mass of seawater) × 100 = (35 ÷ 1000) × 100.

Answer: 3.5%.

Quick check

1. Which gas from the air makes clean rainwater slightly acidic? Answer: Carbon dioxide, which forms carbonic acid in water.

Exam focus

Explain differences in composition by linking each type of water to its source: gases from the air, ions from rock weathering, and concentration of ions by evaporation. Remember that clean rain has a pH of about 5.6, not 7.

Advanced insight

The ocean's composition is roughly steady today because ions are removed as fast as rivers add them — through shell-building organisms, formation of sediments and reactions at hot springs on the sea floor. Scientists express this balance as the "residence time" of each ion: sodium stays in the ocean for tens of millions of years, while calcium is removed much faster.

Summary

Natural water always contains dissolved substances. Rain holds dissolved gases, including carbon dioxide, and is slightly acidic. River water gains ions such as Ca²⁺, Mg²⁺ and HCO₃⁻ from weathered rocks, in amounts that depend on local geology. Seawater is salty because rivers deliver ions and evaporation removes only water; sodium and chloride dominate at about 35 g of salts per kg.

Practice questions

1. Name two ions commonly found in river water that flows over limestone. Answer: Calcium ions (Ca²⁺) and hydrogencarbonate ions (HCO₃⁻). 2. Explain why the sea is salty but rain is not. Answer: Rivers carry dissolved ions into the sea; when seawater evaporates, only water leaves, so the ions accumulate. Rain forms from this evaporated water, which contains no salts. 3. Why does river water usually contain more dissolved solids than rainwater? Answer: As the water passes through soil and over rocks, it dissolves minerals by chemical weathering, gaining ions. 4. Why do calcium ions not build up in seawater as much as sodium ions? Answer: Marine organisms use calcium to build calcium carbonate shells and skeletons, removing it from the water.