Seawater contains many dissolved substances. A scientist collects seawater samples from two locations: one near a busy shipping lane and one from a remote area of the open ocean far from human activity. The scientist measures the concentration of several dissolved ions in each sample. Describe what is meant by the term 'solution' and explain why seawater is considered a complex solution containing many different dissolved substances.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seawater is sometimes described as a complex solution. It contains dissolved salts, gases, and other substances that have entered the oceans through a variety of natural and human processes over millions of years.
Model answer (5 marks)
A solution is a homogeneous mixture formed when a solute dissolves in a solvent.
In seawater the solvent is water.
Seawater therefore contains many different solutes – for example sodium chloride, magnesium sulphate, calcium carbonate and other salts, as well as dissolved gases such as CO₂ and O₂.
The variety of solutes comes from rivers that carry dissolved minerals and ions from the weathering of rocks on land into the sea.
Atmospheric gases also dissolve into seawater, adding further complexity.
In seawater the solvent is water.
Seawater therefore contains many different solutes – for example sodium chloride, magnesium sulphate, calcium carbonate and other salts, as well as dissolved gases such as CO₂ and O₂.
The variety of solutes comes from rivers that carry dissolved minerals and ions from the weathering of rocks on land into the sea.
Atmospheric gases also dissolve into seawater, adding further complexity.
Examiner tips
- Define ‘solution’ and identify water as the solvent. List at least two types of solutes (salts and gases). Explain the source of these solutes – rivers and the atmosphere. Use correct terminology such as ‘solvent’, ‘solute’, ‘dissolved’ and ‘complex solution’.
Common mistakes
- Confusing ‘solution’ with ‘mixture’ or ‘suspension’. Failing to name water as the solvent. Omitting the role of rivers or atmospheric gases. Using vague terms like ‘many substances’ without specifying salts or gases.
Mark scheme (5 marks)
- A solution is a mixture formed when a solute dissolves in a solvent
- In seawater, water is the solvent
- Seawater contains many different solutes / dissolved substances such as sodium chloride or other named salts or dissolved gases
- Rivers carry dissolved minerals / ions into the sea from the weathering or erosion of rocks on land
- Gases such as carbon dioxide or oxygen dissolve from the atmosphere into seawater, adding to its complexity
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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