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pH and Salinity » Solvent, Solute, and Solution Concepts

What you'll learn this session

Study time: 30 minutes

  • Understand what solvents, solutes and solutions are in marine environments
  • Learn how water acts as the universal solvent in the ocean
  • Explore how salinity affects marine ecosystems and organisms
  • Discover the relationship between pH and dissolved substances
  • Examine real-world examples of solutions in marine science
  • Understand concentration and saturation in seawater

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Introduction to Solvent, Solute and Solution Concepts

The ocean is essentially one massive solution - a mixture where substances are completely dissolved in water. Understanding how different chemicals dissolve and interact in seawater is crucial for marine science. This knowledge helps us understand everything from why fish can breathe underwater to how coral reefs form and why some areas of the ocean are more acidic than others.

Key Definitions:

  • Solvent: The substance that does the dissolving (in the ocean, this is water).
  • Solute: The substance being dissolved (like salt, oxygen, or carbon dioxide).
  • Solution: The mixture formed when a solute dissolves in a solvent (seawater is a solution).
  • Concentration: How much solute is dissolved in a given amount of solvent.
  • Saturation: The point where no more solute can dissolve in the solvent.

🌊 Water: The Universal Solvent

Water is called the "universal solvent" because it can dissolve more substances than any other liquid. This happens because water molecules are polar - they have a slightly positive end and a slightly negative end. This polarity allows water to surround and separate the particles of many different substances, pulling them apart and dissolving them.

How Solutions Work in Marine Environments

In the ocean, water acts as the solvent for countless different solutes. These include salts (like sodium chloride), gases (like oxygen and carbon dioxide), nutrients (like nitrates and phosphates) and even waste products from marine organisms. The way these substances dissolve and interact affects everything from water density to the availability of nutrients for marine life.

Types of Solutes in Seawater

Seawater contains many different types of dissolved substances, each playing important roles in marine ecosystems:

🧩 Salts

Sodium chloride (table salt) makes up about 85% of dissolved salts. Other salts include magnesium chloride, calcium carbonate and potassium chloride. These give seawater its salty taste and affect its density.

💨 Gases

Oxygen allows marine animals to breathe, while carbon dioxide affects pH levels. Nitrogen is also present but less reactive. The amount of dissolved gases changes with temperature and pressure.

🌱 Nutrients

Nitrates, phosphates and silicates are essential for marine plant growth. These nutrients often limit where and how much marine life can thrive in different ocean areas.

Concentration and Its Effects

The concentration of different solutes in seawater varies around the world and has major effects on marine life. Concentration is usually measured in parts per million (ppm) or parts per thousand (ppt). For example, average seawater has a salinity of about 35 ppt, meaning there are 35 grams of dissolved salts in every 1000 grams of seawater.

Case Study Focus: The Dead Sea

The Dead Sea has a salinity of about 340 ppt - nearly 10 times saltier than normal seawater! This extreme concentration makes the water so dense that people float easily, but it's too salty for most marine life to survive. This shows how concentration affects both the physical properties of water and its ability to support life.

Factors Affecting Solubility

Several factors determine how much of a substance can dissolve in seawater and how quickly it dissolves:

🌡 Temperature Effects

Warmer water generally dissolves more solids (like salts) but fewer gases (like oxygen). This is why tropical waters often have less dissolved oxygen than polar waters, affecting where different marine species can live.

💧 Pressure Effects

Higher pressure allows more gas to dissolve in water. Deep ocean waters can hold more dissolved gases than surface waters. This is important for deep-sea organisms that depend on dissolved oxygen.

pH and Solution Chemistry

The pH of seawater is directly related to the dissolved substances it contains. When carbon dioxide dissolves in seawater, it forms carbonic acid, which lowers the pH (makes it more acidic). This process, called ocean acidification, is happening as more CO₂ from human activities dissolves in the ocean.

The Carbon Dioxide-pH Connection

Carbon dioxide is highly soluble in water, especially cold water. When CO₂ dissolves, it undergoes a chemical reaction:

CO₂ + H₂O → H₂CO₃ (carbonic acid)

This carbonic acid then releases hydrogen ions (H⁺), which make the water more acidic and lower its pH.

🔴 High CO₂

More dissolved CO₂ means more carbonic acid forms, leading to lower pH (more acidic water). This can harm shell-forming organisms like corals and molluscs.

🟢 Balanced CO₂

Normal levels of dissolved CO₂ maintain seawater's natural pH of about 8.1, which is slightly alkaline and suitable for most marine life.

🔵 Low CO₂

Less dissolved CO₂ can lead to higher pH, but this rarely occurs naturally in the ocean. It might happen in areas with lots of photosynthesis by marine plants.

Case Study Focus: Coral Reef Bleaching

Coral reefs are particularly sensitive to changes in seawater chemistry. When the ocean becomes more acidic due to increased dissolved CO₂, it becomes harder for corals to build their calcium carbonate skeletons. Combined with warming temperatures, this leads to coral bleaching - where corals expel their colourful algae partners and turn white. The Great Barrier Reef has experienced several major bleaching events in recent years due to these changing solution properties.

Saturation and Precipitation

Sometimes seawater becomes saturated with certain substances, meaning it can't dissolve any more. When this happens, the excess solute may precipitate out (form solid particles). This process is important in forming marine sediments and some types of marine life.

Examples of Saturation in Marine Environments

Different areas of the ocean can become saturated with different substances:

🪨 Calcium Carbonate Saturation

In warm, shallow waters, seawater often becomes saturated with calcium carbonate. This allows organisms like corals, molluscs and some types of algae to easily extract calcium and carbonate ions to build their shells and skeletons.

Oxygen Supersaturation

In areas with lots of photosynthesis by marine plants, water can become supersaturated with oxygen. Tiny bubbles of oxygen may even form, creating a fizzy effect in the water.

Practical Applications in Marine Science

Understanding solution concepts helps marine scientists in many ways. They can predict how pollutants will spread, understand why certain areas have different types of marine life and monitor changes in ocean chemistry over time.

Monitoring Ocean Health

Scientists regularly measure the concentration of different solutes in seawater to assess ocean health:

📈 Nutrient Levels

High nutrient concentrations can indicate pollution from agricultural runoff, leading to algal blooms that deplete oxygen and harm marine life.

🌡 Temperature-Solubility Tracking

As ocean temperatures rise, the solubility of gases like oxygen decreases, potentially creating "dead zones" where marine life cannot survive.

🔎 pH Monitoring

Regular pH measurements help scientists track ocean acidification and predict its effects on marine ecosystems, especially coral reefs and shell-forming organisms.

Case Study Focus: The Gulf of Mexico Dead Zone

Every summer, a large "dead zone" forms in the Gulf of Mexico where dissolved oxygen levels become too low to support most marine life. This happens because excess nutrients (mainly nitrogen and phosphorus) from agricultural runoff create massive algal blooms. When these algae die and decompose, they use up the dissolved oxygen in the water. This case study shows how human activities can change the solution chemistry of marine environments with devastating effects on marine ecosystems.

Summary

Understanding solvent, solute and solution concepts is fundamental to marine science. Water's role as the universal solvent makes life in the ocean possible, but changes in what's dissolved in seawater can have major effects on marine ecosystems. From the salts that give seawater its properties to the gases that marine animals breathe, every dissolved substance plays a role in the complex chemistry of our oceans. As human activities continue to change ocean chemistry through pollution and climate change, this knowledge becomes increasingly important for protecting marine environments.

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