🌊 Marine Science Study Notes: Sea Water - pH and Salinity (Section 2.2)
Hello Marine Scientists! This chapter is super important because it explains two key properties of seawater: pH (how acidic or alkaline the water is) and salinity (how salty it is). These factors control where marine life can survive. Let's dive in!
1. The Building Blocks of Sea Water: Elements, Compounds, and Mixtures (Syllabus 2.2.1)
To understand what makes up sea water, we need to quickly review three basic chemistry terms:
A. Elements and Compounds
- Element: A pure substance that cannot be broken down into simpler substances. It consists of only one type of atom.
Example: Oxygen (\(O\)) is an element essential for marine respiration. - Compound: A substance formed when two or more different elements are chemically bonded together in a fixed ratio.
Example: Calcium carbonate (\(CaCO_3\)) is a compound that makes up the shells of corals and molluscs.
B. Mixtures
A mixture is made of two or more substances that are mixed together but are not chemically combined. You can separate them easily (like sand and water).
- Example: Sea water is the ultimate mixture! It is primarily water, with lots of dissolved salts, gases, and other substances mixed in.
Quick Takeaway: Sea water is a mixture. Its components (like oxygen and calcium carbonate) are elements or compounds.
2. Understanding pH: Acidity and Alkalinity (Syllabus 2.2.2 & 2.2.3)
The pH scale measures how acidic or alkaline (basic) a liquid is. It runs from 0 to 14.
A. Describing pH
- Neutrality: pH = 7. Pure water is neutral.
- Acidity: pH is less than 7 (pH < 7). The lower the number, the more acidic the substance (e.g., vinegar, lemon juice).
- Alkalinity (or Basicity): pH is greater than 7 (pH > 7). The higher the number, the more alkaline the substance (e.g., baking soda, soap).
Did you know? Healthy open-ocean water is naturally slightly alkaline, typically having a pH of about 8.1. Marine organisms need this stable pH to survive and build their shells.
B. Investigating pH (PA 2.2.3, 2.2.4)
You can use universal indicator or a pH meter to measure the pH of water samples like sea water, fresh water, and rain water in the lab (PA 2.2.3).
Ocean Acidification: The Effect of Carbon Dioxide (CO₂)
When carbon dioxide from the atmosphere dissolves into sea water, it makes the water more acidic (lower pH).
- What happens? Adding CO₂ to sea water causes the pH to decrease.
- Why is this a problem? Many marine organisms, especially those that build skeletons or shells out of calcium carbonate (like corals and shellfish), struggle to survive and grow when the water becomes even slightly more acidic. This process is known as ocean acidification.
Quick Takeaway: Sea water is slightly alkaline (pH ≈ 8.1). Adding atmospheric CO₂ lowers this pH, threatening shell-building organisms.
3. The Composition of Sea Water: Salts and Solutions (Syllabus 2.2.5)
Sea water is often called a "universal solution" because water is great at dissolving things.
A. Key Terms for Solutions
- Solvent: The substance that does the dissolving. In sea water, the solvent is water.
- Solute: The substance that is dissolved. In sea water, the main solutes are salts.
- Solution: The homogeneous mixture formed when the solute dissolves in the solvent (Solvent + Solute = Solution). Sea water is a solution.
B. Dissolving and Solubility
- Dissolve: The process where a solute mixes completely with a solvent.
- Soluble: A substance that can dissolve in a solvent (like sodium chloride in water).
- Insoluble: A substance that cannot dissolve in a solvent (like sand in water).
C. The Salts in Sea Water
The dissolved solutes are collectively called salts. The most abundant salts in sea water include:
- Sodium chloride (table salt, the most common salt)
- Magnesium sulfate
- Calcium carbonate (important for corals and shells)
D. Investigating Solubility (PA 2.2.6)
You may investigate the effect of temperature on the solubility of a salt (solute) in water. Generally, for solids like salts, increasing the temperature usually increases how much solute can be dissolved in the water.
Quick Takeaway: Water is the solvent, and the dissolved salts (like sodium chloride) are the solutes, forming the sea water solution.
4. Salinity: How Salty is the Ocean? (Syllabus 2.2.7 & 2.2.8)
Salinity is simply the concentration of dissolved salts in water.
A. Measurement of Salinity
- Unit: Salinity is measured in parts per thousand (ppt).
- What does 'ppt' mean? If the salinity is 35 ppt, it means there are 35 grams of dissolved salts in every 1000 grams (or 1 kg) of sea water.
- Typical Ocean Salinity: The average salinity of the open ocean is about 35 ppt.
B. Comparing Salinities Around the World (Syllabus 2.2.8)
Salinity varies greatly depending on location:
- Typical Salinity: The Pacific Ocean generally has a typical salinity of around 35 ppt.
- High Salinity: The Red Sea has a very high salinity (often above 40 ppt). This is because it is in a hot, dry region where evaporation is very high, and there is almost no fresh water input from rivers.
- Low Salinity: The Baltic Sea has a low salinity (sometimes below 10 ppt). This is due to large amounts of run-off and fresh water input from many surrounding rivers, combined with low rates of evaporation.
Quick Takeaway: Salinity is measured in ppt. High evaporation makes salinity rise (Red Sea), while lots of freshwater input makes it fall (Baltic Sea).
5. Environmental Factors Affecting Salinity (Syllabus 2.2.9)
Salinity is constantly changing based on environmental factors:
A. Factors that INCREASE Salinity (making it saltier)
- Evaporation: When water evaporates (turns into gas) due to high temperature, the pure water leaves, but the dissolved salts are left behind. This increases the concentration of salts.
- Erosion of rocks on land: Rain and rivers weather rocks, dissolving minerals (salts) and carrying them into the ocean.
- Temperature: High temperatures increase evaporation, leading to higher salinity.
B. Factors that DECREASE Salinity (making it less salty)
- Precipitation (Rain/Snow): Adds fresh water directly to the ocean surface, diluting the salts.
- Run-off: Fresh water flowing from land (rivers, streams) into the ocean surface, diluting the salts.
- Melting of ice sheets and glaciers: When large land-based ice masses melt, they release huge amounts of fresh water into the sea. (Note: Melting sea ice doesn't change overall salinity much, but melting land ice does).
Don't worry if this seems tricky at first—just remember: if you add fresh water, salinity goes down; if you take away fresh water, salinity goes up!
Quick Takeaway: Salinity is a balancing act between evaporation (removes water, increases salt) and precipitation/run-off (adds water, decreases salt).
6. Estuaries: Where Fresh Meets Salt (Syllabus 2.2.10 & 2.2.11)
Estuaries are critical habitats where these salinity changes are most dramatic.
A. Defining an Estuary
An estuary is a partly enclosed body of water where a river flows into the sea. Think of it as the transition zone between fresh water and the marine environment.
B. Salinity Changes during the Tidal Cycle
Estuaries are tidal bodies of water, meaning the water level and movement are affected by the tides.
- High Tide: During high tide, a larger volume of salty sea water flows into the estuary, mixing with the river water. Salinity levels in the estuary increase.
- Low Tide: During low tide, the sea water recedes, and the flow of fresh river water dominates. Salinity levels in the estuary decrease significantly.
Because of this constant mixing and fluctuation, organisms living in estuaries (like mussels and crabs) must be highly adapted to tolerate rapid and large changes in salinity.
Quick Review: pH and Salinity
pH: How acidic/alkaline water is (scale 0-14). Sea water is slightly alkaline (≈ 8.1). Carbon dioxide lowers pH.
Salinity: Concentration of dissolved salts (ppt). Affected by evaporation (increases salinity) and run-off/rain (decreases salinity).
Estuaries: Mixing zones where salinity changes constantly due to tides.