Henry's Law
Henry’s Law is the key principle that explains how oxygen dissolves in water, and it's highly relevant in areas like environmental science, biology, and aquatic chemistry.________________________________________Henry’s Law – Definition
Henry's Law states:
The amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas above the liquid, at a constant temperature.Mathematically:
C=kH⋅PC = k_H \cdot PC=kH⋅P
Where:
• CCC = concentration of dissolved gas (mol/L or mg/L),• kHk_HkH = Henry's Law constant (depends on the gas, solvent, and temperature),• PPP = partial pressure of the gas above the liquid (usually in atmospheres or mmHg).________________________________________Henry’s Law Applied to Oxygen in Water
In the case of oxygen dissolving in water:
• The concentration of dissolved oxygen (DO) increases when the partial pressure of oxygen (O₂) in the air increases.
• Colder water can hold more oxygen because Henry’s constant decreases with lower temperature (meaning more gas dissolves).
• At higher altitudes, there's less atmospheric pressure → less oxygen in the air → less oxygen dissolved in water.________________________________________Real-World Implications
1. Aquatic Life: Fish and other organisms rely on oxygen dissolved in water. Henry’s Law explains why:
o DO drops in warm water → fish become stressed.o High elevation lakes have less DO.o Aeration (adding air/oxygen to water) helps increase DO by increasing the partial pressure of oxygen.
2. Environmental Monitoring:
o Scientists monitor dissolved oxygen levels to assess water quality.o Thermal pollution (like warm water from industrial discharge) lowers DO and harms ecosystems.3. Decompression Sickness (The Bends):o Divers absorb more nitrogen (not oxygen) at high pressure underwater.o Upon surfacing quickly, nitrogen comes out of solution—Henry's Law in action.________________________________________Key Takeaways
• More pressure = more dissolved oxygen.• Higher temperature = less dissolved oxygen.• Henry’s Law helps explain oxygen behavior in aquatic environments, biology, and gas exchange systems.
Understanding how temperature and pressure affect the solubility of oxygen in water is crucial for various scientific and environmental applications. Below are explanations and visual representations illustrating these relationships.
Oxygen Solubility vs. Temperature
As temperature increases, the solubility of oxygen in water decreases. This inverse relationship means that colder water can hold more dissolved oxygen than warmer water.
Oxygen solubility in fresh water at different temperatures. perrybrake.comFor example, at sea level (1 atm pressure): • At 0°C (32°F), water can dissolve approximately 14.6 mg/L of oxygen.• At 20°C (68°F), this decreases to about 9.1 mg/L.• At 35°C (95°F), the solubility further drops to around 7.0 mg/L. This trend is significant for aquatic life, as warmer waters may not provide sufficient oxygen for certain species to thrive. Oxygen Solubility vs. Pressure According to Henry's Law, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. Therefore, as pressure increases, the amount of oxygen that dissolves in water also increases.
Oxygen solubility in water at various pressures. USGS+1The Engineering ToolBox+1waterontheweb.org For instance, at a constant temperature: • At 1 atm pressure, the solubility might be 9 mg/L.• At 2 atm, it increases to approximately 18 mg/L.• At 3 atm, the solubility can reach around 27 mg/L. This principle is utilized in practices like hyperbaric oxygen therapy and affects how aquatic organisms respond to changes in water depth. Combined Effects of Temperature and Pressure Both temperature and pressure simultaneously influence oxygen solubility. In deep waters, higher pressures can increase oxygen solubility, but if the water is also warmer at those depths, the temperature effect may offset the pressure effect. Understanding the balance between these factors is essential in fields like oceanography and environmental science. For comprehensive tables and calculators to determine oxygen solubility under varying conditions, resources like the U.S. Geological Survey's DOTABLES provide valuable tools. U.S. Geological Survey+2USGS+2USGS+2 These visual aids and explanations should help clarify how temperature and pressure impact the solubility of oxygen in water.
Oxygen solubility in fresh water at different temperatures. perrybrake.comFor example, at sea level (1 atm pressure): • At 0°C (32°F), water can dissolve approximately 14.6 mg/L of oxygen.• At 20°C (68°F), this decreases to about 9.1 mg/L.• At 35°C (95°F), the solubility further drops to around 7.0 mg/L. This trend is significant for aquatic life, as warmer waters may not provide sufficient oxygen for certain species to thrive. Oxygen Solubility vs. Pressure According to Henry's Law, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. Therefore, as pressure increases, the amount of oxygen that dissolves in water also increases.
Oxygen solubility in water at various pressures. USGS+1The Engineering ToolBox+1waterontheweb.org For instance, at a constant temperature: • At 1 atm pressure, the solubility might be 9 mg/L.• At 2 atm, it increases to approximately 18 mg/L.• At 3 atm, the solubility can reach around 27 mg/L. This principle is utilized in practices like hyperbaric oxygen therapy and affects how aquatic organisms respond to changes in water depth. Combined Effects of Temperature and Pressure Both temperature and pressure simultaneously influence oxygen solubility. In deep waters, higher pressures can increase oxygen solubility, but if the water is also warmer at those depths, the temperature effect may offset the pressure effect. Understanding the balance between these factors is essential in fields like oceanography and environmental science. For comprehensive tables and calculators to determine oxygen solubility under varying conditions, resources like the U.S. Geological Survey's DOTABLES provide valuable tools. U.S. Geological Survey+2USGS+2USGS+2 These visual aids and explanations should help clarify how temperature and pressure impact the solubility of oxygen in water.
