Under which combination of pressure and temperature are gases most soluble in water, according to basic gas solubility principles?

Introduction / Context:
The solubility of gases in liquids such as water is an important topic in physical chemistry and environmental science. It helps explain phenomena like why carbonated drinks go flat when warmed and opened, and why cold water can hold more dissolved oxygen for aquatic life than warm water. This question asks you to recall the conditions of pressure and temperature that make gases most soluble in water, a classic concept summarised by Henry law and general solubility rules.

Given Data / Assumptions:

• The solute is a gas and the solvent is liquid water.
• We are varying two variables: pressure above the solution and temperature of the solution.
• We assume no chemical reaction between gas and water that would permanently bind the gas.
• We focus on physical dissolution as in oxygen in water or carbon dioxide in soft drinks.

Concept / Approach:
Henry law states that at a given temperature, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. This means that increasing pressure pushes more gas molecules into the liquid phase, increasing solubility. Temperature has the opposite effect for most gases in liquids: as temperature increases, gas solubility decreases, because the extra thermal energy helps gas molecules escape from the liquid back into the gas phase. Therefore, the condition that maximises gas solubility is high pressure combined with low temperature.

Step-by-Step Solution:
Step 1: Consider the effect of pressure. Higher pressure above the liquid forces more gas molecules into the solution, so solubility increases with pressure. Step 2: Consider the effect of temperature. Most gases become less soluble in water as temperature rises because they are more likely to escape into the gas phase. Step 3: Low temperature therefore favours higher solubility of gases in water. Step 4: Combine the two effects. The best condition for maximum solubility is when pressure is high and temperature is low. Step 5: Among the options, high pressure and low temperature is the only pair that satisfies both requirements simultaneously.

Verification / Alternative check:
Everyday examples confirm this principle. Carbonated drinks are bottled under high pressure and kept cold to keep carbon dioxide dissolved. When you open a warm bottle, the pressure above the liquid drops and the higher temperature encourages gas to escape, so the drink fizzes vigorously and goes flat quickly. In lakes and rivers, cold winter water can hold more dissolved oxygen, which is why fish often do better in cooler, oxygen rich waters. These observations align perfectly with the rule that gas solubility in water increases with pressure and decreases with temperature.

Why Other Options Are Wrong:
- Low pressure and low temperature: Low pressure reduces gas solubility even though low temperature helps, so this is not optimal.
- Low pressure and high temperature: Both low pressure and high temperature decrease gas solubility, so this is the worst possible combination.
- High pressure and high temperature: High pressure helps solubility but high temperature has the opposite effect, so the overall solubility is not maximum.
- Moderate pressure and high temperature: Neither variable is at the optimum, so this cannot lead to maximum solubility compared with high pressure and low temperature.

Common Pitfalls:
Students sometimes remember only one factor, such as pressure, and forget the role of temperature, or vice versa. Another common mistake is to assume that all solubility increases with temperature, which is generally true for solids but often false for gases. A helpful way to remember is to think of gas molecules as wanting to escape: heating gives them energy to leave, while pressure pushes them back into the water. Therefore, cold and high pressure is the ideal combination to keep gases dissolved.

Final Answer:
Gases are most soluble in water under conditions of High pressure and low temperature.