Question

If trihalomethanes are easily removed from water by aeration (bubbling with air), what does this imply about the vapor pressure of THMs compared to water?

Short answer

The fact that trihalomethanes (THMs) are easily removed from water through aeration implies that the vapor pressure of THMs is higher compared to that of water. This higher vapor pressure means that THMs tend to evaporate more easily from water, making them more susceptible to removal through aeration.

Step-by-step solution

Analyze aeration process

Aeration is a process where water is brought into contact with air through bubbling or spraying to remove dissolved gases and volatile compounds. The efficiency of the aeration process for a specific compound depends on its vapor pressure. The higher the vapor pressure, the more easily the compound is vaporized from the water and removed by aeration.

Review vapor pressure definition

Vapor pressure is a measure of a substance's tendency to evaporate or vaporize. It indicates the pressure at which the substance exists in a gaseous state in equilibrium with its liquid state at a given temperature. High vapor pressure typically means a substance can evaporate readily, whereas low vapor pressure means the substance tends not to vaporize as easily.

Compare vapor pressure of THMs with water

The exercise states that Trihalomethanes (THMs) are easily removed from water through aeration (bubbling with air). Since aeration is more efficient for substances with higher vapor pressure, this implies that the vapor pressure of THMs is higher compared to that of water. As a result, THMs tend to evaporate more easily from water, making them more susceptible to removal through aeration.

Key concepts

Understanding Trihalomethanes

Trihalomethanes (THMs) are chemical compounds that can form when chlorine, used for disinfecting water, reacts with natural organic matter present in the water supply. These compounds are of concern as they have been identified as potential carcinogens and therefore pose a risk to human health when present in drinking water.

The most common THMs include chloroform, bromodichloromethane, dibromochloromethane, and bromoform. One of the primary methods for reducing THM levels in water is through the process of aeration. The ease with which THMs can be removed by aeration indicates that they have a higher tendency to vaporize than water itself, which is reflected in their higher vapor pressure.

This ability to vaporize easily makes it easier for THMs to transition from their liquid state in water to a gas phase when air is bubbled through the water. As THMs are considered volatile organic compounds (VOCs), their removal through aeration improves water quality and safety for consumption.

The Aeration Process Explained

The aeration process is a purification technique used to treat water and remove unwanted chemicals, such as volatile organic compounds and dissolved gases. This is achieved by exposing water to air, either by physically mixing the water with air or by forcing air through the water in the form of bubbles, which is known as bubbling aeration.

The effectiveness of aeration depends on the physical properties of the contaminants, particularly their vapor pressure. During aeration, compounds with a high vapor pressure will volatilize into the air more readily than those with a low vapor pressure. This means that compounds like THMs which have a high vapor pressure compared to water will be preferentially transferred from the liquid phase into the gas phase and then can be vented away from the water.

Factors Influencing Aeration Efficiency

• Temperature of water: Higher temperatures typically increase the vapor pressure of contaminants, improving aeration efficiency.
• Surface area between water and air: The larger the surface area, the more effective the aeration process.
• Duration of aeration: Longer aeration times allow more extensive removal of volatile compounds.

Equilibrium States in Vapor Pressure

At a given temperature, each substance has a characteristic pressure at which it can exist in equilibrium as both a liquid and a gas; this characteristic pressure is known as the substance's vapor pressure. Equilibrium is the state where the rate of evaporation of the liquid equals the rate of condensation of the gas, resulting in a stable system.

Understanding equilibrium states is crucial in explaining why substances with a high vapor pressure, such as THMs, are more readily removed by aeration. If a substance's vapor pressure is higher than the ambient pressure, the substance will tend to evaporate until either the ambient pressure increases or its vapor pressure decreases due to cooling or a phase change. For THMs, the equilibrium state favors the gaseous form over the liquid when exposed to air, which is why they can be effectively removed from water during the aeration process.

When water containing THMs is aerated, the vapor pressure of the THMs drives them out of the water and into the air, pushing the equilibrium state to favor the gaseous state, thus allowing for their efficient removal. This process is an excellent illustration of how understanding the principles of chemistry, such as equilibrium and vapor pressure, enables us to devise methods to ensure the safety and purity of drinking water.