Question

(a) Explain why the concentration of dissolved oxygen in freshwater is an important indicator of the quality of the water. (b) Find graphical data in the text that show variations of gas solubility with temperature, and estimate to two significant figures the percent solubility of \(\mathrm{O}_{2}\) in water at \(30^{\circ} \mathrm{C}\) as compared with \(20^{\circ} \mathrm{C} .\) How do these data relate to the quality of natural waters?

Short answer

(a) Dissolved oxygen (O2) in freshwater is important because it supports the survival and health of aquatic organisms, indicating good water quality. Low concentration of O2 may signal pollution or environmental issues affecting the ecosystem. (b) Referring to the graphical data, estimate the solubility of O2 in water at 30°C and 20°C and compute the percentage difference using the formula: Percentage difference = \(\frac{\textrm{Solubility at 30}^\circ\textrm{C} - \textrm{Solubility at 20}^\circ\textrm{C}}{\textrm{Solubility at 20}^\circ\textrm{C}} \times 100\%\) Higher temperatures typically lead to lower O2 solubility, negatively impacting water quality and aquatic ecosystems. Monitoring gas solubility at different temperatures is essential for maintaining healthy aquatic environments.

Step-by-step solution

(a) Importance of dissolved oxygen concentration in freshwater

Dissolved oxygen (O2) is an essential component of the water for the survival and health of aquatic organisms. A higher concentration of dissolved oxygen in water indicates good water quality, while a low concentration of dissolved oxygen can lead to adverse effects on aquatic life and may indicate water pollution or other environmental issues. Oxygen dissolves in water through the interaction of water with the atmosphere and photosynthesis by aquatic plants. Good water quality supports a diverse ecosystem and helps maintain a balanced aquatic environment.

(b) Finding and analyzing graphical data

To find the graphical data showing variations in gas solubility with temperature, refer to the relevant text or diagram in the given material. The graph should display the solubility of different gases, including oxygen, as a function of temperature.

(b.1) Estimating percent solubility of O2 at 30°C compared to 20°C

Analyze the graph to determine the solubility of O2 in water at 30°C and 20°C. Keep in mind that it's an estimate, so the values will likely not be exact. Next, calculate the percentage difference between the solubility of O2 in water at the two temperatures. Use the following formula: Percentage difference = \(\frac{\textrm{Solubility at 30}^\circ\textrm{C} - \textrm{Solubility at 20}^\circ\textrm{C}}{\textrm{Solubility at 20}^\circ\textrm{C}} \times 100\%\)

(b.2) Relating data to the quality of natural waters

The comparison between the solubility of O2 at 30°C and 20°C provides insight into the effects of temperature on dissolved oxygen levels and, ultimately, water quality. Generally, gas solubility, including oxygen, decreases with increasing temperature. Therefore, a rise in temperature leads to a decrease in dissolved oxygen levels, which can negatively impact the quality and health of natural waters. This can result in potential harm to aquatic life, reducing biodiversity and aquatic ecosystem health. So, monitoring gas solubility at various temperatures is essential to ensure aquatic ecosystems' overall health and quality.

Key concepts

Water Quality Indicators

Water quality is crucial for maintaining healthy ecosystems and ensuring the safety of the water for both human use and aquatic life. Among the several indicators of water quality, dissolved oxygen (DO) levels play a pivotal role.

Dissolved oxygen is the amount of oxygen present in water and is essential for the survival of fish, invertebrates, and other aquatic organisms. It is introduced into the water through natural processes such as atmospheric diffusion and photosynthesis by water plants.

Here are some key points about dissolved oxygen as a water quality indicator:

• A high DO level generally indicates good water quality, supporting a robust ecosystem.
• Low DO levels can be a sign of pollution, excessive nutrients, or other problems, and can lead to hypoxia or anoxic conditions, which are harmful to aquatic life.
• Measuring DO is also a way to check the effectiveness of wastewater treatment and the health of water bodies.

The concentration of dissolved oxygen can vary daily and seasonally, and it is affected by temperature, salinity, atmospheric pressure, and the presence of organic materials and microorganisms. Monitoring dissolved oxygen is, therefore, a fundamental aspect of water quality assessments.

Gas Solubility with Temperature

The solubility of gases in water is an important concept, especially when discussing the quality and health of aquatic ecosystems. A key factor influencing this is temperature.

Generally, gas solubility in water decreases as the temperature increases. This is because higher temperatures provide more energy to water molecules, which in turn disrupts the ability of the gas molecules to remain dissolved. Specifically for dissolved oxygen:

• At higher temperatures, the kinetic energy of water molecules increases, which can force oxygen molecules out of the water.
• As a result, warmer water holds less dissolved oxygen compared to cooler water.

This relationship is particularly important to consider in the context of climate change and thermal pollution, as rising temperatures can impact the levels of dissolved oxygen and thereby the health of aquatic environments.

Effect of Temperature on Aquatic Ecosystems

Temperature has far-reaching effects on aquatic ecosystems, influencing not only the solubility of gases like oxygen but also the metabolic rates of organisms, the availability of nutrients, and overall habitat conditions.

Aquatic organisms are, for the most part, cold-blooded, meaning their body temperature is regulated by their environment. Here’s an overview of how temperature can impact them:

• Warmer waters can increase metabolic rates, which in turn raises the oxygen demand of these organisms.
• Elevated temperatures can lead to harmful algal blooms, reducing water quality.
• Temperature also affects the reproduction, growth, and migration patterns of aquatic life.

Thus, understanding and monitoring the temperature is vital for managing and preserving the delicate balance of aquatic ecosystems. The decrease in dissolved oxygen concentration with increasing temperature emphasizes the need for regulations and measures to help mitigate thermal impacts on water bodies.