Aquatic Ecosystem - Chemical Reactions and Water Quality PDF

# Chapter 1: Aquatic Ecosystem ## 1-1 Chemical Reactions and Their Impact on Water Quality ### Get Ready Have you ever thought when you drink a glass of water, about the chemical reactions that may occur within this vital liquid? Water is not just a transparent liquid; it is a medium in which m...

# Chapter 1: Aquatic Ecosystem ## 1-1 Chemical Reactions and Their Impact on Water Quality ### Get Ready Have you ever thought when you drink a glass of water, about the chemical reactions that may occur within this vital liquid? Water is not just a transparent liquid; it is a medium in which many chemical compounds may react, affecting the quality of water and the health of living organisms that depend on it. In this chapter, we will learn about the hydrosphere and the water cycle in nature, as well as some of the basic physical properties and chemical reactions that occur in water, and how these properties and reactions can affect the components of the environment. ### Learn Water has unique properties that support life. Water can dissolve many chemicals and can exist in all three states of matter - solid, liquid, and gaseous states - within the range of known temperatures on the Earth’s surface. Water is essential to the continuation of life on Earth. All living cells have a membrane that separates the organism from its environment. Water passes from the environment to the inside of the living cell through this membrane, carrying the substances needed to produce energy, as well as eliminating waste products to the outside. ### The Hydrosphere on Earth: The hydrosphere distinguishes Earth from other planets in the solar system. About 70% of the Earth’s surface is covered by water. About 97% of this water is found in the oceans, seas, and salt lakes as salt water. The fresh water is found in rivers, freshwater lakes and groundwater represents approximately 1%. And the reminder part represents the frozen water in polar regions, mountain peaks and glaciers. Egypt is characterized by its diverse aquatic environments, which include the Nile River, the Gulf of Suez, the Gulf of Aqaba, the Red Sea, the Mediterranean Sea, and many salt and freshwater lakes. ## Chemical Reactions and Their Impact on Water Quality ### Water Cycle in Nature Water exists on or near the Earth's surface in a state of continuous change between its three states. Water is constantly moving from one place to another in many different paths that form a nearly closed system called the water cycle in nature or the hydrologic cycle. The water cycle as a system is capable of changing the Earth's surface physically, chemically, and biologically. The water cycle in nature mainly includes the process of evaporation, which contributes to the formation of clouds and the process of rain or snowfall. In addition to other biological processes such as transpiration in plants, respiration in plants and animals, and water leakage through the pores of soil and sedimentary rocks to form groundwater. Water vapor in clouds may react chemically with compounds in the air, forming some acids that fall as acid rain, which decomposes rocks. ### Chemical Structure of Water: Water is composed of the two elements hydrogen and oxygen, in the ratio of 2: 1 by volume, respectively. Oxygen represents 88.89% of the mass of the water molecule and hydrogen represents 11.11%. The two hydrogen atoms are connected to the oxygen atom by two covalent bonds with an angle of about 104.5° between them. ### Chemical Properties of Water: Water does not exist on Earth in a pure form as it contains many ions and chemicals that interact with it in different ways. Here are three of the main properties of water: #### 1- Water Polarity: The oxygen atom is characterized by its higher electronegativity than the hydrogen atom, so the bonding electrons are attracted towards the oxygen atom, forming a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atom, which is known as the polarity of the water molecule. The polarity of water molecules causes them to bond with other water molecules or polar molecules of other substances to form hydrogen bonds, which gives water the ability to dissolve many salts and break them down into hydrated ions. #### Example: Dissolving Sodium Chloride Salt in Water: $NaCl_{(s)} + H_2O \rightleftharpoons Na^+_{(aq)} + Cl^-_{(aq)}$ #### The ability of water molecules to form hydrogen bonds with each other is also a key reason pure water has a higher boiling point of 100°C at normal atmospheric pressure than compounds of similar structure, such as hydrogen sulphide, which boils at -61°C. ### 2- Hydrolysis (Hydration): A small percentage of water molecules exist as hydrogen ions (H+) and hydroxide ions (OH-). As a result of chemical reactions of water with different compounds, hydrolysis of some salts, present in natural water, may occur. This hydrolysis affects the balance of these ions in water, leading to acidity or alkalinity of the water. #### A Practical Example: When table salt (NaCl) is added to water, it dissociates into sodium ion (Na+) and chloride ion (Cl) and the salt ions remain in solution without binding to water ions, making the solution neutral because the concentration of hydrogen ions (H+) is equal to the concentration of hydroxide ions (OH-). In the case of sodium bicarbonate salt (NaHCO3), hydrolysis of the salt leads to a decrease in the concentration of hydrogen ions (H+) and an increase in the concentration of hydroxide ions (OH-), making the salt solution basic. The opposite happens when ammonium chloride salt (NH4Cl) dissolves in water; it hydrolyses and causes a decrease in the concentration of hydroxide ions and an increase in the concentration of hydrogen ions, making the salt solution acidic. ### 3- Acid-Base Balance (Equilibrium): The acid-base balance in water depends on the relationship between the concentrations of hydrogen ions (H+) and hydroxide ions (OH-). This relationship can be recognized by the pH value of the solution. It is a scale that ranges from 0 to 14. If the concentration of H+ increases, the water becomes acidic and the pH value is less than 7, if the concentration of OH- increases, the water becomes basic and the pH value is greater than 7, while if the concentration of the two ions is equal, the water is neutral and the pH value is equal to 7. #### pH Value: It is the measure of the acidity or basicity of liquids or solutions. The pH value of pure water is about 7, which is considered neutral. However, this value may vary in natural environments, affecting the organisms that live in them. #### The pH Value of Water from Different Sources: 1. **Seawater:** The pH value of seawater generally ranges from 7.5 to 8.4, depending on the region in which the sea is located and the environmental factors surrounding it. 2. **Fresh Water (Rivers and Lakes):** The pH value varies and normally ranges from 6.5 to 8.5 3. **Distilled Water:** The pH value is around 7, because it is free of most of the impurities and ions that contribute to the acidity or alkalinity of natural water sources. 4. **Groundwater:** The pH of groundwater varies from one region to another depending on several factors, the most important factor is the rock structure of the area. Groundwater is either neutral or alkaline, and its pH value varies due to exposure to salts of certain rocks such as calcium carbonate or magnesium carbonate. 5. **The pH of the clouds is generally slightly acidic, with values ranging from 4.5 to 5, due to the presence of carbon dioxide and other acidic gases dissolved in the rain droplets.** These values can vary depending on different environmental factors, and human activities in that area which can affect the pH level when forming clouds or rainwater ## Practical Activity: Measuring the pH Values in Different Water Samples: To measure the pH value of different water samples (sea water, river water, and spring water), you can perform the following experiment: **Required Materials:** 1. Water samples (seawater, river water, and spring water) 2. A pH meter or pH test strips. 3. Cups for the samples. 4. Distilled water (for calibration) 5. Stirring rod **The Procedures for the Experiment:** 1. **Calibration:** Calibrate the pH meter according to the manufacturer’s instructions using distilled water. 2. **Sample Preparation:** Number the beakers according to the type of water and place a small amount of this type in each beaker. 3. **Testing:** Immerse the electrode of the calibrated pH meter in each sample and record the reading when it stabilizes. 4. **Measuring by Using Test Strips:** When using test strips, dip the strip into each sample for few seconds, then compare its colour against the attached chart to determine the approximate pH value. ## Research Activity With a group of your colleagues, do research using data that show the different pH values of clouds and rainfall in different regions and the reasons for this. Examples for these regions are, a. Industrial cities b. Agricultural areas c. Coastal cities To minimize the negative impacts on water quality and on the health of living organisms because of hydrolysis of salts and its effects on water, it is important to closely monitor salinity levels as well as the changes in ionic structure of natural water bodies. Proper waste disposal minimizes the addition of harmful salts to water bodies and maintains water quality for wildlife habitats and human consumption.