Prep.2 Unit One - Lesson Four 2021 PDF

# Science ## Prep.2 ### First Term #### Unit One - Lesson Four ##### Water Name: *** # Structure of Water ## Formation of Polar Water Molecule Water molecule is formed of one oxygen atom (O) with two hydrogen atoms (2H) joined together by two single covalent bonds, the angle between them...

# Science ## Prep.2 ### First Term #### Unit One - Lesson Four ##### Water Name: *** # Structure of Water ## Formation of Polar Water Molecule Water molecule is formed of one oxygen atom (O) with two hydrogen atoms (2H) joined together by two single covalent bonds, the angle between them is 104.5° Bonds between water molecules Between polar water molecules originate a type of weak electrostatic attraction known as hydrogen bond Water molecule (G.R). Due to the higher electronegativity of oxygen compared with hydrogen. ## Hydrogen Bond It is a type of weak electrostatic attraction that originates between the molecules of some polar compounds. Hydrogen bonds between water molecules are weaker than covalent bonds between the atoms in the same molecules. However hydrogen bonds are considered to be the most important factors which are responsible for the abnormality of water properties. # Properties of Water ## Some Properties of Water ### A Physical Properties 1. It exists in three states 2. It is a good polar solvent 3. Rising of its boiling & its freezing points 4. Its density decreases on freezing ### B Chemical Properties 1. It has a neutral effect on both of litmus papers 2. Electrolysis of water ## A Physical Properties ### 1: Water Exists in Three States Water is an unique matter due to its existence in three states at the ordinary temperature which are: - Solid State: Ice - Liquid State: Water - Gaseous State: Water Vapor ## 2: Water Is a Good Polar Solvent ### Activity 1: To prove that water is a good polar solvent **Observation:** Both sugar and table salt dissolve in water, while oil does not dissolve. **Conclusion:** - Water is a good polar solvent for: 1. Most ionic compounds such as sodium chloride (table salt) 2. Some covalent compounds such as sugar which can form hydrogen bonds with water. - Most covalent compounds as oil cannot dissolve in water, because it cannot form hydrogen bonds with water. **Give reason:** - **Dissolving of table salt in water:** Because water is a good polar solvent for most ionic compounds (as table salt). - **Dissolving of sugar in water although it is among covalent compounds:** Because sugar molecules forms hydrogen bonds with water molecules. - **Oil doesn't dissolve in water:** Because it is a covalent compound which cannot form hydrogen bonds with water. ## 3: Rising of water boiling point and its freezing point It was supposed that: - The boiling point of water is less than 100°C. - The freezing point of water is less than 0°C. But in the normal atmospheric pressure: - Pure water boils at 100°C. - Pure water freezes at 0°C. This is due to the presence of hydrogen bonds between its molecules. ## 4: Water density decreases on freezing Water is not like other matters, where its density in solid state (ice) is lower than its density in liquid state (water) Besides when the temperature of water becomes less than 4°C, water molecules are collected together by hydrogen bonds forming large-sized hexagonal ice crystals with many spaces between them, so its volume increases and thus its density decreases. Ice floats on the water surface (G.R). Because the density of ice is less than the density of water. ## B. Chemical Properties ### 1: Water has a neutral effect on both of litmus papers **Activity 2:** Water has a neutral effect on both of litmus papers: - **Observation:** The color of the two papers (blue and red) doesn't change. **Conclusion:** The pure water has a neutral effect on both of litmus papers (blue and red ). ### 2: Electrolysis of water **Activity 3:** Electrolysis of water: - **Observation:** 1. The evolved gas above the cathode burns with a pop sound, while the evolved gas above the anode makes the glowing splint more glowing. 2. The volume of the evolved gas above the cathode is twice the volume of the evolved gas above the anode. - **Conclusion:** - Hydrogen gas evolves at the cathode. - Oxygen gas evolves at the anode. The acidified water decomposes by electricity into its two elements (hydrogen and oxygen), the volume of hydrogen gas is twice the volume of oxygen gas at a ratio 2:1 by volume respectively. # Water Pollution - The continuous increase in agricultural, industrial, and developmental activities leads to water pollution. **Water pollution can be defined as follows:** - **Water pollution:** It is the addition of any substance into the water which causes continuous gradual change in water properties affecting the health and the life of living creatures. ## Water Pollutants and Their Harms - Environmental pollutants are divided generally into two kinds, which are: 1. **Natural pollutants:** - Natural phenomena. - Examples: - Lightning accompanied by thunder storms. - Death of living organisms. - Volcanic eruptions. 2. **Artificial pollutants:** - Different human activities. - Examples: - The overuse of chemical insecticides and fertilizers - Throwing sewage, factories wastes, and leakage of petroleum oil in the seas and rivers. - Burning coal and oil which leads to the formation of acidic rains and smog. # Types of Water Pollution - Water pollution is classified into 4 main types as shown in the following table: | Types of water pollution | Causes (origins) | Harms (damages) | |---------------------------|---------------------|-------------------------------------------------------------------------------------------------------| | Biological pollution | Mixing animals and human wastes with water | The infection with many diseases such as: bilharzia, typhoid, and hepatitis. | | Chemical pollution | Discharging factories’ wastes and sewage in seas, rivers, and canals. | The increase in some elements concentration causing great harms as:<br>1. The death of brain cells: When eating continuously fish whose bodies contain high concentration of lead.<br>2. Blindness: When drinking continuously from water containing high concentration of mercury.<br>3. Increases the infection rates of liver cancer: When drinking continuously from water containing arsenic. | | Thermal Pollution | Rising of temperature of some marine zones which use water for cooling the nuclear reactors. | Destroy the marine creatures found in these zones due to the separation of the dissolved oxygen in water. | | Radiant Pollution | - Dumping the atomic wastes in oceans and seas.<br>- Leakage of radioactive materials from nuclear reactors. | For illustration:<br>Increase the infection rates of cancer. | # Protection of Water from Pollution - There are many behaviors that must be taken in consideration to protect water from pollution in Egypt, such as: 1. Prevention of getting rid of sewage, wastes of factories, and dead animals in rivers or canals. 2. Disinfection of the drinking water tanks found on the roofs of buildings in a periodical manner. 3. Don’t store the tap water in empty plastic bottles (G.R.), because plastic reacts with chlorine gas which is used as water disinfectant, leading to the increase in the infection rates of cancer. 4. Spreading environmental awareness among people to protect water from pollution. 5. Developing the stations of water purification and doing a periodical analysis to determine the rate of water validity for drinking.