Topic 10
3 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Pure water contains only H2O molecules. Potable water is safe to drink but contains dissolved substances. Water is treated by sedimentation, filtration, and chlorination.
Explanation
Water treatment steps: 1. Sedimentation: large particles settle to the bottom. 2. Filtration: water passes through sand/gravel beds to remove smaller particles. 3. Chlorination: chlorine gas is added to kill bacteria and microorganisms. Testing water purity: - Boiling point: pure water boils at exactly 100 degrees C. Impure water boils above 100 degrees C. - Melting point: pure water melts at exactly 0 degrees C. Impure water melts below 0 degrees C. Distillation: - Used to obtain pure water from impure/salt water. - Water is boiled, steam rises, condensed back to liquid. Dissolved substances remain behind.
Practice Questions
Describe the three main stages in the treatment of river water to make it safe to drink. [3 marks]
Key Idea
Clean dry air is approximately 78% N2, 21% O2, 1% Ar, 0.04% CO2. Air pollutants include CO, SO2, NOx, and particulates.
Explanation
Composition of clean dry air: - Nitrogen (N2): approximately 78% - Oxygen (O2): approximately 21% - Argon (Ar): approximately 1% - Carbon dioxide (CO2): approximately 0.04% Air pollutants and their sources: - Carbon monoxide (CO): incomplete combustion of fuels. Toxic, binds to haemoglobin. - Sulfur dioxide (SO2): burning fossil fuels containing sulfur. Causes acid rain. - Nitrogen oxides (NOx): high temperature combustion. Cause acid rain and photochemical smog. - Carbon dioxide (CO2): combustion of fossil fuels. Greenhouse gas, global warming. - Particulates (soot): incomplete combustion. Respiratory problems. - Methane (CH4): livestock, rice paddies, landfill. Greenhouse gas. Acid rain: - SO2 + H2O to H2SO3 to H2SO4 (sulfuric acid). - NOx + H2O to HNO3 (nitric acid). - Effects: damages buildings (limestone), kills fish in lakes, damages trees.
Practice Questions
Explain how sulfur dioxide causes acid rain and describe two harmful effects of acid rain. [4 marks]
Key Idea
Greenhouse gases (CO2, CH4, H2O vapour) trap heat in the atmosphere. Increasing CO2 from fossil fuel combustion is causing global warming.
Explanation
The greenhouse effect: 1. Short-wave radiation from the Sun passes through the atmosphere and warms the Earth's surface. 2. The Earth emits long-wave (infrared) radiation back upwards. 3. Greenhouse gases (CO2, CH4, H2O, N2O) absorb this infrared radiation and re-emit it in all directions, including back to Earth. 4. This keeps the Earth warmer than it would otherwise be. Enhanced greenhouse effect: - Burning fossil fuels increases CO2 levels. - Deforestation reduces CO2 absorption by trees. - Livestock farming increases CH4. - Consequences: global warming, rising sea levels, more extreme weather events, changes in rainfall patterns, loss of habitats.
Practice Questions
Explain how increasing levels of carbon dioxide in the atmosphere contribute to global warming. [3 marks]