CHEMICAL SYSTEMS - THE WATER CYCLE Learner Note: Examiners regularly comment that it is evident that the section on chemical systems is neglected. This lack of knowledge is carried forward to grade 11 and 12. It is crucial that this section is mastered. SECTION A: TYPICAL EXAM QUESTIONS Question 1: 20 minutes (Taken from MED Nov 2009) How does the cost of bottled water, both short term and long term, compare with other water purification options? Bottled water is simple water from some source that a company has placed in a bottle for resale. Bottled water can have minimal or no processing (natural spring or mineral water) or it can be completely filtered and demineralised to nearly pure water and then have minerals added back to make it taste better. Keep it clean According to the university of Illinois If you choose bottled water after careful investigation, you have to be faithful in maintaining the hygiene of your bottled water or you may increase the exposure to bacteria. Bacteria grow best in warm moist areas, like the threaded cap of an unrefrigerated bottle of water. They will begin to grow as a soon as you break the seal. If ingested these bacteria can cause gastrointestinal problems and other health risks. The key is maintaining the cleanliness of your bottles and storing them properly. Follow these hints: 1. Store the bottle in the refrigerator at a temperature above freezing but less than 4 C 2. Wipe the seal with a clean cloth after each use 3. Avoid any type of build up in the cap 4. If your bottle is refillable, make sure it is well cleaned and rinsed before refilling. If possible recycle the bottle and obtain a fresh, sterile, sealed bottle. 1.1 List three advantages and three disadvantages of bottled water (6) (From the answer it must be clear why it is an advantage give an example of each advantage / disadvantage). 1.2 What would the properties, (microscopic and macroscopic) be between zero and 4 C (4) (Microscopic molecules and between molecules, invisible; macroscopic visible to the naked eye). Page 1 of 8
Learner Note: Use the graph of boiling and melting points to explain what happens at each stage of the temperature change. Hydrogen bonding is not the bond within the molecule between the hydrogen and oxygen atom, it is the name given to the force that keeps water molecules attracted to each other. 1.3 What is pure water? (2) (What is the definition of a pure substance?) 1.4 Water has very specific molecular properties. It takes 4200 J of heat to raise the temperature of 1 kg water by 1 C. How much heat is required to raise the temperature of 75 kg of water to by 1 C? (3) (15) Question 2: 10 minutes Give one term for each of the following descriptions: (These are useful terms to memorise as part of theory and terminology, know these!) 2.1 Water evaporates into the atmosphere from large areas on the earth s surface (1) 2.2 Water evaporates from leaves of plants (1) 2.3 Water cools down to form drops in the atmosphere (1) 2.4 Water drops fall back to the ground as rain, hail or snow (1) 2.5 Water seeps through the ground (1) 2.6 The energy that is transferred form a body of higher temperature to a body of lower temperature (1) 2.7 The amount of heat required to raise the temperature of 1 kg of a substance by 1 C (1) 2.8 The heat absorbed or released when water changes phases (1) 2.9 Amount of heat required for evaporation (1) 2.10 Amount of heat required for melting (1) (10) Learner Note: Regular repetition of the terminology will improve the understanding and correct use of the terms in answering questions. Page 2 of 8
SECTION B: SOLUTIONS AND HINTS Question 1 1.1 Advantages: It can be used as an emergency water source in case the primary water source fails or is contaminated It is a convenient source of safe water if outside of homeall water sources are not safe It does not contain chlorine and may contain a mix of minerals may taste better than untreated tap water Most bottled water contains fewer contaminants than untreated tap waterpeople will be protected against water bourne diseases (Again no marks are given for vague answers, quote an example of where bottled water is used and state why it is an advantage in the given situation. The advantage must be clear) Disadvantages: Cost very expensive to buy more than tap water Using bottled water require the transport and moving of containers and bottles of water It has an environmental impact producing bottles uses resources, unless recycled cause waste and a disposal problem Transporting bottles of water uses resources and causes further wear and tear on our roads 1.2 Macroscopic clear colourless appearance, melts at 0 C, is more dense at 4 than at 0 C. (Learn the melting and boiling-point of water and what the significance of each is at 0 C it melts or solidifies, at 100 C it boils or condensates) Microscopic Water molecules have hydrogen bonds as intermolecular forces between the molecules, hydrogen bonds are relatively strong (Learn that the intermolecular force between water molecules is H-bonding, it is applicable in many sections in the chemistry section) 1.3 Water is made of water molecules, no other substances are present 1.4 Heat required = 75 x 4200 = 315 000 J Page 3 of 8
Question 2: 2.1 Evaporation 2.2 Transpiration 2.3 Condensation 2.4 Precipitation 2.5 Percolation 2.6 Heat 2.7 Specific heat capacity 2.8 Latent heat 2.9 Heat of vaporisation 2.10 Heat of fusion Page 4 of 8
SECTION C: ADDITIONAL CONTENT NOTES CHEMICAL SYSTEMS - THE WATER CYCLE The amount of water on earth stays more or less the same. How is it then possible to have water shortages? Not all the water molecules on earth are available for domestic use because water is found in three different phases namely: solid (ice), liquid (water), and gas (water vapour). To explain the water cycle we shall refer to the sketch from the internet. 2. THE WATER CYCLE The water cycle has no starting or ending point. During the water cycle the water constantly changes phase. To get the water to change from a liquid to water vapour (a gas), the sun supplies the energy. The process that takes place is called evaporation. It is caused when the water molecules gains enough energy from the sun to escape from the liquid into the air. During evaporation the potential energy of the particles increases and the kinetic energy of the particles decreases. If during evaporation no energy is supplied from outside, the temperature decreases. To keep evaporation going, energy must be supplied from outside. It also happens that snow and ice sublimise to water vapour. Sublimation is when a solid change directly to a gas. The water vapour and the water which is transpired form plants and also evaporated from the soil will be taken away by the rising air current. The water vapour in the air cools down and condensate to form clouds when the water accumulates around dust particles. The water falls back to the earth in the form of rain, hail and snow. This process is called precipitation. Page 5 of 8
Most of the precipitation is absorbed by the ground. The remainder of the precipitate (rain) runs of on the surface to rivers and back to the sea where it will evaporate again. The water that is absorbed by the ground will then be absorbed by the roots of plants. The water will then be released by the plant through transpiration and the droplets on the leaves will evaporate to repeat the cycle. The rest of the absorbed water will accumulate in underground lakes and rivers. This water will end up in rivers or will be absorbed by the roots of plants and will be back in circulation. Water is stored in various places on earth. In the oceans In glaciers and ice caps Underground Lakes, dams, rivers, inland seas Atmospheric water. PROCESSES OF THE WATER CYCLE: Evaporation: water evaporates from the surface of pools, dams, rivers, lakes and oceans. This process accounts for most of the atmospheric water in the world. Condensation: This is the opposite process to evaporation. Particles of water accumulate to form tiny water droplets during condensation as water changes state from its vapour into its liquid. Water condenses when the temperature decreases. The water condenses around dust particles to form clouds. Precipitation: When it rains the clouds drop water on the earth by means of precipitation. The amount of precipitation varies from place to place. The amount of rivers and water on the earth in a specific area determine the amount of evaporation and precipitation. Surface Run-off: Melted snow, hail and rain run off along the ground to meet with rivers and streams. Subsurface Run-off / percolation: The water seeps through the ground to underground rivers. This subsurface run-off is vital for plant life. Transpiration: Plants transpire through holes, called stomata, which are found on the under surface of leaves. PROPERTIES OF WATER. Most of the water found is in the liquid phase. Very strong intermolecular forces hold the water molecules close to one another as they slip and slide over each other in the water. Page 6 of 8
The density of water is the highest at 4 C. At this temperature the molecules move very slowly and the force of attraction increases. The moment it freezes the bonds enclosed air and the density decreases to such an extent that the ice floats on water. Energy is needed to change the phases of water. The molecules will gain energy while the temperature increases. The moment the molecules have enough energy to escape from their current phase, all the added energy is used to change the phase and no increase in temperature takes place. The energy needed to increase the temperature is called specific heat. The energy needed to change a phase is called latent heat. Water is a polar molecule; it means that the electron cloud of the covalent bond will be closer to the oxygen than the hydrogen because oxygen is more electronegative. The strong bonds between the water molecules (not the hydrogens and oxygen in a single molecule) is called hydrogen bonds. This bonds cause a high melting and boiling point because this bond is the strongest non-metallic intermolecular force. Page 7 of 8
SECTION D: HOMEWORK Learner Note: These questions are important to show the application of the terminology and concepts dealt with in this section. Attempt all questions after learning the terminology. Question1: 15 minutes Use water as an example to explain each of the following terms give one example where the term applies in everyday life in terms of an advantage or disadvantage 1.1 Cohesion (3) 1.2 Adhesion (3) 1.3 Hydrophilic (3) 1.4 Hydrophobic (3) 1.5 Meniscus (3) (15) Question 2: 5 minutes Capillary action is of great importance in the water cycle and it also has disadvantages. Discuss this statement discuss 3 points (6) (6) SECTION E: SOLUTIONS TO SESSION 16 HOMEWORK Question 1 1.1 V 1 = V 2 + V 3 Sum of the voltmeters along a path equals potential difference across cells Resistors in series are potential dividers V 2 = V 3 = V 4 Potential difference over branches in parallel is the same, Potential difference over any resistors in parallel to each other is the same 1.2 A 1 = A 4 Current in the series circuit is the same A 2 = A 3 A 4 Resistors are current dividers, branches have identical resistance so equal current in branch A 2 + A 3 = A 4 = A 1 Resistors in parallel are current dividers The SSIP is supported by Page 8 of 8