Chemistry and Society
Senior 1 · Chemistry
Chemistry and Society
Lesson 3 of 4
25 min
Lake Kivu, on Rwanda's western border, looks like any other beautiful lake. Yet part of Rwanda's electricity comes from the lake itself. About 300 m below the surface the water holds a huge amount of dissolved gas, mostly methane, , and carbon dioxide, . How can gas be taken out of water, and how does it become electricity? A bottle of fizzy drink holds the first clue.
Figure 3.1: Lowering the pressure on a fizzy drink makes the dissolved carbon dioxide come out as bubbles.
Warning
Safety. Use a syringe with no needle. Point the tip away from faces: if the cap slips off, drink can spray out. Do not drink anything used in the experiment.
Carbon dioxide was dissolved in the drink under pressure when the bottle was filled. Pulling the plunger lowered the pressure inside the syringe, so the gas could no longer stay dissolved and came out as bubbles. When the plunger went back, the pressure rose again and many bubbles shrank or disappeared.
This is the key rule: the higher the pressure, the more gas a liquid can hold dissolved. Deep in Lake Kivu the weight of 300 m of water above creates a very high pressure, so the deep water holds a great deal of dissolved gas, like an unopened bottle of soda.
Definition
Dissolved gas: a gas that has mixed into a liquid so completely that it cannot be seen, like the carbon dioxide in an unopened fizzy drink or the methane in Lake Kivu's deep water. Lowering the pressure lets it come out as bubbles.
Definition
Methane: the simplest fuel gas, : one carbon atom joined to four hydrogen atoms. It has no colour or smell and burns with a hot blue flame. It is the main gas in natural gas and in biogas.
At an extraction platform, deep gas-rich water is pumped up a long pipe (Figure 3.2). As it rises the pressure falls, and gas bubbles out, just as in the syringe. On the platform the gas is separated from the water and washed to remove most of the carbon dioxide. The methane is then piped to a power station on the shore, and the degassed water is sent back down deep so that the layers of the lake are not disturbed.
Figure 3.2: Extracting methane from Lake Kivu (not to scale horizontally).
At the power station the methane burns in air. Burning gives out a lot of heat, which drives the engines that turn the electricity generators:
The amount of methane in the lake is estimated at about 65 cubic kilometres. Before large power stations were built, a small amount of Kivu gas was used to fire the boilers of a brewery in Gisenyi. The KivuWatt power plant was generating 26 MW of electricity by 2016, and further projects have been planned.
Worked example
Problem. Show that the equation for burning methane, , has the same number of each kind of atom on both sides.
Solution. A big number in front multiplies the whole formula; a small number after a symbol counts only that atom.
| Atom | Left side | Right side |
|---|---|---|
| C | 1 (in ) | 1 (in ) |
| H | 4 (in ) | (in ) |
| O | (in ) |
Answer: 1 carbon, 4 hydrogen and 4 oxygen atoms on each side. Atoms are only rearranged in a reaction, never created or destroyed.
Warning
Two dangerous gases. Methane mixed with air explodes if it is lit, so gas plants ban naked flames and check for leaks. Carbon dioxide is not poisonous in small amounts, but a large cloud of it pushes away the air and can suffocate people and animals. Scientists monitor Lake Kivu closely because a sudden release of its deep gas would be a disaster.
Rwanda grows between 20,000 and 22,000 tonnes of coffee a year, almost all of it Arabica, on about 400,000 small farms. At a washing station the red cherries go through a series of changes, some physical and some chemical:
Table 3.1: The chemistry of coffee processing
| Step | What happens | Type of change |
|---|---|---|
| Sorting in water | unripe and damaged cherries float and are removed | physical (separation) |
| Pulping | a machine squeezes the beans out of the skin and flesh | physical |
| Fermentation in tanks | microbes break down the sticky layer (mucilage) on the beans into new substances | chemical |
| Washing | water carries away the broken-down mucilage | physical |
| Drying on raised racks | sun and air evaporate water until the beans are dry enough to store | physical |
| Roasting (by the buyer) | heat turns sugars and proteins into brown, fragrant new substances | chemical |
Worked example
Problem. A washing station spreads 100 kg of wet parchment coffee on its racks. The beans are 55% water. They are dried until they are only 12% water. (a) What mass of dry matter is in the beans? (b) What is the mass of the coffee after drying? (c) What mass of water evaporated?
Solution.
Answer: about 49 kg of water, nearly half the starting mass, must evaporate. That is why drying on racks takes many days.
Rwanda mines ores of tin (cassiterite), tantalum (coltan) and tungsten (wolframite). In 2020 minerals earned the country about 733 million US dollars, making mining the largest export earner after tourism. Chemistry is needed to separate the ore from sand and rock, to test how pure it is, and to extract the metal. Tin, for example, is extracted by heating its ore, tin(IV) oxide, with carbon:
Table 3.2: Chemistry in Rwanda's economy
| Sector | Chemistry involved | Example |
|---|---|---|
| Energy | separating and burning methane | Lake Kivu gas power |
| Farming and exports | fertilisers, fermentation, drying | coffee, tea, pyrethrum |
| Mining | separating ores, testing purity, extracting metals | tin, tantalum, tungsten |
| Construction | making cement and lime | cement from Rusizi |
| Food processing | pasteurising milk, making juices, preserving | dairies and juice factories |
| Water supply | removing mud, killing germs | treatment plants on the Nyabarongo |
Worked example
Problem. Most Rwandan coffee is sold abroad as raw green beans and roasted in other countries. Rwanda first exported roasted beans in 2018. Give one argument for roasting more coffee in Rwanda before export, and one argument against.
Solution.
Conclusion: roasting at home earns more only if the coffee can be packed and delivered fast; a good answer weighs both sides.
Tip
To link chemistry to the economy, trace the path raw material → change → product → money. The change in the middle is where chemistry adds value.
Warning
Common mistake: writing that methane is "made" at the platform. The methane was already in the lake, dissolved in the deep water; the platform only separates it from the water, a physical change. The chemical change happens later, when the methane burns.
BBC Click: The 'Killer Lake' Powering Rwanda (4 min). Look for the barge where gas is separated, and listen for why the gas makes the lake dangerous.
Practice
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
