Acids, bases and the environmentIB MYP Chemistry: Revision notes
Section 1
Acid rain
Rain is naturally slightly acidic (about pH 5.6) because carbon dioxide dissolves in it. Acid rain is rain with a lower pH than this.
Burning fossil fuels releases sulfur dioxide (from sulfur in coal and oil) and nitrogen oxides (formed in hot engines). These gases dissolve in rain water to form acids such as sulfuric acid and nitric acid. Wind can carry the gases hundreds of kilometres, so the rain may fall in a different country.
Effects of acid rain:
- lakes become acidic and fish and other animals die
- trees lose leaves and are damaged
- limestone and marble buildings are worn away
- metal bridges and railings corrode faster
Section 2
Acid rain and buildings
Limestone and marble are made of calcium carbonate, which is a base. Acid rain reacts with it:
calcium carbonate + sulfuric acid → calcium sulfate + water + carbon dioxide
The surface fizzes away slowly, so statues and buildings lose their detail. Reducing sulfur dioxide emissions (for example by removing sulfur from fuels or filtering chimney gases) reduces the damage.
Whenever a question mentions limestone, marble, chalk or shells, think: carbonate plus acid gives a salt, water and carbon dioxide.
Section 3
Ocean acidification
The oceans absorb about a quarter of the carbon dioxide we release. Carbon dioxide dissolves in seawater and reacts with water to form carbonic acid. This lowers the pH of the sea, which is called ocean acidification.
Corals, shellfish and some plankton build shells and skeletons from calcium carbonate. In more acidic water these dissolve or become weaker, and the animals find it harder to grow. This harms coral reefs and the food webs and fishing communities that depend on them.
The main way to slow ocean acidification is to reduce carbon dioxide emissions.
Section 4
Treating acidic soil and lakes with lime
Plants grow best in a certain pH range. If soil is too acidic, crops grow poorly. Farmers spread lime to neutralise the acid. Lime is calcium oxide (quick lime), calcium hydroxide (slaked lime) or calcium carbonate (powdered limestone). All are bases.
Lime can also be added to acidic lakes to raise their pH so that fish can survive.
Drawbacks: it costs money, has to be repeated, and too much lime makes the soil too alkaline.
Section 5
Antacids
The stomach makes hydrochloric acid (about pH 1 to 2) to digest food and kill bacteria. Too much acid causes pain called indigestion.
Antacids are medicines that contain bases such as magnesium hydroxide, calcium carbonate or sodium hydrogencarbonate. They neutralise the extra acid.
magnesium hydroxide + hydrochloric acid → magnesium chloride + water
Antacids use weak bases, not strong alkalis such as sodium hydroxide, because strong alkalis are corrosive and unsafe to swallow. Carbonate antacids also release carbon dioxide, which can cause belching.
Do not say an antacid 'removes' the acid. It reacts with the acid and neutralises it.
Section 6
Evaluating the impacts
Many questions ask you to weigh up the effects of an action. A good answer:
- gives at least one benefit and one drawback
- covers different areas: environmental, economic (cost) and social (people's lives and health)
- ends with a justified conclusion that refers to both sides
Example: cutting sulfur dioxide from power stations protects lakes, forests and buildings (environmental, economic), but fitting filters costs money, which could raise electricity prices (economic, social).
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Acids, bases and the environment
- Researchers in Sweden find that the pH of a lake has fallen from 6.8 to 4.9 over thirty years. Many of the fish have died and nearby trees are losing their leaves. A coal-burning power station, which has no gas filters on its chimney, operates 200 km upwind of the lake.Explain how burning coal can make the rain acidic.2 marks
- A student in Cairo compares three antacid tablets, X, Y and Z, sold for indigestion. For each test she crushes one tablet, adds it to 25 cm³ of 0.10 mol dm⁻³ hydrochloric acid, which represents stomach acid, stirs for two minutes and measures the pH with a pH meter. The acid on its own has a pH of 1.0. After tablet X the pH is 3.2, after tablet Y it is 5.1 and after tablet Z it is 6.9. She concludes that tablet Z is the best antacid.Suggest one improvement to the method that would make the conclusion more reliable, and explain how it helps.2 marks
- Marine scientists monitor the seawater around a coral reef in Australia. Over 40 years the average pH of the surface seawater has fallen from 8.11 to 8.04, while the amount of carbon dioxide in the air has risen. Corals and shellfish build their skeletons and shells from calcium carbonate.Explain how an increase in carbon dioxide in the air lowers the pH of seawater.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).