Exam practice

As rain falls, it dissolves some of the carbon dioxide in the air. Carbon dioxide and water form weak carbonic acid (CO₂ + H₂O → H₂CO₃), so even clean rain has a pH of about 5.6, slightly below neutral.

Ammonium chloride is formed by the reaction of a strong acid (hydrochloric acid) and a weak base (ammonium hydroxide). The strong acid wins, so a solution of ammonium chloride is slightly acidic: it turns blue litmus red.

Sulphuric acid is a strong acid: in water nearly all its molecules give hydrogen ions, and it also pulls water out of materials. It attacks the fibers of cloth (and skin) and burns holes in them. The citric acid in lemon juice is a weak acid that gives very few hydrogen ions, too few to damage cloth.

Magnesium hydroxide is a mild base (an antacid, sold as milk of magnesia). It neutralizes the extra hydrochloric acid in the stomach that causes the burning pain:
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
It is a weak, nearly insoluble base, so it doesn’t harm the stomach itself.

An acid is a substance that gives hydrogen ions (H⁺) when it is dissolved in water. Acids taste sour, turn blue litmus red, and react with metals, carbonates and bases. Examples: hydrochloric acid, sulphuric acid, citric acid.

A base is a metal oxide or metal hydroxide, which reacts with an acid to form a salt and water. A base that dissolves in water (an alkali) gives hydroxide ions (OH⁻). Examples: sodium hydroxide, calcium hydroxide, magnesium oxide.

A salt is the compound formed, along with water, when an acid reacts with a base: the hydrogen of the acid is replaced by a metal (or ammonium). Examples: sodium chloride, copper sulphate, sodium carbonate.

1. Burning fossil fuels (coal, petrol, diesel) in vehicles, power stations, brick kilns and factories, which release sulphur dioxide and oxides of nitrogen.
2. Harmful gases released by factories and industries.
3. Natural sources: volcanic eruptions, lightning, and the decay of dead plants and animals.

These gases react with water in the air to form sulphuric acid and nitric acid, which fall with the rain.

From its effects: it makes lakes and rivers acidic so that fish and their eggs die; it damages leaves and makes soil poor, so crops grow badly and forests die; and it slowly dissolves marble and limestone monuments, statues and temples, and corrodes metal bridges and railings.

1. Use less fossil fuel: walk, cycle, use public and electric transport, and keep engines well tuned.
2. Use renewable energy such as hydroelectricity and solar power.
3. Build factories away from homes, and fit their chimneys with filters that remove sulphur dioxide.
4. Plant trees and protect forests, and make and follow laws on air pollution.

Acid rain makes the soil too acidic. I would suggest: (1) test the soil’s pH; (2) spread lime (calcium hydroxide or powdered limestone) or wood ash, which are bases, to neutralize the acid; (3) add plenty of compost and manure to put back nutrients and make the soil healthier; and (4) in the long run, reduce the air pollution that causes the acid rain.

• i.A turns methyl orange red and litmus red, so A is an acid: phenolphthalein stays colorless in an acid, so X = colorless. B turns litmus blue and phenolphthalein pink, so B is a base: methyl orange turns yellow in a base, so Y = yellow.
• ii.A, the acid. Acids react with carbonates to give a salt, water and carbon dioxide, e.g. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Pure iron is soft and rusts quickly, and rust spoils food and the utensil. Steel, especially stainless steel, is harder and stronger, and stainless steel doesn’t rust, doesn’t react with acidic foods like lemon or tomato, and is easy to clean. So steel utensils are cleaner, safer and last longer.

Rainwater is soft: it has almost no dissolved calcium or magnesium salts, because it hasn’t flowed through rocks. With nothing to turn the soap into scum, all the soap goes into making lather.

Well water is groundwater. On its way down through the soil and rocks, it dissolves salts of calcium and magnesium (from limestone and other minerals). These dissolved salts make the water hard.

Alloys have better properties than pure metals: they are harder and stronger, many don’t rust or corrode, and their color and melting point can be chosen for the job. So they last longer and are more useful: stainless steel for utensils, duralumin for aircraft, brass for taps.

The hardness of water is the property of water, caused by dissolved salts of calcium and magnesium, that stops soap from lathering easily and makes it form a scum.

Dissolved salts of calcium and magnesium:

• their bicarbonates (calcium bicarbonate, magnesium bicarbonate) cause temporary hardness;
• their sulphates and chlorides (calcium sulphate, magnesium sulphate, calcium chloride, magnesium chloride) cause permanent hardness.

The well water is hard. I would suggest:

1. Boil the water and let it cool before washing (this removes temporary hardness), or add a little lime (Clark’s method) and let the solid settle.
2. Add a little washing soda to the water; it removes permanent hardness as well.
3. Use detergent instead of soap: detergents lather even in hard water.
4. Or use soft rainwater if it has been collected.

Tube-well water is hard. The calcium and magnesium salts in it react with soap to form an insoluble, grayish scum, which sticks in the fibers of the cloth and makes white clothes look dull.
Suggestions: soften the water first with washing soda (or by boiling, if the hardness is temporary), use a detergent instead of soap, rinse the clothes well, or wash them in rainwater.

Draw a tank like Figure 10.9, with a layer of gravel at the bottom, then coarse sand, then sodium zeolite (sodium aluminium silicate).
Hard water is poured in at the top. As it trickles down through the zeolite, the calcium and magnesium ions in the water change places with the sodium ions in the zeolite: the calcium and magnesium stay in the zeolite and the sodium passes into the water. The water that flows out at the bottom has almost no calcium or magnesium left, so it is soft. When the zeolite is used up, a strong solution of sodium chloride is passed through it, which swaps the sodium back in, and the zeolite can be used again.
In words: sodium zeolite + calcium salt → calcium zeolite + sodium salt.

An alloy is a homogeneous mixture of a metal with one or more other metals or with a non-metal, made by melting them together. Examples: brass (copper and zinc), bronze (copper and tin), steel (iron and carbon).

• i.Steel: iron and a little carbon (a non-metal).
• ii.Stainless steel: iron, chromium and nickel (with a little carbon).
• iii.Brass: copper and zinc.
• iv.Bronze: copper and tin.

1. They are harder and stronger than pure metals.
2. Many of them don’t rust or corrode.
3. They last longer.
4. Their color can be made more attractive (brass, rose gold, the pink alloy of silver and zinc).
5. Their melting point can be made lower or higher for a particular job (solder melts easily).
6. Light but strong alloys (duralumin) can be made for aircraft and vehicles.

Bronze is made of copper and tin. (Many “bronze” medals today are mostly copper, with some zinc as well as tin.)