Acids, bases and pH
Acids release ions in water (proton donors); pH below 7. Bases neutralise acids (proton acceptors); soluble bases are alkalis, releasing , pH above 7. Acids react with metals (→ salt + hydrogen), bases (→ salt + water) and carbonates (→ salt + water + carbon dioxide). Strong acids ionise completely; weak acids only partly.
≈ 40 minProperties, uses and alloys
Metals are usually shiny, dense, malleable, ductile, good conductors of heat and electricity, and have high melting points. Many react with oxygen, water and dilute acids. An alloy is a mixture of a metal with other elements. The different-sized atoms disrupt the regular layers, so they can't slide easily: alloys are harder and stronger than pure metals. Examples: steel (iron + carbon), stainless steel (+ chromium, nickel), brass (copper + zinc).
≈ 30 minWater
Chemical tests show water is present: anhydrous copper(II) sulfate turns from white to blue; anhydrous cobalt(II) chloride from blue to pink. The physical test for pure water: it boils at exactly and melts at . Natural water contains useful substances (oxygen, minerals) and pollutants (sewage, nitrates, phosphates, metal compounds, plastics). Tap water is treated by sedimentation, filtration, chlorination (and sometimes carbon to remove tastes).
≈ 30 minFertilisers
Plants need nitrogen (N) for proteins and growth, phosphorus (P) for roots, and potassium (K) for flowers and fruit. NPK fertilisers supply these as soluble salts such as ammonium nitrate, ammonium phosphate and potassium chloride. Ammonia from the Haber process is neutralised by acids to make them. Fertilisers boost crop yields, but run-off causes eutrophication.
≈ 25 minGroup I and group VII
Group I (alkali metals): soft, low-density metals that react with water to make an alkaline hydroxide and hydrogen. Reactivity increases down the group because the outer electron is further from the nucleus and lost more easily. Group VII (halogens): diatomic non-metals (, , , ) that get darker and less reactive down the group. A more reactive halogen displaces a less reactive one from its salt.
≈ 40 minOxides
Metal oxides are usually basic: they react with acids to form salts. Non-metal oxides are usually acidic: they react with bases, and soluble ones give acidic solutions. Amphoteric oxides (, ) react with both acids and bases. Neutral oxides (CO, NO, ) react with neither.
≈ 25 minReactivity series
The reactivity series lists metals from most to least reactive: K, Na, Ca, Mg, Al, (C), Zn, Fe, (H), Cu, Ag, Au. More reactive metals form positive ions more easily. Metals above hydrogen react with dilute acids; metals above magnesium react with cold water. A more reactive metal displaces a less reactive one from a solution of its compound, e.g. .
≈ 35 minTests for ions and gases
Flame tests: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green. Sodium hydroxide: blue ppt, green ppt, red-brown ppt, / white ppt (dissolves in excess), ammonia on warming. Anions: carbonate + acid → ; halides + acidified silver nitrate → white (Cl⁻), cream (Br⁻), yellow (I⁻); sulfate + acidified barium chloride/nitrate → white ppt. Gases: H₂ squeaky pop; O₂ relights glowing splint; CO₂ limewater milky; Cl₂ bleaches damp litmus; NH₃ turns damp red litmus blue.
≈ 45 minPreparation of salts
A salt forms when the hydrogen of an acid is replaced by a metal (or ammonium). The acid gives the second part of the name: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate. Soluble salts are made by adding excess insoluble base, metal or carbonate to acid, filtering, then crystallising; or by titration when using an alkali. Insoluble salts are made by precipitation: mix two solutions, filter, wash and dry.
≈ 45 minAir quality and climate
Clean dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide. Pollutants include carbon monoxide (incomplete combustion; toxic), sulfur dioxide (burning sulfur in fossil fuels; acid rain), oxides of nitrogen (car engines; acid rain, smog), particulates (lung damage). Carbon dioxide and methane are greenhouse gases: they absorb infrared radiation, causing global warming and climate change. Catalytic converters, flue-gas desulfurisation and renewable energy reduce emissions.
≈ 40 minTransition elements and noble gases
Transition elements (the central block, e.g. Fe, Cu, Ni, Cr) are hard, dense metals with high melting points. They form coloured compounds, have ions with variable oxidation numbers (iron(II) and iron(III)) and are often catalysts (iron in the Haber process). Noble gases (group VIII/0) have full outer shells, so they are unreactive and exist as single atoms; helium is used in balloons, argon in light bulbs and welding.
≈ 25 minRedox
Oxidation is gain of oxygen, loss of electrons, or an increase in oxidation number. Reduction is loss of oxygen, gain of electrons, or a decrease in oxidation number. They always happen together: a redox reaction. The oxidising agent is itself reduced; the reducing agent is itself oxidised.
≈ 35 minCorrosion of metals
Rusting is the corrosion of iron and steel. It needs both oxygen and water, and forms hydrated iron(III) oxide. Salt speeds it up. Barrier methods (paint, oil, grease, plastic, tin plating) keep oxygen and water away. Galvanising coats iron with zinc; zinc is more reactive, so even if scratched it corrodes first (sacrificial protection).
≈ 30 minExtraction of metals
Most metals are found as compounds in ores. Metals below carbon are extracted by reduction with carbon (heating with coke). Metals above carbon must be extracted by electrolysis of the molten compound, which costs more energy. Iron is made in the blast furnace from haematite (), coke and limestone. Aluminium is made by electrolysing aluminium oxide dissolved in molten cryolite.
≈ 45 min