Everything from the 7 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Acids turn blue litmus red, turn methyl orange red and leave thymolphthalein colourless. They react with metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide. Bases are oxides or hydroxides of metals; soluble bases are alkalis. Alkalis turn red litmus blue, methyl orange yellow and thymolphthalein blue, and release ammonia from ammonium salts. Acids contain H+ ions and alkalis contain OH− ions.
Metal + acid gives hydrogen; carbonate + acid gives carbon dioxide. Mixing these up is the commonest slip. Also, a base is not always an alkali: say "alkali" only for a base that dissolves in water.
The pH scale runs from 1 to 14. pH 7 is neutral; below 7 is acidic and above 7 is alkaline. Universal indicator gives a different colour for each pH, from red (strongly acidic) through green (neutral) to purple (strongly alkaline). The lower the pH, the higher the concentration of H+ ions. In neutralisation, H+ and OH− ions react to form water, so the pH moves towards 7.
A lower pH means a higher concentration of H+ ions, not a lower one. And pH 7 means neutral only if the solution contains no excess acid or alkali; "more acidic" means a smaller pH number.
Extended. An acid is a proton donor and a base is a proton acceptor, because an H+ ion is a proton. A strong acid is completely dissociated in water: HCl(aq) → H+(aq) + Cl−(aq). A weak acid is only partially dissociated: CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq). At the same concentration a weak acid has fewer H+ ions, a higher pH and reacts more slowly.
Strong and concentrated are different ideas. Strength is about how completely the acid dissociates; concentration is about how much acid is dissolved per dm3. A dilute solution of a strong acid and a concentrated solution of a weak acid are both possible.
Basic oxides are oxides of metals, such as calcium oxide and copper(II) oxide; they react with acids to form a salt and water. Acidic oxides are oxides of non-metals, such as carbon dioxide and sulfur dioxide; they react with bases to form a salt and water, and many dissolve in water to give acidic solutions. Extended: amphoteric oxides, such as aluminium oxide and zinc oxide, react with both acids and bases.
Not every metal oxide is simply basic: aluminium and zinc oxides are amphoteric. And "acidic oxide" means it reacts with bases; it does not contain hydrogen ions itself. Silicon dioxide is acidic even though it does not dissolve in water.
A salt forms when the hydrogen of an acid is replaced by a metal or ammonium ion. To make a soluble salt from an insoluble solid: add excess solid (metal, base or carbonate) to warm acid, filter off the excess, heat the filtrate to the point of crystallisation, leave to cool, then filter, wash and dry the crystals. For sodium, potassium and ammonium salts, react an acid with an alkali using a titration, then repeat without the indicator and crystallise.
Filter before crystallising, to remove the excess solid; then filter again after crystallising, to separate the crystals from the remaining solution. And the salt is in the filtrate in the first filtration, not in the residue.
General rules: all sodium, potassium and ammonium salts are soluble; all nitrates are soluble; chlorides are soluble except silver and lead(II) chlorides; sulfates are soluble except barium, calcium and lead(II) sulfates; carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium (calcium hydroxide is slightly soluble). Extended: an insoluble salt is made by precipitation: mix two solutions that contain its ions, then filter, wash and dry the precipitate.
For an insoluble salt the product you want is the residue on the filter paper, not the filtrate: the opposite of the soluble salt method. Nitrates are useful starting materials because they are always soluble; for example, use lead(II) nitrate, not lead(II) chloride, as the source of lead ions.
A hydrated substance is chemically combined with water; an anhydrous substance contains no water. Water of crystallisation is the water molecules present in the crystals of a hydrated salt, as in CuSO4·5H2O. Heating a hydrated salt drives off this water: blue hydrated copper(II) sulfate becomes white anhydrous copper(II) sulfate. Extended: the number of water molecules, x, can be found from the masses before and after heating.
Hydrated crystals look and feel dry: the water of crystallisation is locked inside the crystal structure, not wetting the surface. And removing it is a chemical change, shown by the colour change, not simply drying.