SPI Chemistry · Formula sheet
Acids, Bases and Salts formulas
1 formula, 5 reference tables and 12 common traps for SPI Chemistry Acids, Bases and Salts, grouped by subtopic.
Acids, Bases and Indicators
Learn this subtopic in the notesWhat acids and bases are
Reactions of an acid
Strong and weak acids and bases, and the pH scale
| Substance | Strong or weak | Kind |
|---|---|---|
| Hydrochloric acid, | Strong acid | Hydracid (no oxygen) |
| Sulphuric acid, | Strong acid | Oxyacid |
| Nitric acid, | Strong acid | Oxyacid |
| Ethanoic acid, | Weak acid | Organic acid |
| Carbonic acid, | Weak acid | Oxyacid |
| Sodium hydroxide, | Strong base | Alkali |
| Ammonium hydroxide, | Weak base | Alkali The one weak base to remember. |
Strong or weak is about splitting into ions, not about how much acid is in the water.
Indicators
| Indicator | In an acid | In a base |
|---|---|---|
| Blue litmus | Turns red | Stays blue |
| Red litmus | Stays red | Turns blue |
| Phenolphthalein | Colourless | Pink Colourless with phenolphthalein can mean acidic or neutral. |
| Methyl orange | Red | Yellow |
| Turmeric | Stays yellow | Reddish brown |
| Universal indicator | Red to orange (pH below 7) | Blue to violet (pH above 7) |
Universal indicator is green at pH 7.
Common traps
Calling every base an alkali
An alkali is a base that dissolves in water. Copper oxide is a base but does not dissolve, so it is not an alkali. Every alkali is a base; not every base is an alkali.
Forgetting that acids need water
An acid shows its acidic nature only in water, where it gives ions. Dry hydrogen chloride gas does not change the colour of dry blue litmus.
Confusing concentrated with strong
Concentrated means a lot of acid in a little water. Strong means the acid splits completely into ions. Dilute hydrochloric acid is still a strong acid.
Reading oxyacid or hydracid as a strength
Those words only say whether the acid contains oxygen. Hydrochloric acid is a hydracid and strong; carbonic acid is an oxyacid and weak.
Testing an acid with red litmus
Red litmus stays red in an acid, so it shows nothing. Use blue litmus to detect an acid and red litmus to detect a base.
Taking 'no colour' with phenolphthalein as neutral
Phenolphthalein is colourless in both acids and neutral solutions. Only pink tells you something for certain: the solution is basic.
Salts and Their Uses
Learn this subtopic in the notesNormal salts, acid salts and the nature of a salt solution
| Parent acid | Parent base | Solution | Example |
|---|---|---|---|
| Strong | Strong | Neutral, pH 7 | Sodium chloride, potassium nitrate |
| Strong | Weak | Acidic, pH below 7 | Ammonium sulphate, Ammonium salts of strong acids are acidic: ammonium hydroxide is a weak base. |
| Weak | Strong | Basic, pH above 7 | Sodium carbonate, sodium ethanoate |
The stronger parent decides which way the solution leans.
Salts made from common salt
| Common name | Formula | Made from | Used for |
|---|---|---|---|
| Caustic soda | Electrolysis of brine | Soaps, paper | |
| Baking soda | Brine, ammonia and carbon dioxide | Baking powder, antacid | |
| Washing soda | Heating baking soda, then crystallising | Cleaning, removing hardness of water Washing soda is a carbonate; baking soda is a hydrogencarbonate. | |
| Bleaching powder | Chlorine and dry slaked lime | Bleaching cloth, disinfecting water | |
| Plaster of Paris | Heating gypsum at 373 K | Plaster for broken bones |
The first four start from common salt; plaster of Paris does not.
Tests for ions and gases
| Looking for | Add or do | You see |
|---|---|---|
| Nitrate | Iron(II) sulphate, then concentrated sulphuric acid | A brown ring The sulphate here is part of the reagent: the test finds nitrate. |
| Sulphate | Barium chloride solution | White precipitate |
| Chloride | Silver nitrate solution | White precipitate |
| Carbonate | Dilute acid, gas through lime water | Lime water turns milky |
| Hydrogen gas | A burning splinter | A pop sound |
Barium for sulphate, silver for chloride, iron(II) sulphate for nitrate.
Common traps
Expecting an acid salt to give an acidic solution
An acid salt is named for the replaceable hydrogen it keeps. Sodium hydrogencarbonate is an acid salt, yet its solution is mildly basic, because its parent acid (carbonic acid) is weak.
Calling every salt neutral
Only a salt of a strong acid and a strong base is neutral. A salt of a strong acid and a weak base is acidic, and a salt of a weak acid and a strong base is basic.
Swapping baking soda and washing soda
Baking soda is sodium hydrogencarbonate, . Washing soda is sodium carbonate with ten waters, .
Thinking slaked lime comes from common salt
Slaked lime, , is made from limestone (quicklime and water). Bleaching powder uses slaked lime, but only its chlorine comes from common salt.
Swapping the sulphate and chloride tests
Barium chloride finds sulphate. Silver nitrate finds chloride. Both give a white precipitate, so the reagent is what tells them apart.
Reading the brown ring as a sulphate test
The brown ring test uses iron(II) sulphate and sulphuric acid as reagents, but the ion it detects is nitrate.