Chemistry
🔒 Log in to trackAcids, bases, pH, common compounds and reactions
🔒 Log in to trackAcids taste sour and bases feel slippery. Indicators and the pH scale tell them apart. This lesson also holds the fixed tables that exams love: natural acids, common names of chemicals, and a few everyday reactions.
Acids, bases and indicators
An acid releases H⁺ ions in water and tastes sour. A base releases OH⁻ ions and feels slippery. A base that dissolves in water is an alkali.
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Blue turns red | Red turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow (no change) | Reddish-brown |
Tip: Phenolphthalein is pink only in a base. In acid it stays colourless.
The pH scale
The pH scale runs from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is basic. Each step is a ten-fold change in H⁺.
| Item | pH |
|---|---|
| Gastric juice (HCl) | about 1.2 to 2 |
| Lemon juice | about 2 to 2.5 |
| Pure water | 7 |
| Human blood | about 7.4 (slightly basic) |
| Milk of magnesia | about 10 |
- Tooth decay starts when mouth pH falls below about 5.5.
- Acid rain has pH below about 5.6.
- Antacids such as milk of magnesia neutralise extra stomach acid.
- An ant or bee sting injects formic acid. Rub baking soda on it. A wasp sting is alkaline, so use vinegar.
Natural sources of acids
| Source | Acid |
|---|---|
| Vinegar | Acetic |
| Lemon, orange | Citric |
| Tamarind, grapes | Tartaric |
| Tomato, spinach | Oxalic |
| Curd, sour milk | Lactic |
| Apple | Malic |
| Ant sting, nettle | Formic |
| Stomach | Hydrochloric |
Common names and formulae
| Common name | Chemical name and formula |
|---|---|
| Baking soda | Sodium hydrogen carbonate, NaHCO₃ |
| Washing soda | Na₂CO₃·10H₂O |
| Bleaching powder | Calcium oxychloride, CaOCl₂ |
| Plaster of Paris | CaSO₄·½H₂O |
| Gypsum | CaSO₄·2H₂O |
| Quicklime / slaked lime | CaO / Ca(OH)₂ |
| Limestone, marble, chalk | CaCO₃ |
| Caustic soda / caustic potash | NaOH / KOH |
| Blue vitriol | CuSO₄·5H₂O |
| Green vitriol | FeSO₄·7H₂O |
| White vitriol | ZnSO₄·7H₂O |
| Oil of vitriol | H₂SO₄ |
| Muriatic acid | HCl |
| Epsom salt | MgSO₄·7H₂O |
| Potash alum (fitkari) | K₂SO₄·Al₂(SO₄)₃·24H₂O, purifies water |
| Hypo | Na₂S₂O₃, used in photography |
| Laughing gas | N₂O |
| Marsh gas | CH₄ |
| Lunar caustic | AgNO₃ |
| Heavy water | D₂O |
| Dry ice | Solid CO₂ |
| Aqua regia | Conc. HNO₃ : conc. HCl in 1 : 3; dissolves gold and platinum |
Watch: Aqua regia is 1 part HNO₃ to 3 parts HCl. Do not reverse it.
Reactions in daily life
- Neutralisation: acid + base gives salt + water.
- Acid + carbonate gives salt, water and CO₂. CO₂ turns lime water milky. This is the standard CO₂ test.
- Oxidation is gain of oxygen or loss of electrons. Reduction is the opposite.
- Rusting needs both oxygen and moisture. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O.
- Rancidity is oxidation of fats and oils. Chips packets are flushed with nitrogen to stop it.
- Hard water: boiling removes temporary hardness (bicarbonates). Washing soda or ion exchange removes permanent hardness (chlorides and sulphates).
- Chlor-alkali process: electrolysis of brine gives NaOH, Cl₂ and H₂.
- Plaster of Paris sets into gypsum when water is added. It is used for casts on broken bones.
- Water of crystallisation: CuSO₄·5H₂O is blue. Heating drives off the water and it turns white. Adding water brings the blue back.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Common name and chemical formula
The question says 'baking soda is', 'chemical name of washing soda', 'which is CaO', or gives a formula for a common name.
Learn the table in pairs.
Baking soda NaHCO₃ pairs with washing soda Na₂CO₃·10H₂O.
Quicklime CaO, lime water Ca(OH)₂, limestone CaCO₃.
Gypsum CaSO₄·2H₂O loses water to give plaster of Paris CaSO₄·½H₂O.
These names are fixed by tradition, so recall by pairs is the only route.
What is the chemical formula of baking soda?
Show solutionHide solution
Baking soda is sodium hydrogen carbonate.
Washing soda is the other one, Na₂CO₃·10H₂O.
NaHCO₃
Natural source and acid
The question says 'tamarind contains', 'the acid in an ant sting', or gives a match-the-column of foods and acids.
Recall the kitchen map.
Tamarind and grapes: tartaric. Tomato and spinach: oxalic. Apple: malic.
Ant sting and nettle: formic. Curd: lactic. Stomach: hydrochloric.
For a sting, use baking soda to neutralise.
Each food has one main acid that gives its sour taste.
Which acid is present in tamarind?
Show solutionHide solution
Tamarind is not citrus.
Grapes share its acid.
Tartaric acid
Indicators and pH values
The question asks 'phenolphthalein turns pink in', turmeric with a base, the pH of blood, or whether a given pH is acidic.
Recall the indicator colours.
pH below 7 is acid, 7 is neutral, above 7 is base.
Blood is about 7.4. Tooth enamel dissolves below pH 5.5.
A lower pH means a stronger acid. Each unit is ten times.
The pH scale counts hydrogen ions on a log scale, so one step means ten times.
Solution A has pH 3 and solution B has pH 5. How much more acidic is A?
Show solutionHide solution
The pH differs by 2 units.
Each unit is ten times, so 10 × 10 = 100.
100 times more acidic
Salt chemistry and standard reactions
The question says 'CO₂ turns lime water', 'plaster of Paris is made from', chlor-alkali products, or quicklime with water.
Acid plus carbonate gives CO₂.
CO₂ with lime water gives white CaCO₃, which looks milky.
Heated gypsum gives plaster of Paris. Water sets it back to gypsum.
Brine electrolysis gives NaOH, Cl₂ and H₂.
Each of these is a fixed reaction, so link the input to the product.
Why does lime water turn milky when CO₂ is passed through it?
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CO₂ reacts with Ca(OH)₂.
Insoluble CaCO₃ forms.
Calcium carbonate forms, a standard test for CO₂
Uses of baking soda, washing soda, bleaching powder
The question asks which chemical is an antacid, removes hardness, or disinfects water, or why cakes rise.
Baking soda: antacid, baking powder, soda-acid extinguisher.
Washing soda: removes permanent hardness; used for glass and borax.
Bleaching powder: disinfects drinking water, bleaches cotton.
Match the job to the chemical.
Each chemical has a few well-known uses. Exams test only those.
Permanent hardness of water is removed by adding which chemical?
Show solutionHide solution
Permanent hardness comes from chlorides and sulphates.
Washing soda precipitates calcium and magnesium.
Washing soda (Na₂CO₃·10H₂O)
Vitriols and water of crystallisation
The question asks 'oil of vitriol is', 'blue vitriol is', or why blue copper sulphate turns white on heating.
Recall: oil is H₂SO₄, blue is CuSO₄·5H₂O, green is FeSO₄·7H₂O, white is ZnSO₄·7H₂O.
The dot-water is water of crystallisation.
Heating drives it off. Moistening restores it.
Fixed water molecules inside the crystal cause its colour.
Blue vitriol (CuSO₄·5H₂O) is heated strongly. What colour does it turn?
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It loses its five waters.
The anhydrous salt is white.
White; adding water turns it blue again
Formula sheet
pH + pOH = 14 at 25 °C
PoP → gypsum
Shortcuts that save time
Vinegar-Acetic, Lemon-Citric, Tamarind-Tartaric, Tomato-Oxalic, Curd-Lactic, Apple-Malic, Ant-Formic. Matching first letters helps: Tamarind and Tartaric, Formica (Latin for ant) and Formic.
Which acid is present in tomato?
Show solutionHide solution
Tomato and spinach share one acid.
That acid is oxalic.
Oxalic acid
Blue vitriol is copper sulphate. Green is iron (ferrous) sulphate. White is zinc sulphate. Oil of vitriol is H₂SO₄.
Mistakes to avoid
Where most students lose marks on this subtopic.
Swapping baking soda and washing soda.
Baking soda is NaHCO₃. Washing soda is Na₂CO₃·10H₂O.
Writing aqua regia as 3 HNO₃ : 1 HCl.
It is 1 part HNO₃ to 3 parts HCl.
Thinking rust forms in dry air.
Rust needs both oxygen and moisture.
Saying tamarind has citric acid.
Tamarind has tartaric acid. Citric acid is in lemon.
Removing permanent hardness by boiling.
Boiling removes only temporary hardness. Use washing soda for permanent hardness.
Quick revision
Read this the night before the exam.
pH below 7 acid, 7 neutral, above 7 base. Blood is 7.4.
Tooth decay starts below pH 5.5. Acid rain is below about 5.6.
Phenolphthalein: colourless in acid, pink in base.
Ant sting: formic acid. Tamarind: tartaric. Tomato: oxalic.
Baking soda NaHCO₃. Washing soda Na₂CO₃·10H₂O. Caustic soda NaOH.
Gypsum CaSO₄·2H₂O gives plaster of Paris CaSO₄·½H₂O.
Milky lime water means CO₂. Aqua regia is HNO₃ : HCl = 1 : 3.
Practice: 22 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.