Indicators & chemical properties of acids and bases
Lemon juice tastes sour and soap feels slippery. Both change the colour of special dyes called indicators.
An indicator shows by colour change whether a substance is acidic or basic. Acids (sour, turn blue litmus red) and bases (bitter, soapy, turn red litmus blue) react with metals, carbonates and each other. A base that dissolves in water is called an alkali.
Equation / formula : Acid + metal → salt + hydrogen. Acid + metal carbonate → salt + water + carbon dioxide. Acid + base → salt + water (neutralisation). Base + non-metal oxide → salt + water.
In the figure : Phenolphthalein is colourless in acid and in neutral water.
Rule 1 : Acids and bases with metals and carbonates — Active metals give hydrogen gas (pop sound with a flame). Carbonates and bicarbonates give carbon dioxide (turns lime water milky).
- Easy : Zn + 2HCl → ZnCl₂ + H₂
- Medium : Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ ; NaHCO₃ + HCl → NaCl + H₂O + CO₂
- For exams : CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (milky); excess CO₂: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (clear again).
Rule 2 : Neutralisation — An acid and a base cancel each other and form a salt and water.
- Easy : NaOH + HCl → NaCl + H₂O
- Medium : Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O
- For exams : Base + non-metal oxide: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Antacid (Mg(OH)₂) neutralises excess stomach HCl.
Never taste unknown chemicals. Use indicators or litmus paper instead.
Saying all bases are alkalis. Only bases that dissolve in water are alkalis (NaOH, KOH yes; Cu(OH)₂ no).
Bubbles with metal → H₂. Bubbles with carbonate → CO₂. Memory clue : "ABC": Acid turns Blue litmus to red; "BRB": Base turns Red litmus to Blue.
Spot the difference
| Point | Acid | Base |
|---|---|---|
| Taste | sour | bitter |
| Blue litmus | turns red | no change |
| Red litmus | no change | turns blue |
Practice questions
- 1Which litmus turns red in acid?
- 2Name the gas formed when zinc reacts with hydrochloric acid.
- 3What colour does phenolphthalein show in a base?
- 4Write the equation for NaHCO₃ + HCl.
- 5Why does lime water turn milky when CO₂ is passed?
- 6What is neutralisation? Give one example.
- 7A student tests a gas: it burns with a pop. Which gas, and which two reactants could give it?
- 8Why is milk of magnesia taken for acidity?
Show answers (try them yourself first)
- 1Blue litmus
- 2Hydrogen
- 3Pink
- 4NaHCO₃ + HCl → NaCl + H₂O + CO₂
- 5Insoluble CaCO₃ forms.
- 6Acid + base → salt + water; e.g. NaOH + HCl → NaCl + H₂O
- 7Hydrogen; an active metal (Zn) and an acid (HCl).
- 8It is a weak base; it neutralises extra stomach acid.
Acids and bases in water (ions, dilution)
Dry glucose does not conduct electricity, but lemon juice in water does. The difference is ions.
In water an acid releases H⁺ (hydrogen) ions, and a base releases OH⁻ (hydroxide) ions. In water H⁺ joins a water molecule to form H₃O⁺ (hydronium). A strong acid/base ionises almost completely; a weak one only partly. Dilution means adding water to lower ion concentration.
Equation / formula : HCl → H⁺ + Cl⁻ NaOH → Na⁺ + OH⁻ H⁺ + OH⁻ → H₂O. Always add acid to water slowly, never water to acid.
In the figure : The last card is the safety rule: dissolving acid releases a lot of heat.
Rule 1 : Why dry acid does not show acidity — H⁺ ions are released only when an acid dissolves in water. Dry HCl gas does not change dry litmus; HCl solution does.
- Easy : Glucose and alcohol have hydrogen but give no H⁺, so they are not acids.
- Medium : Dilute HCl lights the bulb in a conductivity test; glucose solution does not.
- For exams : Acetic acid (weak) lights the bulb dimly; HCl (strong) lights it brightly, as it gives more ions.
Rule 2 : Dilution and concentration — Adding water lowers the ion concentration per unit volume; the substance itself is unchanged in amount.
- Easy : Adding water to lemon juice makes it less sour.
- Medium : Diluting an acid raises its pH towards 7 but never beyond 7.
- For exams : Mixing is exothermic for NaOH pellets and conc. H₂SO₄ in water; so add slowly with stirring.
Strength (how fully it ionises) and concentration (how much is dissolved) are different ideas.
Writing H₃O⁺ and H⁺ as different things. H⁺ in water is really H₃O⁺; both show acidity.
Ions decide: acids give H⁺, bases give OH⁻. Memory clue : "A-H, B-OH": Acid–H⁺, Base–OH⁻; and "Acid into water, like a Little drop at a time".
Spot the difference
| Point | Strong acid | Weak acid |
|---|---|---|
| Ionisation | almost complete | partial |
| Example | HCl, H₂SO₄, HNO₃ | acetic, citric |
Practice questions
- 1Which ion do all acids give in water?
- 2Which ion do bases give?
- 3Why is acid added to water and not water to acid?
- 4Why does glucose not behave as an acid even though it has hydrogen?
- 5Write the ionisation of NaOH in water.
- 6A bulb glows brightly with HCl and dimly with acetic acid of equal concentration. Why?
- 7Does pH of an acid ever become greater than 7 on dilution? Explain.
- 8Explain with ions why a dry HCl gas does not turn dry blue litmus red but its solution does.
Show answers (try them yourself first)
- 1H⁺ (H₃O⁺)
- 2OH⁻
- 3Acid dissolving in water gives a lot of heat; adding water to acid can make it splash out.
- 4It does not release H⁺ ions in water.
- 5NaOH → Na⁺ + OH⁻
- 6HCl gives more ions (strong acid).
- 7No; it moves close to 7 but stays below 7.
- 8H⁺ ions are released only in water.
pH scale & its importance
Tooth decay, plant growth in soil and the sourness of curd are all linked to pH.
The pH scale (0–14) measures how acidic or basic a solution is by the hydrogen-ion concentration. pH 7 is neutral; below 7 acidic; above 7 basic. Each step is 10 times change in H⁺ concentration. Universal indicator shows pH by colour.
Equation / formula : pH below 7 → acid; pH = 7 → neutral; pH above 7 → base. pH 3 is 100 times more acidic than pH 5.
In the figure : Red end is strongly acidic, violet end strongly basic.
Rule 1 : Using pH to compare — Lower pH means more H⁺ and stronger acidity. A change of one unit is a ten-fold change.
- Easy : Which is more acidic: pH 2 or pH 6? pH 2.
- Medium : pH 3 vs pH 5: two steps, so 10 × 10 = 100 times more acidic.
- For exams : Order of acidity (most to least): lemon juice (2) → tomato (4) → milk (6.5) → pure water (7) → baking soda (8.3).
pH scale & its importance
In the app you can move this figure with your finger.
Open in the appRule 2 : Importance of pH in life — Living things and farming work only within suitable pH ranges.
- Easy : Human body works best near pH 7.0–7.8. Acid rain has pH below 5.6 and harms plants and monuments.
- Medium : Tooth enamel starts to corrode below pH 5.5; toothpaste is basic and neutralises mouth acid.
- For exams : Bee sting contains methanoic acid: rub baking soda. Nettle sting is also acidic. Soil that is too acidic is treated with lime.
Pure water has pH 7 at room temperature. Never judge a solution's pH by taste or touch.
Saying pH 14 is the weakest base. pH 14 is the strongest; pH 7.5 is a very weak base.
Smaller pH = more acidic. Move 1 step = 10 times. Memory clue : "Low and Lemon, High and Hand-soap": low pH = lemon-like acid, high pH = soap-like base.
Spot the difference
| Point | Acidic | Neutral | Basic |
|---|---|---|---|
| pH | 0–6.9 | 7 | 7.1–14 |
| Example | lemon | pure water | soap |
Practice questions
- 1What is the pH of pure water?
- 2Which is more acidic: pH 1 or pH 4?
- 3Name the indicator that shows full range of pH colours.
- 4How many times more acidic is pH 2 than pH 4?
- 5Why is toothpaste basic?
- 6What happens to a bee-sting victim when baking soda is rubbed on the sting?
- 7A soil sample has pH 4.5. Which substance should the farmer add and why?
- 8Explain why acid rain is harmful to marble monuments with an equation.
Show answers (try them yourself first)
- 17
- 2pH 1
- 3Universal indicator
- 4100 times
- 5To neutralise acids in the mouth.
- 6The acid is neutralised and pain reduces.
- 7Lime (CaO / Ca(OH)₂), a base, to neutralise the acid soil.
- 8Marble is CaCO₃: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
Salts & chemicals from common salt
Table salt on the plate is also the raw material for baking soda, washing soda, bleaching powder and caustic soda.
A salt is formed when an acid and a base neutralise each other. Salts of strong acid + strong base are neutral (pH 7); strong acid + weak base are acidic; weak acid + strong base are basic. Electrolysis of brine (chlor-alkali process) gives NaOH, Cl₂ and H₂.
Equation / formula : 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (electricity). NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (heat). Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
In the figure : Cathode gives H₂, anode gives Cl₂; NaOH stays in the solution.
Rule 1 : Uses of the chlor-alkali products — Each product has an industrial use.
- Easy : NaOH: soap, paper. Cl₂: water treatment. H₂: fuel, margarine.
- Medium : Bleaching powder (CaOCl₂) is made from dry slaked lime and chlorine; used to disinfect water and bleach cloth.
- For exams : Hydrogen + nitrogen is also used to make ammonia in fertiliser industry.
Rule 2 : Baking soda and washing soda — Baking soda (NaHCO₃) is mildly basic; heating gives washing soda's parent Na₂CO₃, which crystallises as Na₂CO₃·10H₂O.
- Easy : Baking powder = baking soda + tartaric acid; CO₂ makes cake soft and spongy.
- Medium : Washing soda removes permanent hardness of water.
- For exams : Baking soda is used in antacids and in soda-acid fire extinguishers.
Common salt is NaCl, from sea water or rock salt (brown crystals, mined).
Confusing baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O).
Brine + electricity = NaOH, Cl₂, H₂. Memory clue : "Chlor-ALKALI": Chlorine + ALKALI (NaOH) together, with hydrogen as a free gift.
Spot the difference
| Point | Baking soda | Washing soda |
|---|---|---|
| Formula | NaHCO₃ | Na₂CO₃·10H₂O |
| Use | cooking, antacid | cleaning, hard water |
Practice questions
- 1Name the three products of electrolysis of brine.
- 2Give the formula of baking soda.
- 3Name the compound used to disinfect drinking water.
- 4What happens when baking soda is heated? Write the equation.
- 5Why does baking powder make cake soft?
- 6Predict pH of NaCl solution and explain.
- 7Why is Na₂CO₃ solution basic? Which acid and base made it?
- 8Write balanced equation for bleaching powder preparation and give two uses.
Show answers (try them yourself first)
- 1NaOH, Cl₂, H₂
- 2NaHCO₃
- 3Bleaching powder (chlorine compounds)
- 42NaHCO₃ → Na₂CO₃ + H₂O + CO₂
- 5CO₂ released makes bubbles.
- 67; strong acid + strong base give a neutral salt.
- 7Weak acid (H₂CO₃) + strong base (NaOH).
- 8Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O; disinfecting water, bleaching cloth.
Water of crystallisation & Plaster of Paris
Doctors set a broken bone with a white powder that hardens into a cast when mixed with water.
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt crystal. Blue copper sulphate crystals are CuSO₄·5H₂O; on heating they lose water and become white anhydrous CuSO₄. Plaster of Paris is calcium sulphate hemihydrate CaSO₄·½H₂O.
Equation / formula : CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O (heat / add water). CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O (at 373 K). CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (sets).
In the figure : Heating loses water to give POP; adding water brings it back to hard gypsum.
Rule 1 : Hydrated and anhydrous salts — Crystals look dry but contain fixed water. Removing it changes colour or shape.
- Easy : Blue CuSO₄·5H₂O turns white on heating; add water and blue returns (test for water).
- Medium : Washing soda Na₂CO₃·10H₂O has 10 water molecules per formula unit.
- For exams : Common salt crystals have no water of crystallisation, so they do not change colour on heating.
Rule 2 : Plaster of Paris uses — Mixed with water it sets in a few minutes into a hard mass.
- Easy : Used to set fractured bones and to make toys and decorations.
- Medium : Used for smooth surface finish on walls and for making casts of statues.
- For exams : POP must be stored in a moisture-proof container, otherwise it absorbs water and hardens.
POP is made by heating gypsum at 373 K (100 °C). Heating above this removes all water and the plaster loses its setting property.
Thinking "water of crystallisation" means the crystal is wet. It is chemically bound water in a fixed ratio.
Heat removes water; adding water brings it back (blue ⇌ white). Memory clue : "Gypsum, half, set": gypsum → ½ water → sets again.
Spot the difference
| Point | Gypsum | Plaster of Paris |
|---|---|---|
| Formula | CaSO₄·2H₂O | CaSO₄·½H₂O |
| Water per unit | 2 | ½ |
Practice questions
- 1What is water of crystallisation?
- 2Give the formula of blue vitriol.
- 3What is the formula of Plaster of Paris?
- 4What happens when blue CuSO₄·5H₂O is heated?
- 5Why is POP stored in a moisture-proof container?
- 6Write the equation for setting of POP.
- 7Does common salt contain water of crystallisation? How can you test?
- 8How many water molecules are lost when 1 unit of CuSO₄·5H₂O is fully heated? What colour test shows the water is present again?
Show answers (try them yourself first)
- 1Fixed number of water molecules in a crystal unit.
- 2CuSO₄·5H₂O
- 3CaSO₄·½H₂O
- 4It turns white (anhydrous).
- 5It absorbs water and hardens.
- 6CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
- 7No; it does not change colour on heating.
- 85 molecules; white turns blue again.