Acids, Bases and Salts
Cheat sheetClass 10 · Science
⚑ Panic box
In water: acids → $H_3O^+$; bases → $OH^-$
Neutralisation: $H^+ + OH^- \rightarrow H_2O$
pH: below 7 acidic · 7 neutral · above 7 basic
Carbonates + acids: salt + $CO_2$ + water; $CO_2$ turns limewater milky
Dilution safety: add acid slowly to water, never water to concentrated acid
1. Indicators and reaction patterns
| Indicator / test | Acid | Base | Key cue |
|---|---|---|---|
| Litmus | blue → red | red → blue | Litmus is purple when neutral |
| Turmeric | stays yellow | turns reddish-brown | Soap’s basic solution changes a turmeric stain |
| Olfactory indicators | odour may change | odour may change | Onion, vanilla and clove are examples |
| Burning splint on gas | — | — | Hydrogen burns with a ‘pop’ |
| Gas through limewater | — | — | $CO_2$ makes it milky; excess $CO_2$ clears it |
| Reactants | General result | Key condition / example |
|---|---|---|
| Acid + metal | salt + $H_2$ | e.g. $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$ |
| Base + some metals | salt + $H_2$ | Warm zinc with sodium hydroxide: $2NaOH + Zn \rightarrow Na_2ZnO_2 + H_2$ |
| Acid + carbonate / hydrogencarbonate | salt + $CO_2$ + water | $CO_2$ is confirmed with limewater |
| Acid + base | salt + water | Neutralisation; e.g. $HCl + NaOH \rightarrow NaCl + H_2O$ |
| Metal oxide + acid | salt + water | Metal oxides are basic; e.g. $CuO + 2HCl \rightarrow CuCl_2 + H_2O$ |
| Non-metal oxide + base | salt + water | Non-metal oxides are acidic; $CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O$ |
Carbonate test — $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$
Hydrogencarbonate test — $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$
2. Ions, solutions and strength
- Acid in water — produces $H^+(aq)$, which joins water to form $H_3O^+$; accompanying anion depends on the acid.
- Base in water — produces $OH^-$ ions; soluble bases are alkalis.
- Acidic/basic solutions conduct → mobile ions carry current; glucose and alcohol solutions do not form these ions.
- Dry HCl — no acidic effect without water; it forms ions only in aqueous solution: $HCl + H_2O \rightarrow H_3O^+ + Cl^-$.
- Neutralisation, ionic form — $H^+ + OH^- \rightarrow H_2O$; the remaining ions form the salt.
- Dilution — adding water lowers the number of $H_3O^+$ or $OH^-$ ions per unit volume; dilution of concentrated acid or base releases much heat.
| Compare | Strong | Weak |
|---|---|---|
| Acid | More $H^+$ ions produced | Fewer $H^+$ ions produced |
| Base | More $OH^-$ ions produced | Fewer $OH^-$ ions produced |
Strength depends on ions produced at the same concentration—not simply how concentrated the solution is.
3. pH and everyday links
pH map — $H_3O^+$ concentration ↑ → pH ↓ · $OH^-$ concentration ↑ → pH ↑
- Scale — usually 0–14; lower values are more acidic, higher values more alkaline.
- Universal indicator — mixture of indicators; colour estimates pH.
- Acidic — pH $<7$ · neutral — pH $=7$ · basic — pH $>7$.
| Situation | pH fact / response |
|---|---|
| Body processes | Work within a narrow range, about 7.0–7.8 |
| Acid rain | Rainwater pH below 5.6; can harm aquatic life by lowering river pH |
| Indigestion | Excess stomach acid → mild base antacid, e.g. magnesium hydroxide |
| Tooth decay | Begins below mouth pH 5.5; basic toothpaste helps neutralise acids |
| Bee sting / nettle sting | Acidic sting; mild base such as baking soda can relieve a bee sting |
| Soil | Plant growth needs a suitable pH; lime can treat acidic soil |
4. Salts and their uses
- Salt family — salts sharing a positive ion or a negative ion; e.g. sodium salts or chloride salts.
- Salt solution pH — strong acid + strong base → neutral (pH 7); strong acid + weak base → acidic; strong base + weak acid → basic.
- Common salt — sodium chloride, $NaCl$; obtained from seawater or mined as rock salt; major raw material for useful chemicals.
| Product | Preparation / equation | Main uses or identifying point |
|---|---|---|
| Sodium hydroxide, $NaOH$ | Brine electrolysis: $2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2$ | Chlor-alkali process; chlorine at anode, hydrogen at cathode |
| Bleaching powder, represented $Ca(ClO)_2$ | Chlorine + dry slaked lime | Bleaching; oxidising agent; disinfecting drinking water |
| Baking soda, $NaHCO_3$ | Made using $NaCl$, water, $CO_2$ and ammonia | Mild basic salt; antacid, fire extinguisher, baking |
| Heating baking soda | $2NaHCO_3 \xrightarrow{heat} Na_2CO_3 + H_2O + CO_2$ | Released $CO_2$ helps baked food rise |
| Washing soda, $Na_2CO_3\cdot10H_2O$ | Crystallised hydrated sodium carbonate | Cleaning; glass, soap and paper industries; removes permanent hardness |
| Baking powder | Baking soda + weak edible acid, such as tartaric acid | On heating / wetting, $CO_2$ makes dough soft and spongy |
5. Water in salts and plaster
| Substance | Formula / change | Recall point |
|---|---|---|
| Water of crystallisation | Fixed number of water molecules per formula unit | Hydrated crystals are not simply wet |
| Copper sulphate crystals | $CuSO_4\cdot5H_2O$ (blue) — heating → $CuSO_4$ (white) | Water is driven off; adding water restores blue colour |
| Gypsum | $CaSO_4\cdot2H_2O$ | Heating at 373 K forms plaster of Paris |
| Plaster of Paris (POP) | $CaSO_4\cdot\tfrac{1}{2}H_2O$ | With water → gypsum; hardens into a solid mass |
| POP setting | $2(CaSO_4\cdot\tfrac{1}{2}H_2O) + 3H_2O \rightarrow 2(CaSO_4\cdot2H_2O)$ | Store moisture-proof to prevent premature setting |
6. Traps
All hydrogen-containing compounds are acids→ acids produce $H^+$ / $H_3O^+$ in water; alcohol and glucose do not.Dry HCl turns dry blue litmus red→ water is needed for acidic ions to form.A base must dissolve in water→ only water-soluble bases are alkalis.Higher pH means more $H^+$→ higher pH means less $H_3O^+$ and greater basicity.Add water to concentrated acid→ add acid slowly to water with stirring; mixing is highly exothermic.All salts are neutral→ salt-solution pH depends on the acid and base that formed it.POP can be left exposed to moisture→ it reacts with water and sets as gypsum.