Acids, Bases & Salts — The Complete Class 10 Guide
Understand acids, bases and salts with indicators, the pH scale, neutralisation reactions and everyday salts like baking soda, washing soda and plaster of Paris — explained simply for Class 10.
By the PadhoDost Team · 📖 7 min read · Updated 4 August 2026
Part of Class 10 (CBSE) prep🧠 Lemon vs Soap
Bite into a lemon and your face scrunches up — that sour tang is an acid at work. Rub a bar of soap between wet fingers and it feels slippery and slightly bitter — that is a base. Almost every acid tastes sour and every base feels soapy. Chemistry just gives us a safe way to tell them apart without tasting anything, because in the lab many acids and bases are dangerous.
The Core Idea: It's All About Ions
When an acid dissolves in water it releases hydrogen ions (H+), which actually join water to form hydronium ions (H3O+). When a base dissolves in water it releases hydroxide ions (OH-). A base that dissolves in water is called an alkali (like NaOH and KOH). These invisible ions are what make acids and bases behave the way they do. We detect them using indicators — substances that change colour or smell in acids and bases.
| Indicator | In Acid | In Base |
|---|---|---|
| Blue litmus | Turns red | No change |
| Red litmus | No change | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red / pink | Yellow |
| Onion / vanilla (smell) | Smell remains | Smell fades |
Reactions You Must Know
The pH Scale in Daily Life
The pH scale runs from 0 to 14 and measures how acidic or basic a solution is. pH 7 is neutral (pure water). Below 7 is acidic — the lower the number, the stronger the acid. Above 7 is basic — the higher the number, the stronger the base. pH matters everywhere: our stomach uses HCl (about pH 1-2) to digest food, an antacid (a mild base) cools acidity, plants grow best in soil of the right pH, and tooth decay starts when mouth pH falls below 5.5. Strong acids and bases ionise fully in water; weak ones ionise only partly.
| Salt (common name) | Formula | Everyday Use |
|---|---|---|
| Common salt | NaCl | Cooking; raw material for NaOH, baking soda |
| Baking soda | NaHCO3 | Antacid, baking, fire extinguishers |
| Washing soda | Na2CO3.10H2O | Cleaning, softening hard water |
| Bleaching powder | CaOCl2 | Bleaching cloth/paper, disinfecting water |
| Plaster of Paris | CaSO4.(1/2)H2O | Casts for fractures, making chalk, plastering |
📝 Worked Example: Making a Salt
Question: What is formed when hydrochloric acid reacts with sodium hydroxide?
Acid HCl supplies H+ ions; base NaOH supplies OH- ions.
H+ and OH- combine: H+ + OH- -> H2O (water).
The leftover ions Na+ and Cl- combine: Na+ + Cl- -> NaCl (a salt).
Full equation: NaOH + HCl -> NaCl + H2O.
This is a neutralisation reaction — heat is released and the pH moves toward 7.
Remember These
- ✓Acids give H+ (as H3O+) in water; bases give OH- in water.
- ✓Acids turn blue litmus red; bases turn red litmus blue; phenolphthalein turns pink in base.
- ✓Acid + Base -> Salt + Water is neutralisation.
- ✓pH 7 is neutral, below 7 acidic, above 7 basic; tooth decay starts below pH 5.5.
- ✓Water of crystallisation is the fixed water in a crystal, e.g. CuSO4.5H2O (blue), Na2CO3.10H2O.
⚡ Quick check
Tooth decay begins when the pH of the mouth falls below 5.5. Brushing with toothpaste (which is basic) helps because it:
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