CBSE Class 10 Science • Chapter 2 • Notes and Mind Map
Acids, Bases and Salts Class 10 Notes and Mind Map
A colour-changing indicator, a fizzing carbonate and a packet of baking powder all connect to the chemistry of acids, bases and salts. These Acids, Bases and Salts Class 10 notes explain those connections through ions, reaction patterns, pH and familiar compounds. Use the mind map to bring the chapter’s ideas together for revision.
Begin with what happens when a substance dissolves in water. Then connect its ions with indicator changes, chemical reactions and pH. Finally, organise important compounds by their preparation, formula and use. This sequence helps you understand the chapter rather than memorising unrelated facts.
Acids and Bases: Why Water Matters
In the introductory aqueous model, acids produce hydrogen ions in water. These associate with water molecules to form hydronium ions, H3O+. Bases such as sodium hydroxide furnish hydroxide ions, OH−, in solution.
Read the Ions Behind the Formula
HCl + H2O → H3O+ + Cl−
NaOH(aq) → Na+(aq) + OH−(aq)
These ions explain why the solutions show acidic or basic behaviour and conduct electricity.
Not every hydrogen-containing compound is an acid. Glucose and ethanol contain hydrogen, but they do not release it as hydronium ions in water in the way an acid does.
Base and Alkali
An alkali is a water-soluble base. Sodium hydroxide is an alkali, while copper(II) oxide is a basic oxide that is not soluble in water. All alkalis are bases, but all bases are not alkalis.
Indicators Class 10: Read the Colour Change
| Indicator | Acidic solution | Basic solution |
|---|---|---|
| Litmus | Blue litmus turns red. | Red litmus turns blue. |
| Phenolphthalein | Colourless. | Pink in its usual basic transition range. |
| Methyl orange | Red in its acidic range. | Yellow in its basic range. |
| Turmeric | Remains yellow. | Turns reddish-brown. |
| Universal indicator | Colour depends on pH. | Compare the colour with its pH chart. |
One Colour Does Not Tell the Whole Story
Phenolphthalein is colourless in both acidic and neutral solutions. A colourless result alone cannot distinguish between them. Choose another indicator or use pH paper.
pH Scale Class 10 Short Notes
pH describes the acidity of an aqueous solution. On the familiar school scale at about 25°C, a neutral solution has pH 7, an acidic solution has pH below 7 and a basic solution has pH above 7.
Lower pH means greater hydronium-ion concentration. The scale is logarithmic: a difference of one pH unit corresponds to a tenfold difference in hydronium-ion concentration.
Compare Two Acidic Solutions
A solution of pH 2 has ten times the hydronium-ion concentration of a solution of pH 3.
Both are acidic. The lower pH indicates the greater hydronium-ion concentration.
Strong and Weak Are Different from Concentrated and Dilute
Acid strength concerns the extent of ionisation in water. Concentration concerns how much acid is present in a given volume of solution. A strong acid can be dilute, and a weak acid can be concentrated.
Diluting an acidic solution generally raises its pH towards neutral; diluting a basic solution generally lowers its pH towards neutral. Dilution does not turn a strong acid into a weak acid.
Chemical Properties: Recognise the Product Pattern
| Reaction pattern | Products | Example |
|---|---|---|
| Suitable metal + dilute acid | Salt + hydrogen. | Zn + 2HCl → ZnCl2 + H2 |
| Carbonate + acid | Salt + water + carbon dioxide. | Na2CO3 + 2HCl → 2NaCl + H2O + CO2 |
| Hydrogencarbonate + acid | Salt + water + carbon dioxide. | NaHCO3 + HCl → NaCl + H2O + CO2 |
| Acid + hydroxide base | Salt + water. | HCl + NaOH → NaCl + H2O |
| Basic metal oxide + acid | Salt + water. | CuO + 2HCl → CuCl2 + H2O |
Neutralisation
Neutralisation involves an acid reacting with a base. For a strong acid and hydroxide base, hydronium and hydroxide ions combine to form water:
H3O+ + OH− → 2H2O
Identifying Carbon Dioxide
Carbon dioxide turns limewater milky because insoluble calcium carbonate forms. This connects the gas released in an acid–carbonate reaction with an observable test.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Common Salt as a Starting Material
Sodium chloride is more than table salt. Brine, a concentrated aqueous solution of sodium chloride, is a starting material for important industrial chemicals.
The Chlor-Alkali Process
Electrolysis of brine produces chlorine at the anode, hydrogen at the cathode and sodium hydroxide in solution. The name connects chlorine with the alkali produced.
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Condition: Electrolysis of brine.
Important Compounds: Formulas and Uses
| Compound | Formula | Important use or connection |
|---|---|---|
| Sodium hydroxide | NaOH | Manufacture of soaps, detergents and paper. |
| Bleaching powder | CaOCl2, the conventional school representation. | Bleaching and disinfection. |
| Baking soda | NaHCO3 | Ingredient in baking powder; releases carbon dioxide. |
| Washing soda | Na2CO3·10H2O | Cleaning and removal of permanent water hardness. |
| Gypsum | CaSO4·2H2O | Starting material for plaster of Paris. |
| Plaster of Paris | CaSO4·½H2O | Making casts and moulds; sets after adding water. |
Baking Soda and Baking Powder
Baking soda is sodium hydrogencarbonate. Baking powder contains baking soda and an edible acid, often with a drying ingredient. Carbon dioxide released during baking helps the mixture rise.
2NaHCO3 →
Na2CO3 + CO2 + H2O
Condition: Heating.
Water of Crystallisation
Water of crystallisation is water present in a definite proportion within a crystalline compound. The dot in a hydrate formula indicates this association; it is not a multiplication sign used in a calculation.
Plaster of Paris and Gypsum
Controlled heating of gypsum at about 373 K produces plaster of Paris. Adding water converts it back into gypsum as the material sets.
CaSO4·½H2O + 1½H2O → CaSO4·2H2O
The half-water notation describes a ratio: one water molecule for every two calcium sulphate formula units. It does not mean that an isolated water molecule is cut in half.
pH in Everyday Life
- Digestion: stomach acid provides an acidic environment; antacids neutralise excess acid.
- Teeth: acid produced by mouth bacteria can promote enamel dissolution, commonly discussed below about pH 5.5.
- Soil: plants have suitable pH ranges; soil conditions influence nutrient availability.
- Aquatic ecosystems: substantial pH changes can affect organisms and water chemistry.
How to Revise with the Acids, Bases and Salts Mind Map
Follow the mind map from ions to observations, then to reactions and useful compounds. For every branch, recall one explanation and one example rather than only a keyword.
- Acids and bases: connect their behaviour with hydronium and hydroxide ions.
- Indicators: recall colour changes and what each test can establish.
- pH: connect acidity, dilution and everyday applications.
- Reactions: identify the product pattern before writing the equation.
- Common salt: connect brine with the chlor-alkali process.
- Useful compounds: link each name with its formula, preparation and use.
Cover one branch and explain it from memory. If you confuse two compounds, compare them side by side—for example, baking soda with washing soda, or gypsum with plaster of Paris.
Frequently Asked Questions
Why does dry hydrogen chloride not change dry litmus?
Water is needed for hydrogen chloride to form hydronium ions and show the usual acidic indicator response. Moist blue litmus turns red.
Why do acid solutions conduct electricity?
Their mobile ions carry electric charge through the solution. Electrical conduction is evidence of ions, not simply of hydrogen atoms in a formula.
Why is acid added to water during dilution?
Dilution can release considerable heat. Adding acid slowly to a larger quantity of water allows heat to disperse more effectively and reduces splashing risk.
Does dilution make a strong acid weak?
No. It lowers concentration. Strong and weak describe ionisation behaviour, while concentrated and dilute describe the amount present per volume.
Are all salt solutions neutral?
No. Depending on their ions, salt solutions can be acidic, basic or approximately neutral. The word “salt” does not imply pH 7.
Does neutralisation always leave a solution of pH 7?
No. Final pH depends on the acid and base involved, their amounts and the salt formed. Excess acid or base also affects the result.
Is baking soda acidic because it contains hydrogen?
No. Sodium hydrogencarbonate forms a mildly basic aqueous solution. Hydrogen in a formula does not by itself establish acidic behaviour.
How is baking soda different from washing soda?
Baking soda is NaHCO₃. Washing soda is Na₂CO₃·10H₂O. They have different compositions and uses and should not be treated as interchangeable.
Why should plaster of Paris be stored away from moisture?
It reacts with water to form gypsum and sets. Moisture can therefore cause premature hydration and reduce its usefulness.
What makes copper sulphate crystals blue?
Hydrated copper(II) sulphate crystals are blue. Heating removes water of crystallisation, producing white anhydrous copper sulphate; adding water restores the hydrated form.
Why is the mind map useful after reading the notes?
It helps you recall relationships between ions, indicators, pH, reactions and compounds. Use it as a memory prompt after understanding the explanations.
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