Acids, Bases and the pH Scale
Lipi Shorts
CBSE · Class 9 · Science
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Acids, Bases and the pH Scale
Central idea
An acid is a hand giving away a hydrogen ion (H+); a base is a hand taking one. The pH scale from 0 to 14 is just a scoreboard for how many free H+ ions are floating in the water.
Central idea
An acid releases hydrogen ions (H+) when dissolved in water, for example HCl -> H+ + Cl-. A base does the opposite: it releases hydroxide ions (OH-), like NaOH -> Na+ + OH-, or it accepts an H+, like ammonia. A base that dissolves in water is called an alkali.
Acids give H+ ions; bases give OH- ions (or accept H+).
Acids and bases
pH measures how acidic or alkaline a solution is, from 0 to 14. The more H+ ions, the lower the pH. Each step of 1 means ten times more (or fewer) H+ ions, so pH 3 has 10000 times more H+ than pH 7.
The pH scale
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Strong acids (HCl, H2SO4) split completely into ions in water. Weak acids (ethanoic acid, CH3COOH) only partly split, sitting in an equilibrium. Strength is about how fully it ionises, not how concentrated it is.
Strong and weak
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When an acid meets a base they cancel out to give a salt and water: acid + base -> salt + water. At ion level it is always H+ + OH- -> H2O. The acid decides the salt: HCl gives chlorides, H2SO4 gives sulfates, HNO3 gives nitrates.
Neutralisation
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See it in a diagram
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Myth vs truth
✕ Myth
Strong and concentrated mean the same thing.
✓ Truth
Strong = fully ionised; concentrated = lots dissolved per litre. They are independent: a dilute strong acid can beat a concentrated weak one.
Myths vs truth
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Word meanings
acid
a substance that releases hydrogen ions (H+) in water; pH below 7
base
a substance that accepts H+ or releases OH- ions; pH above 7
alkali
a base that dissolves in water to give OH- ions
pH
a scale from 0 to 14 measuring how acidic or alkaline a solution is
Word meanings
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Themes & message
•
Acids give H+ ions, bases give OH- ions, and pH counts the free H+.
•
Strength (how fully it ionises) is different from concentration.
•
Acid + base -> salt + water; the core ion reaction is H+ + OH- -> H2O.
Themes & message
Key facts at a glance
Acid
Gives H+ ions in water; pH below 7; turns blue litmus red
Base / alkali
Gives OH- ions in water; pH above 7; turns red litmus blue
pH scale
0 (strong acid) to 14 (strong alkali); 7 is neutral
Key facts at a glance
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