Every substance around us — lemon juice, soap, water, vinegar, baking soda — has a chemical nature. Chemists classify substances as acidic, basic (alkaline), or neutral based on how they behave chemically, especially in water.
Acids
An acid is a substance that tastes sour, turns blue litmus paper red, and has a pH below 7.
- Common acids:
- Hydrochloric acid (HCl) — found in stomach
- Sulphuric acid (H2SO4) — used in car batteries
- Citric acid — found in lemons and oranges
- Acetic acid — found in vinegar
Bases
A base is a substance that tastes bitter, feels slippery, turns red litmus paper blue, and has a pH above 7. A base that dissolves in water is called an alkali.
- Common bases:
- Sodium hydroxide / caustic soda (NaOH)
- Calcium hydroxide / slaked lime (Ca(OH)2)
- Baking soda (sodium bicarbonate, NaHCO3)
- Washing soda (Na2CO3)
Neutral Substances
A neutral substance has a pH of exactly 7. It does not change the colour of litmus paper. Pure water is the classic example.
Indicators
An indicator is a substance that changes colour in the presence of an acid or a base. Key indicators:
| Indicator | Colour in Acid | Colour in Base |
|-----------|---------------|----------------|
| Litmus | Red | Blue |
| Turmeric | No change (yellow) | Red / brown |
| China rose | Pink/dark pink | Green |
The pH scale runs from 0 to 14: 0–6 is acidic, 7 is neutral, 8–14 is basic/alkaline.
Neutralisation
When an acid and a base react together, they produce salt and water. This reaction is called neutralisation.
Formula: Acid + Base → Salt + Water
For example: HCl + NaOH → NaCl + H2O
Natural Indicators
Many natural substances can act as indicators: red cabbage juice turns red in acid and green in base; turmeric turns red-brown in alkali; beetroot juice shows colour changes in acids and bases.
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Lemon juice turns blue litmus paper red. This shows lemon juice is acidic (contains citric acid).
A solution of baking soda turns red litmus paper blue and has a slippery feel. This confirms baking soda is a base.
When a few drops of turmeric indicator are added to soap solution, the colour changes to red-brown, showing the soap is basic (alkaline).
Ant sting injects formic acid into skin, causing a burning sensation. Rubbing baking soda (a base) on the sting neutralises the acid and relieves pain. This is a real-life neutralisation reaction.
Farmers add lime (calcium hydroxide) to acidic soil to neutralise the excess acid and make the soil suitable for growing crops.
A universal indicator paper dipped into cola drink turns orange-red, indicating a pH of about 3 — strongly acidic.
Pure distilled water does not change litmus paper colour and has a pH of exactly 7, confirming it is neutral.
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Key Formulas / Facts
- pH < 7 → Acidic
- pH = 7 → Neutral
- pH > 7 → Basic / Alkaline
- Neutralisation: Acid + Base → Salt + Water
Common mistakes
- Assuming all acids are dangerous: citric acid and acetic acid are safe for consumption in food.
- Confusing base and alkali: all alkalis are bases, but not all bases are alkalis (only bases that dissolve in water are alkalis).
- Thinking neutral means no reaction — neutral solutions can still participate in chemical reactions.
Summary
Substances are classified as acidic, basic, or neutral based on their pH and effect on indicators. Acids turn blue litmus red; bases turn red litmus blue; neutral substances do neither. The neutralisation reaction between acids and bases produces salt and water, and has many important practical applications.