Acids, Bases and Salts — The Chemistry of Taste and Touch
"'Sour' taste means ACID. 'Bitter' taste with a soapy feel means BASE. These properties help us identify everyday chemicals."
1. What Are Acids?
Acids are substances that have a SOUR taste and turn BLUE litmus RED.
Natural Acids — Found in Foods
| Acid | Source |
|---|---|
| Citric acid | Citrus fruits (lemon, orange, lime) |
| Acetic acid | Vinegar |
| Tartaric acid | Tamarind, grapes |
| Lactic acid | Curd, sour milk |
| Oxalic acid | Tomatoes |
| Malic acid | Apples |
| Ascorbic acid (Vitamin C) | Amla (Indian gooseberry), citrus fruits |
| Tannic acid | Tea |
AP Context: 'AP is a major producer of LEMONS (citric acid), TAMARIND (tartaric acid), and MANGOES (citric and ascorbic acids). These sour-tasting fruits are RICH in natural acids.'
Mineral Acids
- Hydrochloric acid (HCl) — in stomach
- Sulphuric acid (H₂SO₄) — car batteries
- Nitric acid (HNO₃) — fertilisers
2. What Are Bases?
Bases are substances that have a BITTER taste, feel SOAPY or SLIPPERY to touch, and turn RED litmus BLUE.
| Base | Common Name | Use |
|---|---|---|
| Sodium hydroxide (NaOH) | Caustic soda | Soap, paper, drain cleaner |
| Calcium hydroxide (Ca(OH)₂) | Slaked lime | Whitewash, soil treatment |
| Magnesium hydroxide (Mg(OH)₂) | Milk of magnesia | Antacid |
| Ammonium hydroxide (NH₄OH) | — | Cleaning solutions |
| Potassium hydroxide (KOH) | Caustic potash | Soap making |
Difference Between Bases and Alkalis
'Alkalis are BASES that are SOLUBLE in water. All alkalis are bases, but NOT all bases are alkalis.'
Example: Copper hydroxide (Cu(OH)₂) is a base (turns red litmus blue) but is NOT soluble in water — so it is NOT an alkali.
3. Indicators — Detecting Acids and Bases
Indicator: A substance that changes COLOUR in acidic or basic solutions.
Natural Indicators
| Indicator | Colour in Acid | Colour in Base |
|---|---|---|
| Litmus (from lichen) | RED | BLUE |
| Turmeric (from turmeric root) | YELLOW (no change) | REDDISH-BROWN |
| China rose (Hibiscus) | DARK PINK/MAGENTA | GREEN |
| Red cabbage juice | RED | GREENISH-YELLOW |
| Grape juice | RED | GREEN |
| Beetroot | RED | YELLOW |
Litmus — The Most Common Indicator
- Available as LIQUID or PAPER (blue and red strips).
- Blue litmus: Turns RED in acid. Stays blue in base.
- Red litmus: Turns BLUE in base. Stays red in acid.
- 'Litmus is extracted from LICHENS (a symbiotic association of fungus and algae).'
Turmeric as an Indicator
'In AP kitchens, turmeric is commonly used in cooking. Rub turmeric on your palm and apply soap (basic) — it turns REDDISH-BROWN. This is a NATURAL indicator test!'
China Rose (Hibiscus) Indicator
'Soak china rose petals in warm water. The coloured water turns MAGENTA in acidic solutions and GREEN in basic solutions. This is an easy DIY indicator.'
Synthetic Indicators
| Indicator | Colour in Acid | Colour in Base |
|---|---|---|
| Phenolphthalein | Colourless | PINK |
| Methyl orange | RED | YELLOW |
Olfactory Indicators
Substances whose SMELL changes in acidic or basic conditions:
- Onion: Smell LOST in base (NaOH).
- Vanilla essence: Smell LOST in base.
- Clove oil: Smell LOST in base.
4. Neutralisation — Acid + Base → Salt + Water
Neutralisation reaction: When an acid and a base react, they FORM a salt and water. The reaction is EXOTHERMIC (releases heat).
General equation: Acid + Base → Salt + Water (+ Heat)
Examples:
- HCl + NaOH → NaCl + H₂O (hydrochloric acid + sodium hydroxide → sodium chloride + water)
- H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
'When you take an antacid (base) for acidity (excess acid in stomach), neutralisation happens inside your body!'
Applications of Neutralisation
| Application | Acid Involved | Base Used | Explanation |
|---|---|---|---|
| Antacid for acidity | Stomach HCl | Magnesium hydroxide / Sodium bicarbonate | Excess acid neutralised by base |
| Insect sting | Bee sting — Formic acid | Baking soda (sodium bicarbonate) paste | Neutralises the acid |
| Wasp sting | — | Wasp sting is BASIC — use vinegar (acetic acid) | Vinegar neutralises the base |
| Soil treatment | Acidic soil | Lime (calcium hydroxide) | Neutralises acidity for better crop growth |
| Factory waste | Acidic industrial waste | Lime / Slaked lime | Prevents water pollution |
AP Context: 'AP farmers often apply LIME (chuna) to acidic soils in the coastal and hill regions to neutralise the pH and improve crop yields for groundnut, cotton, and chilli cultivation.'
5. Salts — Products of Neutralisation
Salt: An IONIC compound formed when an acid reacts with a base.
| Acid | Base | Salt formed |
|---|---|---|
| HCl | NaOH | Sodium chloride (table salt) |
| H₂SO₄ | NaOH | Sodium sulphate |
| HNO₃ | KOH | Potassium nitrate |
| HCl | NH₄OH | Ammonium chloride |
Common Salt (NaCl) — The Most Important Salt
- Obtained from SEAWATER (evaporation).
- AP's coastline is a major source of salt production.
- Used in cooking, preserving food, and as a raw material for many chemicals.
6. Common Mistakes and Fixes
| Mistake | Why It Is Wrong | Correct Approach |
|---|---|---|
| All acids are dangerous and corrosive | Natural acids (citric, acetic) are CONSUMED daily | Only STRONG mineral acids are dangerous |
| Bases and alkalis are the same | Alkalis are a SUBSET of bases | All alkalis are bases but NOT vice versa |
| Indicators only change colour in acids | Indicators change in BOTH acids and bases | Each indicator has specific colours for acid and base |
| Neutralisation always produces neutral pH (7) | The salt formed may be acidic or basic | ONLY strong acid + strong base gives pH 7 |
| Turmeric turns red in acids | Turmeric stays YELLOW in acids; turns REDDISH-BROWN in bases | Turmeric is a base indicator, not an acid indicator |
| Lime and lemon are the same | Lime (chuna) = base (Ca(OH)₂). Lemon = acid (citric acid) | They are chemically OPPOSITES |
7. AP SSC Exam Focus
| Topic | Marks | Question Type |
|---|---|---|
| Natural indicators (litmus, turmeric, china rose) | 3-4 | Colour changes, preparation |
| Neutralisation reaction | 3-4 | Word equation, applications |
| Acids and bases — properties | 2-3 | Distinguish, examples |
| Applications in daily life | 2-3 | Antacids, stings, soil treatment |
| AP context: soil pH, salt production | 1-2 | Short notes |
Quick Self-Test
Q1. What colour does blue litmus turn in an acid? A1. Red.
Q2. Give two examples of natural indicators. A2. Litmus (from lichen), turmeric (from turmeric root), china rose (hibiscus).
Q3. Write the neutralisation reaction between HCl and NaOH. A3. HCl + NaOH → NaCl + H₂O.
Q4. Why is baking soda applied on a bee sting? A4. Bee sting injects formic acid (acidic). Baking soda (sodium bicarbonate) is a base that neutralises the acid, giving relief.
Q5. What colour does china rose indicator turn in a basic solution? A5. Green.
Q6. Why do farmers add lime to acidic soil? A6. Lime (calcium hydroxide) neutralises the excess acid in the soil, making it suitable for crop growth.
Q7. Is vinegar an acid or a base? A7. Vinegar contains acetic acid — it is an ACID.
Q8. What is the difference between a base and an alkali? A8. All alkalis are water-soluble bases. Bases that are NOT water-soluble (e.g., copper hydroxide) are alkalis.
