🧪 1. Acids, Bases, and Salts — Classification
⬇
ACIDS 🔴
Sour taste
Turns blue
litmus → RED
pH < 7
E.g. HCl, H₂SO₄, lemon juice, vinegar
BASES 🔵
Bitter taste, soapy
feel
Turns red litmus → BLUE
pH > 7
E.g. NaOH, NH₃, baking soda, soap
SALTS 🟢
Formed by acid +
base
Neutral or slightly acidic/basic
pH ≈ 7
E.g. NaCl, CaCO₃
| Property |
Acid |
Base |
Salt |
| Taste |
Sour (e.g. lemon, tamarind) |
Bitter (e.g. soap, baking soda) |
Variable (salty, bitter) |
| Feel |
No special feel |
Soapy / slippery |
Normal |
| Effect on Litmus |
Blue → Red |
Red → Blue |
No change |
| pH range |
0 – 6 |
8 – 14 |
~7 |
| Reaction |
React with metals to release H₂ gas |
React with acids (neutralisation) |
Formed by acid + base |
| Examples |
HCl, H₂SO₄, Citric acid, Vinegar (acetic acid) |
NaOH, Ca(OH)₂, Baking soda (NaHCO₃) |
NaCl (common salt), CaCO₃, CaSO₄ |
🎨 2. The pH Scale
The pH scale is a number scale from 0 to 14 that tells us how acidic or basic a
solution is. pH = 7 means neutral.
01234567891011121314
pH of common substances:
Battery acid (pH 0–1) • Stomach acid (pH 1–2) • Lemon juice (pH 2) • Vinegar (pH 3) • Tomato (pH 4) • Rain
water (pH 5.6) • Pure water (pH 7) • Blood (pH 7.4) • Egg white (pH 8) • Baking soda (pH
8–9) • Soap (pH 9–10) • Bleach (pH 12–13) • Drain cleaner (pH 14)
📸 AI Image Prompt
A beautiful, colorful horizontal pH scale bar from 0 to 14. The bar is a smooth
gradient from bright red (0 — STRONG ACID) through orange, yellow, light green (7 — NEUTRAL, marked with
a dotted vertical line) and then to dark blue (14 — STRONG BASE). Below each number on the scale, small
icons show everyday substances: 0=battery (pH 0), 1=stomach (gastric acid), 2=lemon, 3=vinegar bottle,
4=tomato, 5=black coffee, 6=milk, 7=pure water drop (special highlight box), 8=egg, 9=baking soda,
10=soap bar, 11=ammonia bottle, 12=bleach, 13=oven cleaner, 14=drain cleaner. Above the bar: "ACIDIC" on
the left in red and "BASIC / ALKALINE" on the right in blue, with a neutral label above 7. Litmus paper
color changes shown: blue turning red for acid, red turning blue for base. Clean infographic style,
white background, large font, visually striking.
Fig. 4.1 — The pH scale with everyday examples
🔬 3. Indicators — How We Detect Acids and Bases
An indicator is a substance that changes colour in the presence of an acid or a
base. It tells us whether something is acidic or basic.
| Indicator |
In Acid |
In Base |
In Neutral |
| Litmus (Red) |
Red (no change) |
Blue |
Red |
| Litmus (Blue) |
Red |
Blue (no change) |
Blue |
| Turmeric |
Yellow (no change) |
Red/Brown |
Yellow |
| China Rose (Gudhal) |
Magenta/Pink |
Green |
Colourless |
| Phenolphthalein |
Colourless |
Pink |
Colourless |
| Methyl Orange |
Red/Orange |
Yellow |
Orange |
Litmus is extracted from lichen. Turmeric, China rose petals, red cabbage juice → all
natural indicators! Try this: add lemon juice to turmeric paper → it stays yellow. Add soap water → turns
red/brown!
⚗️ 4. Neutralisation Reaction
⬇
Example: HCl + NaOH → NaCl + H₂O (Neutralisation)
Real-life applications of neutralisation:
- Antacid tablets (like Eno, Gelusil): Acidic stomach (excess HCl) → take a base
(NaHCO₃) → neutralises acidity → relief!
- Soil treatment: Acidic soil → add slaked lime [Ca(OH)₂] to neutralise. Basic soil →
add compost/organic matter
- Insect stings: Bee sting (formic acid/acidic) → apply baking soda (base) to
neutralise pain. Wasp sting (alkaline) → apply vinegar (acid)
- Factory waste: Acidic industrial waste treated with lime/base before releasing into
environment
📝 5. Quick Revision
- Acids: sour, pH<7, turns blue litmus red. E.g. HCl, lemon juice, vinegar
- Bases: bitter, soapy feel, pH>7, turns red litmus blue. E.g. NaOH, baking soda
- Neutral: pH = 7. E.g. pure water
- Litmus extracted from lichen. Turns red in acid, blue in base
- Turmeric: yellow in acid, red in base. China Rose: pink in acid,
green in base
- Neutralisation: Acid + Base → Salt + Water + Heat
- Bee sting = acidic → treat with baking soda (base)
- Wasp sting = alkaline → treat with vinegar (acid)
- Antacids = base → neutralise excess stomach acid (HCl)
- Acidic soil → add slaked lime [Ca(OH)₂] to make it suitable for crops
