Indicators · pH Scale · Neutralization · Experiments · MCQs
Look around you! The lemon in your kitchen, the soap you wash your hands with, the salt on your dinner table -- they all belong to three important groups of chemicals that we encounter every single day.
In this chapter, we will learn about Acids, Bases, and Salts -- three categories of substances that play a vital role in our daily life, in our body, and in nature!
Substances that taste sour. Found in lemon, vinegar, curd, and even in your stomach!
Substances that taste bitter and feel soapy. Found in soap, baking soda, and antacids.
Formed when an acid reacts with a base. Common salt (NaCl) is the best-known example!
Citric acid in lemon, acetic acid in vinegar (sirka), lactic acid in curd (dahi) -- acids make food tasty!
Antacids (Eno, Digene, Pudin Hara) neutralize excess stomach acid. Medicines use acid-base chemistry!
Farmers test soil pH and add lime (base) or organic matter to make soil suitable for crops.
Soaps and detergents are bases. Window cleaners contain ammonia (a base).
Any matter that has a specific composition and properties (e.g., water, salt, acid).
A uniform mixture where one substance (solute) is dissolved in another (solvent), like salt in water.
A substance that changes colour in the presence of an acid or base, helping us identify them.
Acids taste sour. Bases taste bitter and feel soapy/slippery. (Never taste chemicals in lab!)
An acid is a substance that tastes sour in water solution and turns blue litmus paper red. Acids release hydrogen ions (H⁺) when dissolved in water.
All acids taste sour. Lemon is sour because of citric acid. Curd is sour because of lactic acid.
When blue litmus paper is dipped in an acid, it turns red. This is the key test for acids!
Acids react with metals like zinc and iron to produce hydrogen gas (H₂) with a "pop" sound.
Acids react with bases to form salt + water. This is called neutralization!
Acid solutions conduct electricity because they contain ions (charged particles).
Strong acids like HCl and H₂SO₄ can burn skin and damage materials. Handle with care!
These acids are found naturally in plants, animals, and food:
| Acid | Found In (Source) | Taste/Use |
|---|---|---|
| 🍋 Citric Acid | Lemon (nimbu), Orange (santra), Amla | Sour taste in citrus fruits |
| 🍎 Malic Acid | Apple (seb), Pear | Tart taste in apples |
| 🍇 Tartaric Acid | Grapes (angur), Tamarind (imli) | Sour taste; used in baking powder |
| 🥛 Lactic Acid | Curd (dahi), Yogurt, Sour milk | Makes milk sour; also in muscles after exercise |
| 🐝 Formic Acid | Ant bite, Bee sting, Nettle leaf | Causes pain and itching on skin |
| 🍅 Oxalic Acid | Tomato (tamatar), Spinach (palak) | Slightly sour; used in cleaning |
| 🧈 Acetic Acid | Vinegar (sirka) | Sour taste; used in cooking & pickling |
| 🤮 Hydrochloric Acid (HCl) | Stomach (gastric juice) | Helps digest food; kills bacteria |
| 🍮 Ascorbic Acid | Amla, Orange, Guava | Vitamin C! Keeps us healthy |
These are man-made acids produced in laboratories and factories. They are very strong and dangerous!
Used in cleaning, making medicines, and found in stomach as gastric juice.
Called the "King of Chemicals" because it is used in almost every industry -- batteries, fertilizers, dyes!
Used to make fertilizers, explosives (TNT), and in gold testing (Aqua Regia = HCl + HNO₃).
Squeezing a lemon releases citric acid drops!
A base is a substance that tastes bitter, feels soapy or slippery to touch, and turns red litmus paper blue. Bases release hydroxide ions (OH⁻) when dissolved in water. A base that dissolves in water is called an alkali.
Bases taste bitter. Think of the bitter taste of baking soda or the after-taste of some medicines.
When red litmus paper is dipped in a base, it turns blue. This is the key test for bases!
Bases feel soapy or slippery when rubbed between fingers (because they react with skin oils).
Bases react with acids to form salt + water (neutralization reaction).
Base solutions also conduct electricity because they contain ions.
Strong bases like NaOH (caustic soda) can burn skin. "Caustic" means burning!
| Base (Chemical Name) | Common Name | Found In / Use |
|---|---|---|
| NaOH (Sodium Hydroxide) | Caustic Soda | Making soap, drain cleaner |
| KOH (Potassium Hydroxide) | Caustic Potash | Making liquid soap, batteries |
| NaHCO₃ (Sodium Bicarbonate) | Baking Soda (Meetha Soda) | Cooking, baking, antacid |
| Ca(OH)₂ (Calcium Hydroxide) | Slaked Lime (Chuna) | Whitewashing, lime water, paan |
| Mg(OH)₂ (Magnesium Hydroxide) | Milk of Magnesia | Antacid (Digene, Eno) |
| NH₃ (Ammonia) solution | Ammonia / Liquid NH₃ | Window cleaner, fertilizer |
| Na₂CO₃ (Sodium Carbonate) | Washing Soda (Dhulai Soda) | Washing clothes, making glass |
We know acids are sour and bases are bitter, but we should NEVER taste chemicals to identify them -- that's dangerous! Instead, scientists use special substances called indicators that change colour to tell us if something is an acid or a base.
These indicators are obtained from plants and natural sources:
Blue litmus → turns RED in acid
Red litmus → turns BLUE in base
Extracted from lichen (a plant). Most commonly used indicator!
In acid: Blue → Red
In base: Red → Blue
Neutral: No change
Indian kitchen indicator!
In acid: Yellow (no change)
In base: Yellow → Red/Brown
That's why turmeric stain turns red when you wash with soap!
Petals of hibiscus flower!
In acid: Dark pink / Magenta
In base: Green
Neutral: Light pink
Juice changes colour beautifully!
In acid: Red / Pink
In base: Green / Yellow
Neutral: Purple / Violet
In acid: Colourless
In base: Pink / Magenta
Very useful in lab experiments!
In acid: Red / Pink
In base: Yellow
Used in titration experiments.
The pH scale is a number scale from 0 to 14 that measures how acidic or basic a substance is. pH stands for "potential of Hydrogen".
When an acid reacts with a base, they cancel out each other's properties to form a salt and water. This reaction is called neutralization. Heat is also released (exothermic reaction)!
Ant and bee stings inject formic acid into skin. Applying baking soda (NaHCO₃) -- a base -- neutralizes the acid and reduces pain!
Formic acid (ant sting) + Baking soda (base) → Salt + Water = Relief!
Problem: Formic acid (acidic)
Remedy: Apply baking soda (NaHCO₃) paste -- a base neutralizes the acid!
Problem: Injects methanoic acid
Remedy: Rub with dock leaf (contains base) or apply baking soda!
Problem: Excess HCl in stomach
Remedy: Take antacid -- Eno, Digene, Pudin Hara, Milk of Magnesia (Mg(OH)₂) neutralizes acid!
Problem: Bacteria in mouth produce acid that decays teeth
Remedy: Brush teeth with toothpaste (basic) to neutralize acid!
Acidic soil: Add quick lime/slaked lime (CaO/Ca(OH)₂) -- a base
Basic soil: Add organic matter/compost -- slightly acidic
Problem: Factory waste is often acidic
Remedy: Treated with lime (base) before releasing into rivers to protect aquatic life!
Your stomach produces HCl to digest food. When too much acid is produced, you feel acidity/heartburn. An antacid tablet (base) drops in and neutralizes the extra acid!
Mg(OH)₂ (antacid) + 2HCl (stomach acid) → MgCl₂ + 2H₂O
A salt is a compound formed when the hydrogen (H⁺) of an acid is replaced by a metal or ammonium ion. Salts are produced during neutralization reactions.
The most well-known salt is common salt (NaCl) that we use in food every day! It is formed from hydrochloric acid (HCl) and sodium hydroxide (NaOH).
Sea water, rock salt deposits, salt lakes. India's Sambhar Lake in Rajasthan is a famous salt source!
Cooking, food preservation (pickling, drying), making soap, glass, and chemicals like NaOH and Na₂CO₃.
Essential for nerve function, muscle movement, and water balance. Too little or too much salt is harmful!
| Salt (Formula) | Common Name | How Formed | Use |
|---|---|---|---|
| NaCl | Common Salt / Table Salt | HCl + NaOH | Cooking, preservation |
| NaHCO₃ | Baking Soda (Meetha Soda) | Contains Na⁺ and HCO₃⁻ | Baking, antacid, fire extinguisher |
| Na₂CO₃ | Washing Soda (Dhulai Soda) | From NaCl by Solvay process | Cleaning, softening water, making glass |
| CaSO₄ | Plaster of Paris (POP) | From gypsum (heating) | Making casts, statues, moulds |
| KNO₃ | Saltpetre / Potassium Nitrate | HNO₃ + KOH | Fertilizer, fireworks, gunpowder |
📖 NCERT Textbook Activity
🎯 Aim: Test the acidic, basic, or neutral nature of substances using litmus paper.
📦 Materials: Red & blue litmus paper, dropper, test solutions — lemon juice, tap water, detergent solution, aerated drink, soap solution, shampoo, salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water.
👣 Steps:
📖 NCERT Textbook Activity
🎯 Aim: Make turmeric paper and test solutions with it.
📦 Materials: Turmeric powder, water, blotting paper (or filter paper), test solutions (soap, lemon juice, baking soda, vinegar, tap water, etc.).
👣 Steps:
📖 NCERT Textbook Activity
🎯 Aim: Prepare China rose (gudhal) indicator and test substances.
📦 Materials: China rose (hibiscus) petals, warm water, beaker, test solutions (dilute HCl, NaOH, vinegar, baking soda, tap water, etc.).
👣 Steps:
📖 NCERT Textbook Activity
| Solution | Red Litmus | Blue Litmus | Turmeric Paper | China Rose |
|---|---|---|---|---|
| Dil. HCl | No change | → Red | No change | Dark pink |
| Dil. H₂SO₄ | No change | → Red | No change | Dark pink |
| Dil. HNO₃ | No change | → Red | No change | Dark pink |
| Acetic Acid | No change | → Red | No change | Dark pink |
| NaOH | → Blue | No change | → Red/Brown | Green |
| Ca(OH)₂ | → Blue | No change | → Red/Brown | Green |
| NH₄OH | → Blue | No change | → Red/Brown | Green |
🎯 Aim: Compare the results of litmus, turmeric, and China rose indicators on laboratory acids and bases.
📦 Materials: Dilute HCl, H₂SO₄, HNO₃, acetic acid, NaOH, NH₄OH, Ca(OH)₂, red & blue litmus paper, turmeric paper, China rose solution.
👣 Steps:
📖 NCERT Textbook Activity
🎯 Aim: Demonstrate neutralisation using phenolphthalein indicator.
📦 Materials: Test tube, dilute HCl, phenolphthalein indicator, NaOH solution, dropper.
👣 Steps:
⭐ Extended / Fun Activity
🎯 Aim: Write a secret message using a base and reveal it with a natural indicator.
📦 Materials: Baking soda, water, white paper, thin paintbrush or cotton bud, beetroot.
👣 Steps:
⭐ Extended / Fun Activity
🎯 Aim: Use red cabbage juice as a universal-style indicator that shows different colours at different pH values.
📦 Materials: Red cabbage, water, saucepan, several test tubes, test solutions at various pH (lemon juice, vinegar, tap water, baking soda, soap, lime water).
👣 Steps:
| Acid Name | Formula | Natural Source | Type |
|---|---|---|---|
| Citric Acid | C₆H₈O₇ | Lemon, Orange, Amla | Organic |
| Acetic Acid | CH₃COOH | Vinegar (Sirka) | Organic |
| Lactic Acid | C₃H₆O₃ | Curd (Dahi), Sour Milk | Organic |
| Tartaric Acid | C₄H₆O₆ | Tamarind (Imli), Grapes | Organic |
| Formic Acid | HCOOH | Ant sting, Bee sting | Organic |
| Oxalic Acid | C₂H₂O₄ | Tomato, Spinach | Organic |
| Malic Acid | C₄H₆O₅ | Apple | Organic |
| Hydrochloric Acid | HCl | Stomach (gastric juice) | Mineral |
| Sulphuric Acid | H₂SO₄ | Manufactured | Mineral |
| Nitric Acid | HNO₃ | Manufactured | Mineral |
| Base Name | Formula | Common Name | Use |
|---|---|---|---|
| Sodium Hydroxide | NaOH | Caustic Soda | Soap making, drain cleaner |
| Potassium Hydroxide | KOH | Caustic Potash | Liquid soap, batteries |
| Calcium Hydroxide | Ca(OH)₂ | Slaked Lime (Chuna) | Whitewashing, lime water |
| Magnesium Hydroxide | Mg(OH)₂ | Milk of Magnesia | Antacid |
| Sodium Bicarbonate | NaHCO₃ | Baking Soda | Cooking, antacid |
| Sodium Carbonate | Na₂CO₃ | Washing Soda | Cleaning, glass making |
| Ammonium Hydroxide | NH₄OH | Ammonia Solution | Window cleaner |
| Indicator | Colour in Acid | Colour in Base | Colour when Neutral |
|---|---|---|---|
| Blue Litmus | RED | No change (Blue) | No change (Blue) |
| Red Litmus | No change (Red) | BLUE | No change (Red) |
| Turmeric (Haldi) | No change (Yellow) | RED / BROWN | Yellow |
| China Rose (Gudhal) | Dark Pink | GREEN | Light Pink |
| Phenolphthalein | Colourless | PINK | Colourless |
| Methyl Orange | RED / PINK | YELLOW | Orange |
| Red Cabbage | RED | GREEN | Purple |
| Property | 🔴 Acids | 🔵 Bases |
|---|---|---|
| Taste | Sour | Bitter |
| Touch | Sticky / watery | Soapy / slippery |
| Litmus test | Blue litmus → Red | Red litmus → Blue |
| Turmeric test | No change (yellow) | Turns red/brown |
| pH range | 0 to less than 7 | More than 7 to 14 |
| Contains | H⁺ ions | OH⁻ ions |
| React with metals | Yes, produces H₂ gas | Some react (e.g., NaOH + Al) |
| Examples | HCl, H₂SO₄, lemon, vinegar | NaOH, Ca(OH)₂, soap, baking soda |
Click on any option -- the correct one will be highlighted. Click "Show Answer" for explanation.
Click on each question to reveal the answer.
Click on each statement to reveal the answer.
Bacteria called Lactobacillus convert the sugar (lactose) in milk into lactic acid. As more lactic acid forms, the milk becomes sour and thick -- that's how curd (dahi) is made!
Antacids contain bases like Mg(OH)₂ or NaHCO₃. When you take an Eno or Digene, it neutralizes the excess HCl in your stomach. The fizzing in Eno is CO₂ gas released during the reaction!
Shampoo is basic (pH 8-9) which opens hair cuticles. Conditioner is slightly acidic (pH 4-5) which closes the cuticles back, making hair smooth and shiny! It's acid-base chemistry on your head!
Mixing bleach (basic) with toilet cleaner (acidic) can produce toxic chlorine gas! Different cleaners should never be mixed together -- even if both seem harmless alone.
Our stomach has a pH of about 1.5-2 -- as acidic as battery acid! The inner lining produces a thick mucus layer that protects the stomach walls from being digested by its own acid!
Factories and vehicles release SO₂ and NO₂ gases. These dissolve in rainwater to form sulphuric acid and nitric acid, making the rain acidic (pH below 5.6). Acid rain damages buildings, statues, and kills fish in lakes!