Bite into a raw mango and your mouth puckers up — sour. Lick a spoonful of baking soda mixed in water and it tastes bitter, almost soapy. Squeeze lemon onto a rusty tap and, over days, the rust fades. Your grandmother rubs a paste of baking soda on a bee sting to stop the burning. None of this is magic — it is chemistry happening in your kitchen and garden every single day, and this chapter gives you the tools to explain all of it.
In Grade 6 you learned that materials around us can be classified in many ways — by their state, their use, their properties. Now we go one level deeper and ask a very old chemistry question: is this substance an acid (अम्ल), a base (क्षार), or is it neutral (उदासीन)? Answering this question safely — without tasting random chemicals, which can be dangerous — is exactly what this chapter teaches you to do.
What You’ll Learn
- Why taste is a clue, but never a safe test
- Litmus: nature’s own colour-changing indicator
- Turmeric, red cabbage and other natural indicators
- Olfactory indicators: testing by smell
- Neutralisation: when acids and bases cancel out
- Acids, bases and neutralisation in daily life
- Acids and bases: strong or weak?
- Safety rules when testing substances
- Holistic Lens: connections across subjects
- Know a Scientist: Robert Boyle
- Textbook Questions ki Taiyari
- Extra Practice
- Self-Assessment Quiz
Why Taste Is a Clue, But Never a Safe Test
Long before chemistry labs existed, people already knew that some foods taste sour and others taste bitter. Lemon, tamarind, curd, and unripe fruit all taste sour — they contain acids. Soap, and the water in which baking soda is dissolved, taste bitter and feel slightly slippery on the skin — these are bases. A pinch of common salt dissolved in water tastes neither sour nor bitter — it is neutral.
So could we just taste every substance in the science lab to sort it into acid, base, or neutral? Absolutely not. Many chemicals used in laboratories and factories — like the acid inside a car battery or the sodium hydroxide used to make soap — are corrosive. They can burn your skin, damage your tongue, and even be poisonous. This is exactly why scientists needed a safer way to test a substance without tasting or touching it. That safer way is called an indicator.
Litmus: Nature’s Own Colour-Changing Indicator
An indicator is a substance that changes colour when it comes in contact with an acid or a base. The most famous indicator used in schools is litmus, which is extracted from a small, crusty organism called a lichen. A lichen is actually a partnership between a fungus and an alga living together — you may have seen its yellow-green or greyish patches growing on tree bark and old walls.
Litmus is available in two forms for testing:
- Blue litmus paper — turns red when dipped in an acidic substance.
- Red litmus paper — turns blue when dipped in a basic substance.
- If neither paper changes colour, the substance is neutral.
Suppose you dip a strip of blue litmus paper into lemon juice and a strip of red litmus paper into a soap solution.
Step 1: Blue litmus dipped in lemon juice turns red. This tells us lemon juice is acidic.
Step 2: Red litmus dipped in soap solution turns blue. This tells us soap solution is basic.
Conclusion: Lemon juice → acid. Soap solution → base. This matches what we already suspected from taste and feel, but now we have proof using a safe method.
Litmus can also be used in liquid form, called litmus solution, which is purple in colour. It turns red in acids and blue in bases, working exactly the same way as the paper strips.
Turmeric, Red Cabbage and Other Natural Indicators
Litmus is not the only indicator found in nature. Your own kitchen has several natural indicators hiding in plain sight.
- Turmeric (हल्दी): Turmeric paper or turmeric powder is yellow. When it touches a basic substance, it turns reddish-brown. It does not change colour with acids or neutral substances. This is why a turmeric stain on a white cotton shirt often turns reddish when soap (a base) is scrubbed on it, and turns back to yellow when a mild acid, like lemon juice, is dabbed on the stain — a trick many households already use without realising it is chemistry.
- Red cabbage juice: Naturally purple, red cabbage juice turns pink or red in acids and turns green or yellow in bases.
- China rose (Gudhal) extract: Turns dark pink or magenta in acids and turns green in bases.
Explore More: Testing by Smell (Olfactory Indicators)
- Onion: Its characteristic smell fades when a basic solution is added to a cloth rubbed with onion juice, but the smell remains if an acid is added.
- Vanilla extract: Its sweet smell disappears in the presence of a basic solution.
- Clove oil: Behaves similarly — its odour is masked by bases.
Olfactory indicators are especially useful for visually impaired students and for teaching everyone that science can be “observed” through senses other than sight.
Neutralisation: When Acids and Bases Cancel Out
What happens when you mix an acid and a base together in the right amounts? They react with each other, and in the process, each cancels out the other’s acidic or basic character. The mixture that results is close to neutral. This reaction is called neutralisation.
A simple way to picture this: imagine an acid as a “hot” property and a base as a “cold” property, in a purely chemical sense (not actual temperature). When you carefully combine them, they balance each other, similar to how adding cold water to overly hot water gives you lukewarm water. Neutralisation reactions often produce salt and water as products, and sometimes release a small amount of heat. Note: in science, “salt” is a general term for the new substance formed in this reaction — it does not always mean the common table salt we sprinkle on food!
An ant bite injects a mild acidic substance (formic acid) into the skin, which causes the stinging, burning sensation.
Step 1: Identify the problem — excess acid on the skin.
Step 2: Apply a mild base to neutralise it — a paste of baking soda (sodium bicarbonate) dissolved in a little water is commonly used.
Step 3: The base reacts with the acid, neutralising it, and the burning sensation reduces.
Conclusion: This home remedy works because of a real neutralisation reaction, not because of superstition.
Acids, Bases and Neutralisation in Daily Life
Once you start looking, acids, bases, and neutralisation reactions show up everywhere:
- Indigestion: Our stomach naturally produces hydrochloric acid (HCl) to digest food. Too much acid causes indigestion and a burning feeling. Antacid tablets contain a mild base (such as magnesium hydroxide) that neutralises the excess acid and gives relief.
- Farming and soil treatment: If farmland soil is too acidic, farmers add slaked lime (a base) to neutralise it before sowing crops. If soil is too basic, organic compost or other mild treatments can be used to balance it.
- Factory waste treatment: Factories that release acidic waste water must neutralise it with a base before releasing the water into rivers, to protect fish and other aquatic life.
- Tooth decay: Bacteria in the mouth produce acids from leftover sugar and food, which attack tooth enamel. Toothpaste is mildly basic, which helps neutralise these acids — this is one reason dentists recommend brushing after meals.
Acids and Bases: Strong or Weak?
Not every acid behaves the same way, and not every base behaves the same way either. Some acids are so mild that we eat and drink them every day — the citric acid in oranges, the lactic acid in curd, the acetic acid in vinegar. These are called weak acids. Others are so powerful that a single drop can burn a hole in cloth or skin — sulphuric acid inside a car battery, or hydrochloric acid used in industries. These are called strong acids.
The same idea applies to bases. Soap and toothpaste are weak bases, gentle enough to use on our bodies every day. Sodium hydroxide (NaOH), used to manufacture soap and paper in factories, is a strong base that can cause serious burns if handled carelessly.
This is a useful idea to hold on to: being “acidic” or “basic” is not, by itself, a measure of danger. It is the strength of the acid or base that decides whether it is safe to eat, safe to touch, or dangerous even in small amounts.
Safety Rules When Testing Substances in a Lab
Because some acids and bases can be strong and corrosive, science labs everywhere follow a few non-negotiable safety rules whenever indicators are used to test unknown substances:
- Never taste, smell directly (by putting your nose right over the container), or touch an unknown chemical with bare hands.
- Always use a dropper to add small amounts of the test substance onto litmus paper, rather than dipping the paper directly into a large, unlabelled container.
- Wash your hands thoroughly after any activity involving chemicals, even mild ones like vinegar or baking soda.
- If any chemical touches your skin accidentally, rinse the area immediately with plenty of clean water and inform your teacher.
- Always perform such activities under the supervision of a teacher or a responsible adult, never alone.
Suppose your teacher gives you two unlabelled cups: one contains diluted vinegar, and the other contains a diluted washing soda (baking soda substitute) solution. You are asked to identify each safely.
Step 1: Using a dropper, place one drop from Cup A onto a strip of blue litmus paper, and one drop onto a strip of red litmus paper. Repeat separately for Cup B.
Step 2: Observe: Cup A turns blue litmus red, and does not change red litmus. Cup B turns red litmus blue, and does not change blue litmus.
Step 3: Since Cup A turns blue litmus red, it is acidic — this must be the diluted vinegar. Since Cup B turns red litmus blue, it is basic — this must be the washing soda solution.
Conclusion: Using two strips of litmus, and never tasting either liquid, you have safely and correctly identified both unknown substances.
In a Nutshell
Every acid or base also has a strength — weak ones are common in our food and household items, while strong ones need careful handling in labs and factories. Whenever we test an unknown substance, safety rules like using droppers, never tasting chemicals, and working under supervision keep us protected while we explore.
Substances can be classified as acidic, basic, or neutral. Since tasting chemicals is unsafe, we use indicators — litmus, turmeric, red cabbage juice, China rose extract, and olfactory indicators like onion — to identify them safely. Litmus turns red in acids and blue in bases. When an acid and a base react in suitable amounts, they neutralise each other, forming salt and water. Neutralisation explains many everyday situations: treating indigestion, soothing ant and bee stings, correcting soil pH for farming, and treating factory waste water.
Holistic Lens: One Idea, Many Connections
The acid–base idea does not stay locked inside a chemistry chapter — it connects to almost every subject you study. In biology, your stomach lining is specially designed to survive strong hydrochloric acid while digesting food, and doctors study acid–base balance in blood when treating serious illnesses. In geography and environmental studies, “acid rain,” caused when factory and vehicle smoke mixes with rainwater, is a real environmental problem that damages buildings, soil, and water bodies — and the solution, once again, involves neutralisation using lime. In history, you may have read that ancient civilisations like the Indus Valley people used natural plant dyes and extracts, some of which behaved exactly like the indicators you are learning about today, even though they had no word for “chemistry” as we know it.
Know a Scientist: Robert Boyle
The Irish scientist Robert Boyle (1627–1691) is often credited as one of the earliest scientists to study how plant extracts change colour with acids and bases, laying the groundwork for what we now call indicators. Long before modern laboratories existed, Boyle carefully observed and recorded how syrup made from violet flowers turned red with acids and green with bases — very similar to the red cabbage indicator you may test in your own classroom today. His careful, curiosity-driven approach is exactly the spirit this textbook hopes to build in you.
Textbook Questions ki Taiyari (Complete Study Material)
Your NCERT textbook’s end-of-chapter exercises test a few core ideas repeatedly. Below, we cover each type of question the chapter asks about — in our own original wording — with complete, easy-to-follow explanations, so you walk into your test fully prepared.
1. Explaining why we cannot rely on taste to classify substances.
Tasting is unsafe because many acidic and basic substances used in science and industry are corrosive or poisonous. Some can burn the skin or the inside of the mouth on contact. This is why chemists always use an indicator, never taste, to test an unknown substance.
2. Describing how litmus paper is used and what results mean.
Dip a strip of blue litmus in the test substance. If it turns red, the substance is acidic. Dip a strip of red litmus in the test substance. If it turns blue, the substance is basic. If there is no colour change on either strip, the substance is neutral. Always use both colours of litmus when you are unsure, to avoid missing a result.
3. Naming and explaining natural indicators besides litmus.
Turmeric turns reddish-brown only with bases and shows no change with acids. Red cabbage juice turns pink/red with acids and green/yellow with bases. China rose extract turns dark pink with acids and green with bases. Each of these is a plant-based indicator that works exactly like litmus but shows different colours.
4. Explaining what an olfactory indicator is, with examples.
An olfactory indicator is a substance whose smell changes or disappears when mixed with an acid or a base, allowing us to identify the nature of a substance using our sense of smell instead of sight. Onion and vanilla extract are common examples — their smell fades in the presence of a base.
5. Defining neutralisation and explaining a real-life example in detail.
Neutralisation is the chemical reaction between an acid and a base that cancels out their acidic and basic properties, usually producing salt and water. A real-life example: applying baking soda paste (a base) on an ant bite (which injects an acid) relieves the burning sensation by neutralising the acid on the skin.
6. Explaining how farmers manage soil that is too acidic or too basic.
If soil is too acidic, farmers mix in slaked lime or agricultural lime, which is basic, to neutralise the excess acid and bring the soil closer to neutral, so that crops can grow well. Different crops also need different soil conditions, so farmers test the soil before deciding what to add.
Extra Practice — Original Questions
1. Why does a turmeric stain on cloth turn reddish-brown when soap is rubbed on it?
Soap is a basic substance. Turmeric is a natural indicator that turns reddish-brown in contact with a base, which is why the yellow stain changes colour when soap and water are used to scrub it.
2. If a solution turns both blue litmus and red litmus paper unchanged, what can you conclude about it?
The solution is neutral — it is neither acidic nor basic, since it does not change the colour of either litmus paper.
3. A student dips red litmus paper in a solution and it turns blue. What is the nature of the solution?
The solution is basic, since only basic substances turn red litmus paper blue.
4. Why should olfactory indicators not be used with substances that don’t have any smell?
Olfactory indicators rely on a change or disappearance of smell to show a result. If a substance has no smell to begin with, there is nothing to observe changing, so the test would give no useful information.
5. Explain, using the idea of neutralisation, why toothpaste is helpful against tooth decay.
Bacteria in the mouth produce acids from leftover food particles, and these acids attack tooth enamel. Toothpaste is mildly basic, so brushing helps neutralise these acids, protecting the teeth from decay.
6. A factory releases acidic waste water directly into a river without treatment. Predict one harmful effect, and suggest a chemical solution.
The acidic water can harm or kill fish and other aquatic organisms and disturb the river’s ecosystem. The factory should treat the waste water with a suitable base to neutralise the acid before releasing it into the river.
7. Why is litmus obtained from lichens considered a natural indicator rather than a synthetic one?
Litmus is extracted directly from a living organism (a lichen, a partnership of fungus and alga) found in nature, without being artificially manufactured through industrial chemical processes, which is why it is classified as a natural indicator.
8. Two unlabelled bottles contain vinegar and a soap solution. Describe a complete, safe procedure to identify each.
Dip a strip of blue litmus paper into each bottle. The bottle where blue litmus turns red contains vinegar (an acid). The bottle where blue litmus does not change colour, and where red litmus turns blue instead, contains the soap solution (a base). This procedure avoids tasting or touching either liquid directly.
9. Why might farmers avoid adding too much slaked lime to acidic soil?
Adding too much slaked lime, which is a base, can overcorrect the problem and make the soil too basic instead of neutral, which can also harm crop growth. Farmers need to add the right amount, often based on a soil test, rather than guessing.
10. Why is it wrong to say “all acids are dangerous” or “all bases are safe”?
Danger depends on the strength of the acid or base, not the category alone. Many acids (like citric acid in fruit) and many bases (like soap) are weak and completely safe in daily life, while some acids and some bases can be strong and corrosive. Both categories contain both safe and dangerous examples.
11. A student wants to remove a turmeric stain but only has lemon juice and soap water available at home. Which should they use, and why?
They should use lemon juice, because turmeric turns reddish-brown with bases like soap, which would make the stain look worse or set it in further. Lemon juice, being acidic, does not react the same way and is more likely to help fade the stain rather than darken it.
Self-Assessment — Quick Quiz
Try to answer these on your own before checking. No peeking until you’ve attempted each one!
Q1. What colour does blue litmus paper turn in an acidic solution?
Red.
Q2. Name one natural indicator, other than litmus, that turns reddish-brown in bases and shows no change in acids.
Turmeric.
Q3. What is the general name given to indicators that work through changes in smell?
Olfactory indicators.
Q4. What are the two general products formed when an acid and a base neutralise each other?
Salt and water.
Q5. Why is baking soda paste often applied to an ant bite?
Because the ant bite injects an acidic substance into the skin, and baking soda (a mild base) neutralises this acid, reducing the burning sensation.
Kaizen — Keep Improving
Kaizen is a Japanese idea meaning “continuous, small improvement.” Today, pick just one thing from this chapter and actually try it. For example, make your own turmeric paper: mix a little turmeric powder with water into a thick paste, rub it onto a strip of thick paper, and let it dry completely. Then place a drop of soap water and a drop of lemon juice on separate spots and watch what happens. Small hands-on steps like this, repeated often, build real scientific understanding far better than reading alone.
🌐 Next chapter: Chapter 3 — Electricity: Circuits and their Components (coming soon in this series).