Litmus and acid-base indicators
A magician writes on a plain white sheet with a clear liquid and the secret message turns red. The trick works because some substances change colour when they meet an acid or a base.
Litmus is a natural dye obtained from lichens. It is sold as a solution or as blue and red litmus paper. A substance that shows different colours in acidic and basic solutions is an acid-base indicator. Acids turn blue litmus red, bases turn red litmus blue, and neutral substances change neither.
In the figure : The colour change of litmus shows whether a substance is acidic, basic or neutral.
Rule 1: Read the colour change — Test a sample with both blue and red litmus before deciding.
- Easy : Lemon juice turns blue litmus red, so it is acidic.
- Medium : Soap solution turns red litmus blue, so it is basic.
- Exam-type : Salt solution changes neither paper, so it is neutral.
Rule 2: Litmus is a natural indicator — Litmus comes from a living source and not from a factory.
- Easy : Lichens give litmus.
- Medium : A lichen is a partnership of a fungus and an alga.
- Exam-type : Lichens grow best where the air is clean and rainfall is plentiful.
Acid: blue to red. Base: red to blue. Neutral: no change.
Testing with only one paper. A basic sample does not change blue litmus, so one paper can mislead.
Memory clue: A-B-R — Acid makes Blue go Red; Base makes Red go Blue.
Spot the difference
| Nature | Blue litmus | Red litmus |
|---|---|---|
| Acidic | Turns red | No change |
| Basic | No change | Turns blue |
| Neutral | No change | No change |
Practice questions
- 1What is litmus and where does it come from?
- 2Name two forms in which litmus is available.
- 3What is an acid-base indicator?
- 4What colour change shows that a sample is acidic?
- 5What colour change shows that a sample is basic?
- 6Why do we test with both red and blue litmus papers?
- 7A sample leaves both papers unchanged. What is its nature and give one example?
- 8Why is litmus called a natural indicator?
Show answers (try them yourself first)
- 1A natural dye obtained from lichens.
- 2As a solution and as litmus paper strips.
- 3A substance that shows different colours in acidic and basic solutions.
- 4Blue litmus turns red.
- 5Red litmus turns blue.
- 6A basic sample does not change blue litmus and an acidic one does not change red, so both are needed.
- 7It is neutral, for example sugar solution.
- 8Because it is obtained from a natural source, lichens.
Litmus and acid-base indicators
In the app you can move this figure with your finger.
Open in the appSorting everyday substances
At home you can find lemons, vinegar, soap, baking soda and plain water. By testing them we can place each one into a group: acidic, basic or neutral.
After testing common samples, we sort them into three groups. Group A samples turn blue litmus red (acidic). Group B samples turn red litmus blue (basic). Group C samples change neither (neutral). Acidic edible substances taste sour; bases feel soapy and taste bitter. Never taste an unknown substance.
In the figure : Three groups cover most of the common solutions we test.
Rule 1: Group by litmus result — The litmus result decides the group, not guessing.
- Easy : Vinegar, amla juice and tamarind water are acidic.
- Medium : Washing powder solution and lime water are basic.
- Exam-type : Sugar and salt solutions are neutral.
Rule 2: Taste and touch are hints, not proof — Sour often means acid and soapy often means base, but never taste unknown samples.
- Easy : Acidic food tastes sour.
- Medium : A baking soda solution feels slippery between the fingers.
- Exam-type : Bitter gourd tastes bitter but is not basic, so confirm with an indicator.
Sour = acid, soapy and bitter = base, neither = neutral. Always confirm with litmus.
Tasting unknown substances or thinking all bitter foods are bases.
Memory clue: S-S-N — Sour acid, Soapy base, Neither neutral.
Spot the difference
| Group | Examples | Feature |
|---|---|---|
| Acidic | Lemon juice, vinegar | Sour taste |
| Basic | Soap, baking soda | Slippery feel |
| Neutral | Sugar, salt solution | No effect on litmus |
Practice questions
- 1Name three acidic substances.
- 2Name three basic substances.
- 3Name two neutral substances.
- 4How does an acidic edible substance taste?
- 5How does a basic substance feel to the touch?
- 6Why should unknown substances never be tasted?
- 7Karela is bitter. Can we say it is basic? Explain.
- 8How would you prepare lime water at home?
Show answers (try them yourself first)
- 1Lemon juice, vinegar, tamarind water (any three).
- 2Soap solution, baking soda solution, lime water (any three).
- 3Sugar solution and salt solution.
- 4Sour.
- 5Soapy or slippery.
- 6Some substances are poisonous or harmful.
- 7No. A bitter taste is only a hint; karela is bitter but not basic.
- 8Mix lime (chuna) in water, leave it for about an hour and filter it.
Acids and bases in food and at home
Lemons, curd, tamarind and vinegar all taste sour, and soap feels slippery. Behind these tastes and feelings are particular acids and bases.
Many sour foods contain natural acids: citric acid in lemon, lactic acid in curd, tartaric acid in tamarind and acetic acid in vinegar. Bases are usually bitter, feel soapy and are found in soap, baking soda, lime water and many cleaners. A bitter taste alone does not prove a substance is a base.
In the figure : Different foods have different natural acids.
Rule 1: Foods contain natural acids — Each sour food owes its taste to a particular acid.
- Easy : Lemon juice has citric acid.
- Medium : Curd has lactic acid and vinegar has acetic acid.
- Exam-type : Tamarind has tartaric acid, which gives it a sour taste.
Rule 2: Bases are bitter and soapy — Many cleaning materials are basic.
- Easy : Soap feels slippery because it is basic.
- Medium : Baking soda is a base used in cooking and for relief from ant bites.
- Exam-type : A floor-cleaning liquid can be tested with an indicator to learn whether it is basic.
Name the acid with the food: lemon-citric, curd-lactic, tamarind-tartaric, vinegar-acetic.
Believing every bitter food is basic. Bitter gourd is bitter but not basic.
Memory clue: L-C-T-V — Lemon-Citric, Curd-Lactic, Tamarind-Tartaric, Vinegar-Acetic.
Spot the difference
| Food | Acid in it | Taste |
|---|---|---|
| Lemon | Citric acid | Sour |
| Curd | Lactic acid | Sour |
| Vinegar | Acetic acid | Sharp and sour |
Practice questions
- 1Name the acid present in lemon.
- 2Name the acid present in curd.
- 3Name the acid in tamarind and in vinegar.
- 4Why do soaps feel slippery?
- 5Why is baking soda useful for an ant bite?
- 6Can a bitter food be non-basic? Give an example.
- 7Why should we not taste cleaning liquids to test them?
- 8Name two household bases.
Show answers (try them yourself first)
- 1Citric acid.
- 2Lactic acid.
- 3Tamarind: tartaric acid; vinegar: acetic acid.
- 4Because soaps are basic.
- 5It is a base, so it neutralises the acid injected by the ant.
- 6Yes, bitter gourd is bitter but not basic.
- 7They can be harmful or poisonous.
- 8Baking soda and soap (also lime water).
Natural indicators: red rose and more
Fallen red rose petals are usually thrown away, yet they can show whether lemon juice or soap solution is in a glass. Plants carry their own testing colours.
Many flowers, fruits and vegetables give colours that change in acids and bases, so they work as natural indicators. Red rose extract turns red in acidic solutions and green in basic solutions. Other indicators are beetroot, purple cabbage, jamun and red hibiscus. The colour of hydrangea flowers depends on the soil: blue in acidic soil and pink or red in basic soil.
In the figure : A flower extract can be prepared in a few easy steps.
Rule 1: Prepare the extract carefully — Use fallen petals, hot water under adult supervision, and filter the liquid.
- Easy : Wash the petals and crush them.
- Medium : Pour hot water to cover the petals and cover with a lid for 5–10 minutes.
- Exam-type : The filtrate (liquid after filtration) is the indicator.
Rule 2: Read the colour of the extract — Compare the extract's colour with the colours for acid and base.
- Easy : Red rose extract turns red with lemon juice.
- Medium : Red rose extract turns green with soap solution.
- Exam-type : Samples that do not change the extract are neutral.
Red rose extract: red = acid, green = base, no change = neutral.
Plucking fresh flowers for the extract. Use only fallen petals.
Memory clue: R-A-G-B — Red-Acid, Green-Base.
Spot the difference
| Natural indicator | In acid | In base |
|---|---|---|
| Red rose extract | Red | Green |
| Litmus | Blue turns red | Red turns blue |
| Hydrangea flower | Blue | Pink or red |
Practice questions
- 1How is red rose extract prepared?
- 2What colour does red rose extract show in an acid?
- 3What colour does red rose extract show in a base?
- 4Name two other natural indicators.
- 5What is a filtrate?
- 6Why is hot water used under adult supervision?
- 7What colour of hydrangea suggests acidic soil?
- 8A sample does not change the colour of red rose extract. What is it?
Show answers (try them yourself first)
- 1Wash and crush fallen petals, pour hot water, cover for 5–10 minutes and filter.
- 2Red.
- 3Green.
- 4Beetroot and purple cabbage (or jamun, red hibiscus).
- 5The liquid obtained after filtration.
- 6Hot water can burn, so an adult should help.
- 7Blue.
- 8It is neutral.
Turmeric and smell as indicators
A curry stain on a white shirt suddenly turns reddish-brown when soap is rubbed in. The shirt is telling you that soap is a base and turmeric is an indicator.
Turmeric paper is filter paper dipped in turmeric paste and dried. It turns red in basic solutions and stays yellow in acidic and neutral solutions, so it can test bases but cannot separate acids from neutral substances. Olfactory indicators are substances whose smell changes in acidic or basic solutions, for example finely chopped onion.
In the figure : Turmeric paper tests bases, while smell can also be an indicator.
Rule 1: Turmeric paper detects bases — Yellow turns red in a base and stays yellow otherwise.
- Easy : Soap solution turns turmeric paper red.
- Medium : Lemon juice does not change turmeric paper.
- Exam-type : Turmeric paper cannot tell an acid from a neutral sample.
Rule 2: Smell can show acid or base — Olfactory indicators lose their smell in a base but keep it in an acid.
- Easy : Onion has a strong smell.
- Medium : Onion strips smell different with baking soda than with tamarind water.
- Exam-type : Onion, vanilla essence and clove oil are olfactory indicators.
Turmeric: red = base, no change = acid or neutral. Olfactory: smell tells acid or base.
Using turmeric paper to find an acid. It cannot show acid.
Memory clue: T-R-B — Turmeric Red means Base.
Spot the difference
| Indicator | Acid | Base |
|---|---|---|
| Turmeric paper | Stays yellow | Turns red |
| Olfactory (onion) | Smell stays | Smell fades |
| Litmus | Blue turns red | Red turns blue |
Practice questions
- 1How is turmeric paper made?
- 2What colour does turmeric paper turn in a base?
- 3Why can't turmeric paper identify an acid?
- 4What is an olfactory indicator?
- 5Name two olfactory indicators.
- 6Why does a curry stain change colour when soap is rubbed on it?
- 7What will happen to turmeric paper if lemon juice and tap water are dropped on it?
- 8Which one test would you use to find an acid: turmeric paper or blue litmus? Why?
Show answers (try them yourself first)
- 1Dip filter paper in turmeric paste, dry it and cut strips.
- 2Red.
- 3Acids do not change its colour; it looks the same as with neutral substances.
- 4A substance whose smell changes in acidic or basic solutions.
- 5Onion, vanilla essence or clove oil (any two).
- 6Soap is basic and turns turmeric red.
- 7It does not change colour.
- 8Blue litmus; it turns red in an acid, but turmeric does not change.
Neutralisation: acid meets base
A bitter-sour drink can be made milder by adding a little of something opposite. In science, when an acid meets a base, they weaken each other's effect and form new substances.
Neutralisation is the reaction of an acid with a base in enough quantity to form a solution that is neither acidic nor basic. Acid + Base → Salt + Water + Heat. In an experiment, a lemon juice solution with blue litmus turns red; adding lime water drop by drop eventually turns it blue, showing that the acid has been neutralised.
In the figure : Adding a base to an acid slowly shows neutralisation by a colour change.
Rule 1: Acid + Base → Salt + Water + Heat — Both acid and base lose their special properties.
- Easy : Lemon juice + lime water gives a neutral solution.
- Medium : The heat released is usually small and may not be felt.
- Exam-type : The new substance formed along with water is called a salt.
Rule 2: Add the base in small steps — Drop-by-drop addition shows the exact point when the colour changes.
- Easy : Add lime water drop by drop and swirl.
- Medium : When litmus turns from red to blue, the acid has been neutralised.
- Exam-type : One extra drop of lemon juice brings back the red colour.
Acid + base = salt + water + heat. Colour change tells the stage.
Thinking the product of neutralisation is only water. A salt is also formed.
Memory clue: A-B-S-W-H — Acid + Base gives Salt + Water + Heat.
Spot the difference
| Term | Meaning | Example |
|---|---|---|
| Acid | Turns blue litmus red | Lemon juice |
| Base | Turns red litmus blue | Lime water |
| Salt | New substance formed | Formed with water |
Practice questions
- 1What is neutralisation?
- 2Write the general equation of neutralisation in words.
- 3Why is a base added drop by drop in the lemon-lime experiment?
- 4What shows that the acid has been neutralised?
- 5What happens if one more drop of lemon juice is added at the end point?
- 6Name the products of neutralisation.
- 7Why is the heat from small-scale neutralisation hard to notice?
- 8Is a neutralised solution always neutral? Explain.
Show answers (try them yourself first)
- 1Reaction of an acid with a base to form salt and water with release of heat.
- 2Acid + Base → Salt + Water + Heat.
- 3To find the exact point where the colour changes.
- 4The litmus colour changes from red to blue.
- 5The solution turns slightly acidic and the red colour returns.
- 6Salt, water and heat.
- 7Only a small amount of acid and base is used, so little heat is released.
- 8It is neutral only if enough base has been added to balance the acid.
Neutralisation in daily life
A red ant bite stings, an upset stomach burns, and teeth need cleaning after meals. All three are everyday problems that neutralisation can ease.
Ants inject formic acid, so moist baking soda (a base) relieves the sting. Stomach pain due to too much acid is eased by an antacid, which is a mild base. Toothpaste is basic and neutralises the acids made by bacteria in the mouth. These are examples of neutralisation helping us daily.
In the figure : A mild base can calm the effect of an acid.
Rule 1: Treat an acid with a mild base — A mild base neutralises the acidic effect without harm.
- Easy : Moist baking soda is applied on an ant bite.
- Medium : An antacid tablet eases acidity in the stomach.
- Exam-type : Toothpaste neutralises the acid in the mouth that harms teeth.
Rule 2: Treat a base with a mild acid — A mild acid can reduce the effect of a base.
- Easy : A few drops of lemon juice reduce the basic nature of lime water.
- Medium : Excess lime water added to lemon juice is brought back to neutral by a little more lemon juice.
- Exam-type : Never use strong acids or bases for first aid.
Acid sting needs a mild base; base problem needs a mild acid; use only mild substances.
Using a strong acid or base to neutralise a sting. Strong chemicals can burn.
Memory clue: A-B-M — Acid sting, Base cure, Mild only.
Spot the difference
| Problem | Cause | Remedy |
|---|---|---|
| Red ant bite | Formic acid | Moist baking soda |
| Acidity in stomach | Too much acid | Antacid |
| Tooth decay risk | Acid made by bacteria | Basic toothpaste |
Practice questions
- 1What acid is injected by an ant bite?
- 2Why is moist baking soda applied on an ant bite?
- 3What is an antacid?
- 4How does toothpaste help our teeth?
- 5Why should strong acids or bases not be used for first aid?
- 6Name two daily uses of neutralisation.
- 7What is the nature of baking soda?
- 8Why must baking soda be moist when applied?
Show answers (try them yourself first)
- 1Formic acid.
- 2It is a base, so it neutralises the acid and eases the pain.
- 3A mild base that reduces excess acid in the stomach.
- 4It is basic and neutralises acids formed in the mouth.
- 5They can burn the skin.
- 6Treating ant bites and acidity in the stomach (or using toothpaste).
- 7It is a base.
- 8So that it can spread and act on the affected area.
Neutralisation in soil and in factory waste
A farmer's plants look weak after too much fertiliser. A lake near a factory has fewer fish. In both cases, the surroundings have become too acidic, and neutralisation can help.
When soil becomes too acidic, it is treated with lime (a base). When soil is basic, organic matter like manure and compost leaves is added, since it releases acids. Acidic factory waste is neutralised with a base before it is released into water bodies, so that living things are protected. A neutral soil can still show poor plant growth if nutrients are lacking.
In the figure : Neutralisation protects soil, plants and water bodies.
Rule 1: Fix soil by neutralising it — Choose lime for acidic soil and organic matter for basic soil.
- Easy : Lime is added to acidic soil.
- Medium : Manure and compost are added to basic soil.
- Exam-type : Even neutral soil can give poor growth if nutrients are missing.
Rule 2: Treat factory waste before release — Waste that is acidic is neutralised using a base.
- Easy : Fish in a lake can die if acidic waste enters it.
- Medium : Adding a base neutralises acidic waste before release.
- Exam-type : Treated waste protects water life and drinking water.
Acidic soil needs lime; basic soil needs manure; acidic waste needs a base.
Adding more fertiliser to poor soil without testing. Excess chemical fertiliser can make soil acidic.
Memory clue: L-M-B — Lime for acid soil, Manure for base soil, Base for acidic waste.
Spot the difference
| Situation | Problem | Remedy |
|---|---|---|
| Acidic soil | Poor plant growth | Add lime |
| Basic soil | Poor plant growth | Add manure or compost |
| Acidic factory waste | Harms fish | Neutralise with a base |
Practice questions
- 1What is added to acidic soil?
- 2What is added to basic soil?
- 3Why is organic matter used to treat basic soil?
- 4Why must acidic factory waste be treated before release?
- 5How can excess chemical fertiliser harm soil?
- 6A soil is neutral but plants are weak. Give one possible reason.
- 7Why is testing the soil before treatment important?
- 8Write two examples of neutralisation that protect the environment.
Show answers (try them yourself first)
- 1Lime (a base).
- 2Manure or compost leaves (organic matter).
- 3Organic matter releases acids that neutralise the basic soil.
- 4Acidic waste can kill fish and harm living things in water.
- 5It can make the soil acidic.
- 6The soil may lack nutrients.
- 7To know whether it is acidic or basic, so that the right remedy is used.
- 8Treating acidic soil with lime and treating factory waste with a base.