Phenolphthalein Stays Colourless Until a Base Arrives
Predict indicator colours for acids and bases, write the reactions of acids with metals, carbonates and hydrogencarbonates, name the salt from a neutralisation, and explain why an acid conducts only when dissolved in water.
How can a colour tell you whether a solution is an acid or a base?
Dip a strip of blue litmus paper into lemon juice and it turns red. Dip another into a solution of washing soda and nothing happens — but a red strip dipped in the same solution turns blue.
That is an indicator: a substance whose colour depends on whether its surroundings are acidic or basic. It does not measure how strong the acid is; it answers a yes-or-no question, and it answers it in a form you can see across a room.
Why a single indicator is not enough. Blue litmus turning red proves an acid. Blue litmus staying blue proves only that the solution is not acidic — it might be a base, or it might be plain water. To identify a base you need red litmus, or a different indicator altogether, and that is why every laboratory keeps several.
The chapter then goes beyond colour. Acids and bases are recognised not only by indicators but by what they do: acids release hydrogen from metals, release carbon dioxide from carbonates, and neutralise bases to give a salt and water. Each of those is a test with a visible product, and each is a reaction you can write down.
This page covers the first part of the CBSE Class 10 Science chapter on acids, bases and salts: indicator colours, the reactions of acids with metals and carbonates, neutralisation and the salt formed, and why an acid conducts electricity only in aqueous solution.
That is an indicator: a substance whose colour depends on whether its surroundings are acidic or basic. It does not measure how strong the acid is; it answers a yes-or-no question, and it answers it in a form you can see across a room.
Why a single indicator is not enough. Blue litmus turning red proves an acid. Blue litmus staying blue proves only that the solution is not acidic — it might be a base, or it might be plain water. To identify a base you need red litmus, or a different indicator altogether, and that is why every laboratory keeps several.
The chapter then goes beyond colour. Acids and bases are recognised not only by indicators but by what they do: acids release hydrogen from metals, release carbon dioxide from carbonates, and neutralise bases to give a salt and water. Each of those is a test with a visible product, and each is a reaction you can write down.
This page covers the first part of the CBSE Class 10 Science chapter on acids, bases and salts: indicator colours, the reactions of acids with metals and carbonates, neutralisation and the salt formed, and why an acid conducts electricity only in aqueous solution.
Which colour does each indicator show in an acid and in a base?
Three synthetic indicators carry most of the syllabus, and each has its own pair of colours.
- Litmus: blue litmus turns red in an acid; red litmus turns blue in a base. In a neutral solution neither changes
- Methyl orange: red in an acid, orange when neutral, yellow in a base
- Phenolphthalein: colourless in an acid and in neutral solution, pink in a base
Worked example 1. A solution turns methyl orange yellow and phenolphthalein pink. What is it?
Both results point the same way, so the solution is basic — sodium hydroxide or lime water, for instance.
Worked example 2. A solution leaves phenolphthalein colourless. What can you conclude?
Very little. Phenolphthalein is colourless in acids and in neutral solutions, so the solution could be dilute hydrochloric acid or distilled water. A negative result with phenolphthalein does not identify anything — you would need litmus or methyl orange to separate the two possibilities.
That asymmetry is worth noticing. Phenolphthalein is excellent for detecting the arrival of a base, which is why it is the indicator of choice when a base is added drop by drop to an acid: the moment the acid is used up, the flask turns pink. It is useless for detecting an acid, because it has only one colour on that side.
Natural and olfactory indicators.
- Turmeric is yellow and turns red in a base — which is why a turmeric stain on cloth turns reddish-brown when soap is applied
- Red cabbage extract and china rose petals also change colour and are used in school laboratories
- Onion and vanilla are olfactory indicators: their smell is destroyed by a base but survives in an acid, so they can be used by someone who cannot see a colour change
The everyday check. Soap and detergent solutions are basic, which is why a drop of soap on a turmeric-stained cloth turns it red. Lemon juice, tamarind, curd and vinegar are acidic, which is why adding lemon juice to that same red patch turns it yellow again. The colour change is reversible, because the indicator is not consumed — it only responds to what surrounds it.
- Litmus: blue litmus turns red in an acid; red litmus turns blue in a base. In a neutral solution neither changes
- Methyl orange: red in an acid, orange when neutral, yellow in a base
- Phenolphthalein: colourless in an acid and in neutral solution, pink in a base
Worked example 1. A solution turns methyl orange yellow and phenolphthalein pink. What is it?
Both results point the same way, so the solution is basic — sodium hydroxide or lime water, for instance.
Worked example 2. A solution leaves phenolphthalein colourless. What can you conclude?
Very little. Phenolphthalein is colourless in acids and in neutral solutions, so the solution could be dilute hydrochloric acid or distilled water. A negative result with phenolphthalein does not identify anything — you would need litmus or methyl orange to separate the two possibilities.
That asymmetry is worth noticing. Phenolphthalein is excellent for detecting the arrival of a base, which is why it is the indicator of choice when a base is added drop by drop to an acid: the moment the acid is used up, the flask turns pink. It is useless for detecting an acid, because it has only one colour on that side.
Natural and olfactory indicators.
- Turmeric is yellow and turns red in a base — which is why a turmeric stain on cloth turns reddish-brown when soap is applied
- Red cabbage extract and china rose petals also change colour and are used in school laboratories
- Onion and vanilla are olfactory indicators: their smell is destroyed by a base but survives in an acid, so they can be used by someone who cannot see a colour change
The everyday check. Soap and detergent solutions are basic, which is why a drop of soap on a turmeric-stained cloth turns it red. Lemon juice, tamarind, curd and vinegar are acidic, which is why adding lemon juice to that same red patch turns it yellow again. The colour change is reversible, because the indicator is not consumed — it only responds to what surrounds it.
What happens when an acid meets a metal, a carbonate or a hydrogencarbonate?
A metal gives hydrogen; a carbonate or hydrogencarbonate gives carbon dioxide. In every case a salt is formed as well.
Acid with a metal — salt plus hydrogen.
The test for hydrogen: bring a burning splint to the mouth of the tube and the gas burns with a pop sound. That pop is the standard identification, and describing it is what earns the mark.
Acid with a metal carbonate — salt, water and carbon dioxide.
Acid with a metal hydrogencarbonate — the same three products.
Notice the coefficients. The carbonate needs two acid molecules and the hydrogencarbonate needs only one, because the carbonate carries two sodium atoms to be replaced and the hydrogencarbonate carries one. The formula decides the coefficient, which is the balancing skill from the previous chapter doing real work.
The test for carbon dioxide, in two steps. Pass the gas through lime water and it turns milky, because an insoluble white precipitate forms:
Now keep passing the gas and the milkiness disappears, because the carbonate reacts further to give a soluble hydrogencarbonate:
Both steps are part of the test. An answer that stops at turns lime water milky is incomplete if the question says excess carbon dioxide is passed, and the disappearance of the milkiness is itself a confirmation that the gas really was carbon dioxide.
One everyday appearance of these reactions. The fizzing when baking soda is added to lemon juice or curd is the hydrogencarbonate reaction, and the carbon dioxide it releases is what makes a cake or a dhokla rise. The same reaction relieves an acidic stomach, where baking soda neutralises excess hydrochloric acid.
A boundary case worth knowing. Not every metal gives hydrogen with an acid — copper and silver do not react with dilute hydrochloric acid at all, because they are below hydrogen in the reactivity series. The displacement rule from the previous chapter is still in force: only a metal more reactive than hydrogen can displace it.
Acid with a metal — salt plus hydrogen.
The test for hydrogen: bring a burning splint to the mouth of the tube and the gas burns with a pop sound. That pop is the standard identification, and describing it is what earns the mark.
Acid with a metal carbonate — salt, water and carbon dioxide.
Acid with a metal hydrogencarbonate — the same three products.
Notice the coefficients. The carbonate needs two acid molecules and the hydrogencarbonate needs only one, because the carbonate carries two sodium atoms to be replaced and the hydrogencarbonate carries one. The formula decides the coefficient, which is the balancing skill from the previous chapter doing real work.
The test for carbon dioxide, in two steps. Pass the gas through lime water and it turns milky, because an insoluble white precipitate forms:
Now keep passing the gas and the milkiness disappears, because the carbonate reacts further to give a soluble hydrogencarbonate:
Both steps are part of the test. An answer that stops at turns lime water milky is incomplete if the question says excess carbon dioxide is passed, and the disappearance of the milkiness is itself a confirmation that the gas really was carbon dioxide.
One everyday appearance of these reactions. The fizzing when baking soda is added to lemon juice or curd is the hydrogencarbonate reaction, and the carbon dioxide it releases is what makes a cake or a dhokla rise. The same reaction relieves an acidic stomach, where baking soda neutralises excess hydrochloric acid.
A boundary case worth knowing. Not every metal gives hydrogen with an acid — copper and silver do not react with dilute hydrochloric acid at all, because they are below hydrogen in the reactivity series. The displacement rule from the previous chapter is still in force: only a metal more reactive than hydrogen can displace it.
How do you write a neutralisation and name the salt it produces?
An acid and a base react to give a salt and water. The metal comes from the base and the rest of the salt comes from the acid.
Worked example 1. Sodium hydroxide with hydrochloric acid.
The salt is sodium chloride: sodium from the base, chloride from the acid.
Worked example 2 — an acid with two hydrogens. Sodium hydroxide with sulphuric acid.
The salt is sodium sulphate, and two formula units of the base are needed because the acid supplies two hydrogen atoms.
Worked example 3 — a base with two hydroxides. Calcium hydroxide with nitric acid.
The salt is calcium nitrate.
How to name any salt in two steps.
- Take the metal from the base — sodium, calcium, potassium, magnesium
- Take the ending from the acid — hydrochloric acid gives a chloride, sulphuric acid a sulphate, nitric acid a nitrate, carbonic acid a carbonate
So potassium hydroxide with sulphuric acid gives potassium sulphate, and magnesium hydroxide with hydrochloric acid gives magnesium chloride.
Watch the ratio rather than assuming it is one to one. The number of formula units needed depends on how many hydrogens the acid has and how many hydroxides the base has. Sulphuric acid needs twice as much sodium hydroxide as hydrochloric acid does, and a question about how much base neutralises a given acid is testing exactly that.
Neutralisation in ordinary life.
- An antacid is a mild base that neutralises excess hydrochloric acid in the stomach
- Lime is added to acidic soil so that crops can grow
- Baking soda on a bee sting or an ant bite neutralises the acid injected
- Toothpaste is basic, neutralising the acid that bacteria produce from food left on teeth
And one warning that is examined. The neutralisation of a strong acid by a strong base is exothermic — the mixture becomes warm. So is the dilution of a concentrated acid, which is why acid must always be added slowly to water and never water to acid. Adding water to concentrated acid releases so much heat in one place that the mixture can spit out of the container.
Worked example 1. Sodium hydroxide with hydrochloric acid.
The salt is sodium chloride: sodium from the base, chloride from the acid.
Worked example 2 — an acid with two hydrogens. Sodium hydroxide with sulphuric acid.
The salt is sodium sulphate, and two formula units of the base are needed because the acid supplies two hydrogen atoms.
Worked example 3 — a base with two hydroxides. Calcium hydroxide with nitric acid.
The salt is calcium nitrate.
How to name any salt in two steps.
- Take the metal from the base — sodium, calcium, potassium, magnesium
- Take the ending from the acid — hydrochloric acid gives a chloride, sulphuric acid a sulphate, nitric acid a nitrate, carbonic acid a carbonate
So potassium hydroxide with sulphuric acid gives potassium sulphate, and magnesium hydroxide with hydrochloric acid gives magnesium chloride.
Watch the ratio rather than assuming it is one to one. The number of formula units needed depends on how many hydrogens the acid has and how many hydroxides the base has. Sulphuric acid needs twice as much sodium hydroxide as hydrochloric acid does, and a question about how much base neutralises a given acid is testing exactly that.
Neutralisation in ordinary life.
- An antacid is a mild base that neutralises excess hydrochloric acid in the stomach
- Lime is added to acidic soil so that crops can grow
- Baking soda on a bee sting or an ant bite neutralises the acid injected
- Toothpaste is basic, neutralising the acid that bacteria produce from food left on teeth
And one warning that is examined. The neutralisation of a strong acid by a strong base is exothermic — the mixture becomes warm. So is the dilution of a concentrated acid, which is why acid must always be added slowly to water and never water to acid. Adding water to concentrated acid releases so much heat in one place that the mixture can spit out of the container.
Why does an acid conduct electricity only when it is dissolved in water?
Because conduction needs free ions, and an acid produces them only when water is present to pull its molecules apart.
Dry hydrogen chloride gas does not conduct electricity at all. Dissolve the same gas in water and the solution conducts well. Nothing was added except water, so water must be doing something essential.
What it does is this:
The molecule splits, and the hydrogen ion attaches itself to a water molecule to form the hydronium ion, . That ion is often written simply as , and the is the important part — a bare hydrogen ion never exists on its own in solution.
So the definition of an acid used in this chapter is: a substance that produces hydrogen ions in aqueous solution. And a base is a substance that produces hydroxide ions:
Now the two experiments that prove the point.
- Dry acid does not conduct. Hydrogen chloride gas, or dry crystals of citric acid, leave the bulb in a conductivity circuit unlit. No water, no ions, no current
- A substance with hydrogen but no ionisation does not conduct. A solution of glucose or of alcohol contains plenty of hydrogen atoms, yet the bulb stays dark. Those hydrogens do not leave as ions, so the solution has no charge carriers
That second experiment is the sharper one. It shows that having hydrogen in the formula is not what makes something an acid — **releasing it as in water is. Glucose contains far more hydrogen per molecule than hydrochloric acid does, and it is not an acid at all.
And neutralisation now has a deeper explanation.** When an acid meets a base, the hydrogen ions and the hydroxide ions combine:
That single reaction is what neutralisation really is. The metal ion and the acid's other ion simply remain in solution as the salt — they are spectators. It is also why the heat released is nearly the same for every strong acid with every strong base: the same reaction is occurring in every case.
Dry hydrogen chloride gas does not conduct electricity at all. Dissolve the same gas in water and the solution conducts well. Nothing was added except water, so water must be doing something essential.
What it does is this:
The molecule splits, and the hydrogen ion attaches itself to a water molecule to form the hydronium ion, . That ion is often written simply as , and the is the important part — a bare hydrogen ion never exists on its own in solution.
So the definition of an acid used in this chapter is: a substance that produces hydrogen ions in aqueous solution. And a base is a substance that produces hydroxide ions:
Now the two experiments that prove the point.
- Dry acid does not conduct. Hydrogen chloride gas, or dry crystals of citric acid, leave the bulb in a conductivity circuit unlit. No water, no ions, no current
- A substance with hydrogen but no ionisation does not conduct. A solution of glucose or of alcohol contains plenty of hydrogen atoms, yet the bulb stays dark. Those hydrogens do not leave as ions, so the solution has no charge carriers
That second experiment is the sharper one. It shows that having hydrogen in the formula is not what makes something an acid — **releasing it as in water is. Glucose contains far more hydrogen per molecule than hydrochloric acid does, and it is not an acid at all.
And neutralisation now has a deeper explanation.** When an acid meets a base, the hydrogen ions and the hydroxide ions combine:
That single reaction is what neutralisation really is. The metal ion and the acid's other ion simply remain in solution as the salt — they are spectators. It is also why the heat released is nearly the same for every strong acid with every strong base: the same reaction is occurring in every case.
Exam tip
What layout keeps an acids and bases answer complete?
Give the observation, the balanced equation with states, and the name of the gas or salt formed. Questions in this chapter almost always want all three.
- Name the indicator and both colours: blue litmus turns red, not merely litmus changes colour. The direction of the change is the answer
- Describe the gas test in full: hydrogen burns with a pop; carbon dioxide turns lime water milky, and the milkiness disappears with excess gas
- Write both lime-water equations when a question mentions excess carbon dioxide
- Balance by counting the replaceable hydrogens: a carbonate needs , a hydrogencarbonate needs one, and sulphuric acid needs twice as much base as hydrochloric acid
- Name a salt as metal-from-the-base plus ending-from-the-acid, and state the ending correctly — chloride, sulphate, nitrate
- **Use for every ion** and for a dissolved substance; a bare without is wrong
- **Say acid into water, never water into acid in any dilution question, and give the reason: the process is exothermic
- Include the colour of a precipitate when asked what you would see
The misconception to name. A substance is not an acid because its formula contains hydrogen. Glucose and alcohol both contain hydrogen and neither conducts or turns litmus red, because their hydrogen does not ionise in water.** An acid is defined by producing , and that is the sentence to write when a question asks why is glucose not an acid.
- Name the indicator and both colours: blue litmus turns red, not merely litmus changes colour. The direction of the change is the answer
- Describe the gas test in full: hydrogen burns with a pop; carbon dioxide turns lime water milky, and the milkiness disappears with excess gas
- Write both lime-water equations when a question mentions excess carbon dioxide
- Balance by counting the replaceable hydrogens: a carbonate needs , a hydrogencarbonate needs one, and sulphuric acid needs twice as much base as hydrochloric acid
- Name a salt as metal-from-the-base plus ending-from-the-acid, and state the ending correctly — chloride, sulphate, nitrate
- **Use for every ion** and for a dissolved substance; a bare without is wrong
- **Say acid into water, never water into acid in any dilution question, and give the reason: the process is exothermic
- Include the colour of a precipitate when asked what you would see
The misconception to name. A substance is not an acid because its formula contains hydrogen. Glucose and alcohol both contain hydrogen and neither conducts or turns litmus red, because their hydrogen does not ionise in water.** An acid is defined by producing , and that is the sentence to write when a question asks why is glucose not an acid.
Did you know
Why does a turmeric stain turn red when you wash it with soap?
Spill turmeric on a white kurta and you get a yellow stain. Rub soap on it and the yellow turns reddish-brown — which looks like the stain getting worse. Rinse thoroughly and it fades back to yellow.
Nothing has been made worse. Turmeric is a natural indicator, yellow in acidic and neutral conditions and red in a basic one. Soap and detergent solutions are basic, so the soap is simply telling you what it is. Rinse the soap away and the indicator returns to yellow; touch the patch with lemon juice, which is acidic, and it turns yellow immediately.
So a kitchen contains a working indicator set. Curd, tamarind, lemon juice, vinegar and tomato are acidic; soap, detergent, baking soda, lime water and washing soda are basic. Turmeric, red cabbage and the petals of the china rose will all report which is which.
And one household pair is worth watching for the gas rather than the colour. Add baking soda to vinegar and the mixture fizzes vigorously:
That escaping carbon dioxide is what makes a cake, a dhokla or an idli batter rise, and it is the same gas the lime-water test identifies. A leavening agent is an acid-base reaction being used for its by-product.
There is also an olfactory version of the trick. Chop an onion, put some of it in a base such as dilute sodium hydroxide, and the characteristic smell disappears; put it in an acid and the smell survives. So a person who cannot see a colour change can still identify a base, and that is exactly why olfactory indicators are in the syllabus — an indicator only has to change something detectable, and colour is not the only option.
The point behind all of this. An indicator does not measure acidity; it partitions the world into two halves and tells you which half you are in. Deciding how strongly acidic something is needs a scale rather than a colour switch — and that scale, pH, is where the next part of the chapter begins.
Nothing has been made worse. Turmeric is a natural indicator, yellow in acidic and neutral conditions and red in a basic one. Soap and detergent solutions are basic, so the soap is simply telling you what it is. Rinse the soap away and the indicator returns to yellow; touch the patch with lemon juice, which is acidic, and it turns yellow immediately.
So a kitchen contains a working indicator set. Curd, tamarind, lemon juice, vinegar and tomato are acidic; soap, detergent, baking soda, lime water and washing soda are basic. Turmeric, red cabbage and the petals of the china rose will all report which is which.
And one household pair is worth watching for the gas rather than the colour. Add baking soda to vinegar and the mixture fizzes vigorously:
That escaping carbon dioxide is what makes a cake, a dhokla or an idli batter rise, and it is the same gas the lime-water test identifies. A leavening agent is an acid-base reaction being used for its by-product.
There is also an olfactory version of the trick. Chop an onion, put some of it in a base such as dilute sodium hydroxide, and the characteristic smell disappears; put it in an acid and the smell survives. So a person who cannot see a colour change can still identify a base, and that is exactly why olfactory indicators are in the syllabus — an indicator only has to change something detectable, and colour is not the only option.
The point behind all of this. An indicator does not measure acidity; it partitions the world into two halves and tells you which half you are in. Deciding how strongly acidic something is needs a scale rather than a colour switch — and that scale, pH, is where the next part of the chapter begins.
Exam relevance
How do acids and bases appear in JEE and NEET Chemistry?
This is foundation work for one of the most heavily examined areas of Class 11 and 12 Chemistry.
Where it leads. The Class 11 chapter Equilibrium rebuilds this material quantitatively: the ionisation you met as HCl splits in water becomes an equilibrium with a dissociation constant, strong and weak acids are distinguished by how far that ionisation goes, and pH becomes a calculation rather than a colour. JEE Main and NEET both set numericals on pH, ionisation constants and buffers, and every one of them assumes the ion picture from the last section.
Where the ion definition leads. The definition you use here is the Arrhenius definition. Class 11 replaces it with the Bronsted-Lowry definition — an acid donates a proton, a base accepts one — which explains why ammonia is a base despite having no hydroxide group, and then with the Lewis definition in terms of electron pairs. Knowing that the definition you learn now is the first of three makes the later ones easier to accept.
Where the reactions themselves reappear. Acid with metal, acid with carbonate and neutralisation are used throughout the p-block chapters and in qualitative analysis, where the gas evolved identifies the anion present. The lime-water test appears in both JEE and NEET as an identification step.
Where it appears in Biology. NEET uses the same ideas in digestion — the hydrochloric acid of the stomach and its neutralisation by pancreatic bicarbonate — and in the buffering of blood. The antacid example above is a NEET Biology topic as well as a Chemistry one.
Question types to expect. At this level: predict indicator colours, write and balance the reactions, name the salt, explain conduction. In competitive papers: pH calculations, identification of the gas or precipitate formed, and assertion-reason items on whether a given substance is an acid.
The single trap that costs marks. Assuming hydrogen in the formula makes a substance an acid. It is the standard distractor — glucose, alcohol and methane all contain hydrogen and none is an acid — and the correct reason is always ionisation in aqueous solution.
A second trap. Getting the neutralisation ratio wrong. Sulphuric acid is dibasic, so it needs two formula units of a monoacidic base, and calcium hydroxide needs two of a monobasic acid. Count the replaceable hydrogens and the hydroxides before writing any coefficient.
Board versus competitive emphasis. The CBSE paper marks the observation, the balanced equation, the gas test and the salt's name; a competitive paper marks a pH value or an identification. The transferable habit is thinking in ions — once a reaction is read as meeting , the whole of Class 11 equilibrium follows naturally.
Where it leads. The Class 11 chapter Equilibrium rebuilds this material quantitatively: the ionisation you met as HCl splits in water becomes an equilibrium with a dissociation constant, strong and weak acids are distinguished by how far that ionisation goes, and pH becomes a calculation rather than a colour. JEE Main and NEET both set numericals on pH, ionisation constants and buffers, and every one of them assumes the ion picture from the last section.
Where the ion definition leads. The definition you use here is the Arrhenius definition. Class 11 replaces it with the Bronsted-Lowry definition — an acid donates a proton, a base accepts one — which explains why ammonia is a base despite having no hydroxide group, and then with the Lewis definition in terms of electron pairs. Knowing that the definition you learn now is the first of three makes the later ones easier to accept.
Where the reactions themselves reappear. Acid with metal, acid with carbonate and neutralisation are used throughout the p-block chapters and in qualitative analysis, where the gas evolved identifies the anion present. The lime-water test appears in both JEE and NEET as an identification step.
Where it appears in Biology. NEET uses the same ideas in digestion — the hydrochloric acid of the stomach and its neutralisation by pancreatic bicarbonate — and in the buffering of blood. The antacid example above is a NEET Biology topic as well as a Chemistry one.
Question types to expect. At this level: predict indicator colours, write and balance the reactions, name the salt, explain conduction. In competitive papers: pH calculations, identification of the gas or precipitate formed, and assertion-reason items on whether a given substance is an acid.
The single trap that costs marks. Assuming hydrogen in the formula makes a substance an acid. It is the standard distractor — glucose, alcohol and methane all contain hydrogen and none is an acid — and the correct reason is always ionisation in aqueous solution.
A second trap. Getting the neutralisation ratio wrong. Sulphuric acid is dibasic, so it needs two formula units of a monoacidic base, and calcium hydroxide needs two of a monobasic acid. Count the replaceable hydrogens and the hydroxides before writing any coefficient.
Board versus competitive emphasis. The CBSE paper marks the observation, the balanced equation, the gas test and the salt's name; a competitive paper marks a pH value or an identification. The transferable habit is thinking in ions — once a reaction is read as meeting , the whole of Class 11 equilibrium follows naturally.
Key takeaways
What should you know about acids and bases before the pH scale?
Indicators tell you which side you are on; the reactions tell you what is really happening.
- Blue litmus turns red in an acid; red litmus turns blue in a base, and neither changes in a neutral solution
- Methyl orange: red in acid, yellow in base. Phenolphthalein: colourless in acid, pink in base
- A colourless phenolphthalein result proves nothing — acids and neutral solutions look the same to it
- Turmeric turns red in a base; onion and vanilla are olfactory indicators
- Acid with a metal gives salt and hydrogen, which burns with a pop
- Acid with a carbonate or hydrogencarbonate gives salt, water and carbon dioxide, which turns lime water milky — and the milkiness disappears with excess gas
- A carbonate needs two acid molecules; a hydrogencarbonate needs one
- Neutralisation gives salt and water; the salt takes its metal from the base and its ending from the acid
- Acids conduct only in water, because water is needed to release ; bases release
- Hydrogen in a formula does not make an acid — glucose and alcohol do not ionise and do not conduct
- Always add acid to water, never water to acid, because dilution is exothermic
The sharpest self-test is the lime-water test. Write both equations — the one that makes the lime water milky and the one that clears it again — and then explain in one sentence why the second reaction proves the gas was carbon dioxide.
- Blue litmus turns red in an acid; red litmus turns blue in a base, and neither changes in a neutral solution
- Methyl orange: red in acid, yellow in base. Phenolphthalein: colourless in acid, pink in base
- A colourless phenolphthalein result proves nothing — acids and neutral solutions look the same to it
- Turmeric turns red in a base; onion and vanilla are olfactory indicators
- Acid with a metal gives salt and hydrogen, which burns with a pop
- Acid with a carbonate or hydrogencarbonate gives salt, water and carbon dioxide, which turns lime water milky — and the milkiness disappears with excess gas
- A carbonate needs two acid molecules; a hydrogencarbonate needs one
- Neutralisation gives salt and water; the salt takes its metal from the base and its ending from the acid
- Acids conduct only in water, because water is needed to release ; bases release
- Hydrogen in a formula does not make an acid — glucose and alcohol do not ionise and do not conduct
- Always add acid to water, never water to acid, because dilution is exothermic
The sharpest self-test is the lime-water test. Write both equations — the one that makes the lime water milky and the one that clears it again — and then explain in one sentence why the second reaction proves the gas was carbon dioxide.