Tooth Enamel Starts Dissolving Below a Certain pH
Read the pH scale and rank solutions by acidity, see why pH matters for teeth, digestion, soil and stings, work out whether a salt is acidic or basic from its parents, and learn the four important sodium and calcium compounds.
What does a pH number actually measure?
Part 1 used indicators to answer a yes-or-no question: acid or base. That is useful but crude. Lemon juice and gastric juice both turn blue litmus red, yet one is far harsher than the other, and litmus cannot tell them apart.
The pH scale can. It runs from to and measures how many hydrogen ions a solution contains:
- **pH below — acidic, and the lower the number the more acidic
- pH exactly — neutral, like pure water
- pH above — basic, and the higher the number the more basic
The scale is not an ordinary ruler. Each step of one pH unit means a ten-fold** change in hydrogen-ion concentration, so a solution of pH has ten times as many hydrogen ions as one of pH , and a hundred times as many as one of pH . That is why a small pH difference matters so much, and why the scale can cover such an enormous range in only fourteen steps.
To read pH you use a universal indicator, which is a mixture of dyes giving a different colour at every pH — red through orange and yellow for acids, green at neutral, and blue through violet for bases. A single indicator gives one boundary; a universal indicator gives the whole scale.
This page covers the second part of the CBSE Class 10 Science chapter on acids, bases and salts: the pH scale and strength, the importance of pH in everyday life, acidic and basic salts, and the preparation and uses of four important salts.
The pH scale can. It runs from to and measures how many hydrogen ions a solution contains:
- **pH below — acidic, and the lower the number the more acidic
- pH exactly — neutral, like pure water
- pH above — basic, and the higher the number the more basic
The scale is not an ordinary ruler. Each step of one pH unit means a ten-fold** change in hydrogen-ion concentration, so a solution of pH has ten times as many hydrogen ions as one of pH , and a hundred times as many as one of pH . That is why a small pH difference matters so much, and why the scale can cover such an enormous range in only fourteen steps.
To read pH you use a universal indicator, which is a mixture of dyes giving a different colour at every pH — red through orange and yellow for acids, green at neutral, and blue through violet for bases. A single indicator gives one boundary; a universal indicator gives the whole scale.
This page covers the second part of the CBSE Class 10 Science chapter on acids, bases and salts: the pH scale and strength, the importance of pH in everyday life, acidic and basic salts, and the preparation and uses of four important salts.
How do you rank solutions in order of increasing acidity?
Order them by falling pH — the lowest pH is the most acidic.
Here are typical pH values worth knowing:
- Gastric juice — about , the most acidic thing in your body
- Lemon juice — about
- Vinegar — about
- Tomato juice — about
- Black coffee — about
- Milk — about , very slightly acidic
- Pure water — , neutral
- Blood — about , very slightly basic
- Baking soda solution — about
- Lime water — about to
- Sodium hydroxide solution — close to
Worked example. Arrange milk, lemon juice, lime water and vinegar in order of increasing acidity.
Increasing acidity means decreasing pH, so start from the highest pH and work down:
**The words increasing acidity and increasing pH mean opposite orders, and reading the question wrongly reverses your whole answer. Write which direction you are going before you list anything.
Now the distinction that this section exists to fix: strong is not the same as concentrated.
- Strength is about how completely an acid ionises in water. Hydrochloric, sulphuric and nitric acids ionise almost completely and are strong; acetic, carbonic and citric acids ionise only partly and are weak
- Concentration is about how much acid there is in a given amount of water. Any acid, strong or weak, can be concentrated or dilute
So a very dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid. Strength is a property of the substance and never changes; concentration is a property of the solution and changes the moment you add water. A question asking which is the stronger acid is not asking which solution has the lower pH unless both are at the same concentration.
One boundary case.** Diluting an acid raises its pH towards but never past it — adding water cannot turn an acid into a base. **The pH approaches and stops**, which makes sense once you remember that water itself is neutral.
Here are typical pH values worth knowing:
- Gastric juice — about , the most acidic thing in your body
- Lemon juice — about
- Vinegar — about
- Tomato juice — about
- Black coffee — about
- Milk — about , very slightly acidic
- Pure water — , neutral
- Blood — about , very slightly basic
- Baking soda solution — about
- Lime water — about to
- Sodium hydroxide solution — close to
Worked example. Arrange milk, lemon juice, lime water and vinegar in order of increasing acidity.
Increasing acidity means decreasing pH, so start from the highest pH and work down:
**The words increasing acidity and increasing pH mean opposite orders, and reading the question wrongly reverses your whole answer. Write which direction you are going before you list anything.
Now the distinction that this section exists to fix: strong is not the same as concentrated.
- Strength is about how completely an acid ionises in water. Hydrochloric, sulphuric and nitric acids ionise almost completely and are strong; acetic, carbonic and citric acids ionise only partly and are weak
- Concentration is about how much acid there is in a given amount of water. Any acid, strong or weak, can be concentrated or dilute
So a very dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid. Strength is a property of the substance and never changes; concentration is a property of the solution and changes the moment you add water. A question asking which is the stronger acid is not asking which solution has the lower pH unless both are at the same concentration.
One boundary case.** Diluting an acid raises its pH towards but never past it — adding water cannot turn an acid into a base. **The pH approaches and stops**, which makes sense once you remember that water itself is neutral.
Why does pH matter for teeth, digestion, soil and insect stings?
Because living systems work only within narrow pH ranges, and each of these four examples is a case of pH being pushed outside one.
Tooth decay. Enamel is made of calcium phosphate, the hardest substance in the body, and it does not dissolve in water. It does dissolve in acid. Bacteria in the mouth turn leftover sugar into acid, and when the pH of the mouth falls below about the enamel begins to corrode.
The remedy is neutralisation. Toothpastes are mildly basic, so brushing after eating neutralises the excess acid before it can act. Brushing removes the food and the paste removes the acid — two jobs in one.
Digestion. The stomach produces dilute hydrochloric acid, keeping its contents at about pH to . That acidity activates the enzyme pepsin and kills many swallowed bacteria. When too much acid is produced the result is pain and indigestion, and the cure is again neutralisation — a mild base such as magnesium hydroxide or sodium hydrogencarbonate:
**Why a mild base and not sodium hydroxide. A strong base would neutralise the acid and then attack the stomach lining itself. An antacid has to be weak enough to stop at neutral, which is why milk of magnesia is used and caustic soda never is.
Soil health. Most crops grow well only in a soil whose pH sits near neutral. Soil that is too acidic is treated with quicklime, slaked lime or chalk; soil that is too basic is treated with organic matter, which releases acids as it decays. A farmer testing soil pH before sowing is doing exactly the measurement this chapter describes.
Self-defence by animals and plants. A bee sting and an ant bite inject methanoic acid, which causes the pain. Rubbing on baking soda — a mild base — neutralises it. The stinging hairs of a nettle leaf inject the same acid, and the traditional remedy is to rub the area with a dock leaf, which is basic.
Acid rain.** Rain water is normally slightly acidic because dissolved carbon dioxide forms carbonic acid. When polluting gases push the pH of rain below about it is called acid rain, and when it drains into a river the water's pH falls far enough to threaten aquatic life. Fish and most water organisms survive only in a narrow pH band, which is why a small change in river pH has such large effects.
Tooth decay. Enamel is made of calcium phosphate, the hardest substance in the body, and it does not dissolve in water. It does dissolve in acid. Bacteria in the mouth turn leftover sugar into acid, and when the pH of the mouth falls below about the enamel begins to corrode.
The remedy is neutralisation. Toothpastes are mildly basic, so brushing after eating neutralises the excess acid before it can act. Brushing removes the food and the paste removes the acid — two jobs in one.
Digestion. The stomach produces dilute hydrochloric acid, keeping its contents at about pH to . That acidity activates the enzyme pepsin and kills many swallowed bacteria. When too much acid is produced the result is pain and indigestion, and the cure is again neutralisation — a mild base such as magnesium hydroxide or sodium hydrogencarbonate:
**Why a mild base and not sodium hydroxide. A strong base would neutralise the acid and then attack the stomach lining itself. An antacid has to be weak enough to stop at neutral, which is why milk of magnesia is used and caustic soda never is.
Soil health. Most crops grow well only in a soil whose pH sits near neutral. Soil that is too acidic is treated with quicklime, slaked lime or chalk; soil that is too basic is treated with organic matter, which releases acids as it decays. A farmer testing soil pH before sowing is doing exactly the measurement this chapter describes.
Self-defence by animals and plants. A bee sting and an ant bite inject methanoic acid, which causes the pain. Rubbing on baking soda — a mild base — neutralises it. The stinging hairs of a nettle leaf inject the same acid, and the traditional remedy is to rub the area with a dock leaf, which is basic.
Acid rain.** Rain water is normally slightly acidic because dissolved carbon dioxide forms carbonic acid. When polluting gases push the pH of rain below about it is called acid rain, and when it drains into a river the water's pH falls far enough to threaten aquatic life. Fish and most water organisms survive only in a narrow pH band, which is why a small change in river pH has such large effects.
How do you tell whether a salt will be acidic, basic or neutral?
Look at the acid and the base that formed it. Whichever parent was stronger decides the character of the salt.
- Strong acid + strong base gives a neutral salt, pH — sodium chloride from hydrochloric acid and sodium hydroxide
- Strong acid + weak base gives an acidic salt, pH below — ammonium chloride from hydrochloric acid and ammonium hydroxide
- Weak acid + strong base gives a basic salt, pH above — sodium carbonate from carbonic acid and sodium hydroxide, and sodium acetate from acetic acid and sodium hydroxide
Worked example 1. Predict the nature of sodium sulphate.
Its parents are sulphuric acid, which is strong, and sodium hydroxide, which is strong. Both strong, so the salt is neutral.
Worked example 2. Predict the nature of sodium carbonate solution.
Carbonic acid is weak and sodium hydroxide is strong, so the stronger parent is the base and the solution is basic — which is why washing soda solution is slippery to the touch and turns red litmus blue.
Worked example 3. Predict the nature of ammonium chloride.
Hydrochloric acid is strong and ammonium hydroxide is weak, so the acid wins and the solution is acidic.
The rule in one sentence: the salt takes after its stronger parent, and takes after neither when both are equally matched.
Salts come in families. Salts sharing the same positive ion or the same negative ion belong to one family:
- Sodium chloride and sodium sulphate are both in the sodium family
- Sodium chloride and potassium chloride are both in the chloride family
Where the raw material comes from. Common salt is obtained from sea water and from deposits of rock salt, and it is the starting point for almost every chemical in this chapter. Passing electricity through concentrated brine — the chlor-alkali process — splits it into three useful products at once:
Chlorine comes off at the anode, hydrogen at the cathode, and sodium hydroxide stays in solution. The name chlor-alkali records the two main products, and every one of the four salts in the next section is made from one of them.
- Strong acid + strong base gives a neutral salt, pH — sodium chloride from hydrochloric acid and sodium hydroxide
- Strong acid + weak base gives an acidic salt, pH below — ammonium chloride from hydrochloric acid and ammonium hydroxide
- Weak acid + strong base gives a basic salt, pH above — sodium carbonate from carbonic acid and sodium hydroxide, and sodium acetate from acetic acid and sodium hydroxide
Worked example 1. Predict the nature of sodium sulphate.
Its parents are sulphuric acid, which is strong, and sodium hydroxide, which is strong. Both strong, so the salt is neutral.
Worked example 2. Predict the nature of sodium carbonate solution.
Carbonic acid is weak and sodium hydroxide is strong, so the stronger parent is the base and the solution is basic — which is why washing soda solution is slippery to the touch and turns red litmus blue.
Worked example 3. Predict the nature of ammonium chloride.
Hydrochloric acid is strong and ammonium hydroxide is weak, so the acid wins and the solution is acidic.
The rule in one sentence: the salt takes after its stronger parent, and takes after neither when both are equally matched.
Salts come in families. Salts sharing the same positive ion or the same negative ion belong to one family:
- Sodium chloride and sodium sulphate are both in the sodium family
- Sodium chloride and potassium chloride are both in the chloride family
Where the raw material comes from. Common salt is obtained from sea water and from deposits of rock salt, and it is the starting point for almost every chemical in this chapter. Passing electricity through concentrated brine — the chlor-alkali process — splits it into three useful products at once:
Chlorine comes off at the anode, hydrogen at the cathode, and sodium hydroxide stays in solution. The name chlor-alkali records the two main products, and every one of the four salts in the next section is made from one of them.
How are washing soda, baking soda, bleaching powder and Plaster of Paris made and used?
All four come from common salt or from limestone, and each one's use follows directly from its chemistry.
**Washing soda — sodium carbonate decahydrate, .**
Heating sodium hydrogencarbonate gives anhydrous sodium carbonate, and recrystallising that from water gives the decahydrate:
Uses: in glass, soap and paper manufacture, as a cleaning agent for domestic purposes, and to remove the permanent hardness of water.
**Notice the . Those ten water molecules are the water of crystallisation — part of the crystal's structure even though the solid looks and feels dry. Leaving them out of the formula is a lost mark, and they explain why washing soda crystals weigh so much more than the sodium carbonate they contain.
Baking soda — sodium hydrogencarbonate, .**
Uses: as an antacid, since it is a mild base; in a soda-acid fire extinguisher; and as an ingredient of baking powder. On heating it gives off carbon dioxide, which is what makes a cake or an idli rise.
**Bleaching powder — calcium oxychloride, .**
Made by passing chlorine over dry slaked lime:
Uses: for bleaching cotton and linen in the textile industry and wood pulp in paper factories, for disinfecting drinking water, and as an oxidising agent in chemical industries.
**Plaster of Paris — calcium sulphate hemihydrate, .**
Gypsum heated carefully to about K loses most of its water of crystallisation:
Add water again and it sets into a hard solid of gypsum, which is why it is used for plastering fractured bones, for making toys and decorative material, and for smoothing wall surfaces before painting.
The temperature matters, and this is examined. Heating gypsum above K drives off all the water and leaves anhydrous calcium sulphate, which will no longer set. Plaster of Paris must keep its half molecule of water — that is the part that lets it recombine and harden, and it is why the compound is stored in moisture-proof containers.
A summary thread worth seeing. Chlorine from the chlor-alkali process makes bleaching powder; sodium hydroxide from the same process makes soap; the hydrogen becomes fuel; and the sodium hydrogencarbonate route leads to washing soda. One salt, electricity, and a whole industry.
**Washing soda — sodium carbonate decahydrate, .**
Heating sodium hydrogencarbonate gives anhydrous sodium carbonate, and recrystallising that from water gives the decahydrate:
Uses: in glass, soap and paper manufacture, as a cleaning agent for domestic purposes, and to remove the permanent hardness of water.
**Notice the . Those ten water molecules are the water of crystallisation — part of the crystal's structure even though the solid looks and feels dry. Leaving them out of the formula is a lost mark, and they explain why washing soda crystals weigh so much more than the sodium carbonate they contain.
Baking soda — sodium hydrogencarbonate, .**
Uses: as an antacid, since it is a mild base; in a soda-acid fire extinguisher; and as an ingredient of baking powder. On heating it gives off carbon dioxide, which is what makes a cake or an idli rise.
**Bleaching powder — calcium oxychloride, .**
Made by passing chlorine over dry slaked lime:
Uses: for bleaching cotton and linen in the textile industry and wood pulp in paper factories, for disinfecting drinking water, and as an oxidising agent in chemical industries.
**Plaster of Paris — calcium sulphate hemihydrate, .**
Gypsum heated carefully to about K loses most of its water of crystallisation:
Add water again and it sets into a hard solid of gypsum, which is why it is used for plastering fractured bones, for making toys and decorative material, and for smoothing wall surfaces before painting.
The temperature matters, and this is examined. Heating gypsum above K drives off all the water and leaves anhydrous calcium sulphate, which will no longer set. Plaster of Paris must keep its half molecule of water — that is the part that lets it recombine and harden, and it is why the compound is stored in moisture-proof containers.
A summary thread worth seeing. Chlorine from the chlor-alkali process makes bleaching powder; sodium hydroxide from the same process makes soap; the hydrogen becomes fuel; and the sodium hydrogencarbonate route leads to washing soda. One salt, electricity, and a whole industry.
Exam tip
What layout keeps a pH and salts answer complete?
Give the formula with its water of crystallisation, the balanced equation, and the use — in that order. Questions in this chapter nearly always want a name, a formula and a reason.
- Write the full formula including hydrated water: for washing soda and for Plaster of Paris
- State the direction before ranking: increasing acidity means decreasing pH. Then list
- Name both parents when classifying a salt, and say which is the stronger: hydrochloric acid is strong and ammonium hydroxide weak, so the salt is acidic
- **Keep strong and concentrated apart. Strength is the extent of ionisation; concentration is the amount per unit volume
- Quote the pH threshold with its context** — enamel corrodes below about , rain below about is acid rain
- Say why a mild base is used in an antacid, rather than only naming it
- Give the temperature for Plaster of Paris and say what goes wrong above it
- Match each use to a property: bleaching powder bleaches because it is an oxidising agent
The misconception to name. A higher pH does not mean a stronger acid — it means a weaker one, or a more dilute one. And *pH is not no acid*; it is the extreme acidic end of the scale. When a question gives you two pH values, the smaller number is always the more acidic solution, and writing that sentence out first prevents the most common error in the whole chapter.
- Write the full formula including hydrated water: for washing soda and for Plaster of Paris
- State the direction before ranking: increasing acidity means decreasing pH. Then list
- Name both parents when classifying a salt, and say which is the stronger: hydrochloric acid is strong and ammonium hydroxide weak, so the salt is acidic
- **Keep strong and concentrated apart. Strength is the extent of ionisation; concentration is the amount per unit volume
- Quote the pH threshold with its context** — enamel corrodes below about , rain below about is acid rain
- Say why a mild base is used in an antacid, rather than only naming it
- Give the temperature for Plaster of Paris and say what goes wrong above it
- Match each use to a property: bleaching powder bleaches because it is an oxidising agent
The misconception to name. A higher pH does not mean a stronger acid — it means a weaker one, or a more dilute one. And *pH is not no acid*; it is the extreme acidic end of the scale. When a question gives you two pH values, the smaller number is always the more acidic solution, and writing that sentence out first prevents the most common error in the whole chapter.
Did you know
Why does baking powder contain more than just baking soda?
Baking soda on its own would raise a cake. Heated in the oven it breaks down and releases carbon dioxide, and the bubbles do the lifting:
But look at what is left behind: sodium carbonate, which is washing soda. It is a basic salt, and it tastes bitter. A cake raised with baking soda alone rises perfectly and then tastes wrong.
Baking powder solves it by adding a mild edible acid — usually tartaric acid. Now when the mixture is wetted and heated, the acid neutralises the sodium carbonate as it forms, so the carbon dioxide is still released and nothing bitter survives:
So baking powder is a neutralisation reaction packaged as a powder, designed to go off at the right moment. It is also why a batter left standing too long after mixing rises less — the gas has already escaped before the heat arrives.
The same gas, released deliberately, fights fires. A soda-acid extinguisher holds sodium hydrogencarbonate solution and a sealed bottle of sulphuric acid. Knocking the plunger breaks the bottle, the acid meets the solution, and the carbon dioxide produced forces the water out and smothers the flame:
Carbon dioxide is heavier than air and does not burn, so it forms a blanket over the fire and cuts off the oxygen — the same reason it turns lime water milky and the same reason it makes a cake rise. One gas, three completely different uses, each following from a different one of its properties.
And that is the pattern of this whole chapter. A property explains a use: mild basicity makes an antacid, water of crystallisation makes Plaster of Paris set, an oxidising nature makes bleaching powder bleach. Learn the property and the use is not a separate fact to memorise.
But look at what is left behind: sodium carbonate, which is washing soda. It is a basic salt, and it tastes bitter. A cake raised with baking soda alone rises perfectly and then tastes wrong.
Baking powder solves it by adding a mild edible acid — usually tartaric acid. Now when the mixture is wetted and heated, the acid neutralises the sodium carbonate as it forms, so the carbon dioxide is still released and nothing bitter survives:
So baking powder is a neutralisation reaction packaged as a powder, designed to go off at the right moment. It is also why a batter left standing too long after mixing rises less — the gas has already escaped before the heat arrives.
The same gas, released deliberately, fights fires. A soda-acid extinguisher holds sodium hydrogencarbonate solution and a sealed bottle of sulphuric acid. Knocking the plunger breaks the bottle, the acid meets the solution, and the carbon dioxide produced forces the water out and smothers the flame:
Carbon dioxide is heavier than air and does not burn, so it forms a blanket over the fire and cuts off the oxygen — the same reason it turns lime water milky and the same reason it makes a cake rise. One gas, three completely different uses, each following from a different one of its properties.
And that is the pattern of this whole chapter. A property explains a use: mild basicity makes an antacid, water of crystallisation makes Plaster of Paris set, an oxidising nature makes bleaching powder bleach. Learn the property and the use is not a separate fact to memorise.
Exam relevance
How does pH feed into JEE and NEET Chemistry?
This is foundation work for one of the most calculation-heavy topics in Class 11, and it is examined in both papers.
Where it leads. The Class 11 chapter Equilibrium turns pH from a colour reading into a calculation: , with dissociation constants for weak acids and bases, buffer solutions, and salt hydrolysis — which is precisely the acidic-basic-neutral salt rule of this chapter, now derived quantitatively. JEE Main and NEET both set numericals on pH, on the pH of a salt solution, and on buffers.
Why the ten-fold step matters later. Because the scale is logarithmic, a pH change of is a factor of in hydrogen-ion concentration, and a change of is a factor of . Competitive questions are built on that: given the pH, find the concentration, or given a dilution, find the new pH. The qualitative statement you learn here becomes the arithmetic there.
Where the four salts lead. Sodium and calcium compounds return in Class 11 the s-Block Elements, where the preparation of washing soda, baking soda, bleaching powder and Plaster of Paris is examined again with more detail, including the industrial processes. The equations in this chapter are the same ones, so learning them properly now saves relearning them later.
Where the chlor-alkali process leads. It reappears in Class 12 Electrochemistry as an electrolysis with named electrode reactions. The three products and which electrode each comes from is the part that carries forward.
Question types to expect. At this level: rank by pH, explain a daily-life application, classify a salt, name a preparation and a use. In competitive papers: pH numericals, hydrolysis of a given salt, and assertion-reason items on strong versus concentrated.
The single trap that costs marks. Confusing strength with concentration. It is the standard distractor: a dilute strong acid and a concentrated weak acid can have similar pH values, and a question asking which is the stronger acid is asking about ionisation, not about pH. In JEE this becomes the difference between the dissociation constant and the molarity — two independent quantities.
A second trap. Omitting the water of crystallisation from a formula. and have different formula masses, and a mole calculation built on the wrong one is wrong throughout. Count the hydrated water before any numerical work.
Board versus competitive emphasis. The CBSE paper marks the named use, the balanced equation and the reason; a competitive paper marks a pH value or a classification. **The transferable habit is asking which parent was stronger and how many water molecules are in the formula** — two questions that answer a large share of both papers.
Where it leads. The Class 11 chapter Equilibrium turns pH from a colour reading into a calculation: , with dissociation constants for weak acids and bases, buffer solutions, and salt hydrolysis — which is precisely the acidic-basic-neutral salt rule of this chapter, now derived quantitatively. JEE Main and NEET both set numericals on pH, on the pH of a salt solution, and on buffers.
Why the ten-fold step matters later. Because the scale is logarithmic, a pH change of is a factor of in hydrogen-ion concentration, and a change of is a factor of . Competitive questions are built on that: given the pH, find the concentration, or given a dilution, find the new pH. The qualitative statement you learn here becomes the arithmetic there.
Where the four salts lead. Sodium and calcium compounds return in Class 11 the s-Block Elements, where the preparation of washing soda, baking soda, bleaching powder and Plaster of Paris is examined again with more detail, including the industrial processes. The equations in this chapter are the same ones, so learning them properly now saves relearning them later.
Where the chlor-alkali process leads. It reappears in Class 12 Electrochemistry as an electrolysis with named electrode reactions. The three products and which electrode each comes from is the part that carries forward.
Question types to expect. At this level: rank by pH, explain a daily-life application, classify a salt, name a preparation and a use. In competitive papers: pH numericals, hydrolysis of a given salt, and assertion-reason items on strong versus concentrated.
The single trap that costs marks. Confusing strength with concentration. It is the standard distractor: a dilute strong acid and a concentrated weak acid can have similar pH values, and a question asking which is the stronger acid is asking about ionisation, not about pH. In JEE this becomes the difference between the dissociation constant and the molarity — two independent quantities.
A second trap. Omitting the water of crystallisation from a formula. and have different formula masses, and a mole calculation built on the wrong one is wrong throughout. Count the hydrated water before any numerical work.
Board versus competitive emphasis. The CBSE paper marks the named use, the balanced equation and the reason; a competitive paper marks a pH value or a classification. **The transferable habit is asking which parent was stronger and how many water molecules are in the formula** — two questions that answer a large share of both papers.
Key takeaways
What should you know about pH and salts before the next chapter?
One scale, four applications, one classification rule, and four compounds.
- **pH runs from to **: below acidic, neutral, above basic, read with a universal indicator
- Each pH unit is a ten-fold change in hydrogen-ion concentration
- Increasing acidity means decreasing pH — state the direction before ranking anything
- Strong is about ionisation; concentrated is about amount. A dilute strong acid is still a strong acid
- **Enamel corrodes below about pH ; toothpaste is basic and neutralises the acid
- Gastric juice is about pH to , and an antacid must be a mild base
- Acidic soil is treated with lime, basic soil with organic matter; rain below about pH is acid rain
- Bee and ant stings inject methanoic acid, neutralised by baking soda
- A salt takes after its stronger parent: strong-strong gives neutral, strong acid with weak base gives acidic, weak acid with strong base gives basic
- The chlor-alkali process on brine gives sodium hydroxide, chlorine and hydrogen
- , , and ** — learn each formula with its water of crystallisation, its preparation and its use
The sharpest self-test is the salt rule. Take ammonium chloride, sodium acetate and potassium sulphate, name both parents of each, and predict whether each solution turns litmus red, blue or neither.
- **pH runs from to **: below acidic, neutral, above basic, read with a universal indicator
- Each pH unit is a ten-fold change in hydrogen-ion concentration
- Increasing acidity means decreasing pH — state the direction before ranking anything
- Strong is about ionisation; concentrated is about amount. A dilute strong acid is still a strong acid
- **Enamel corrodes below about pH ; toothpaste is basic and neutralises the acid
- Gastric juice is about pH to , and an antacid must be a mild base
- Acidic soil is treated with lime, basic soil with organic matter; rain below about pH is acid rain
- Bee and ant stings inject methanoic acid, neutralised by baking soda
- A salt takes after its stronger parent: strong-strong gives neutral, strong acid with weak base gives acidic, weak acid with strong base gives basic
- The chlor-alkali process on brine gives sodium hydroxide, chlorine and hydrogen
- , , and ** — learn each formula with its water of crystallisation, its preparation and its use
The sharpest self-test is the salt rule. Take ammonium chloride, sodium acetate and potassium sulphate, name both parents of each, and predict whether each solution turns litmus red, blue or neither.