ScienceCBSE Class 10 · Acids, Bases and Salts

pH Scale and Its Importance in Everyday Life

Q: What is the pH scale in simple words?

A number scale, generally from 0 to 14, showing how acidic or basic a solution is. 7 is neutral, below 7 is acidic and above 7 is basic.

Q: Does a lower pH mean a stronger acid?

A lower pH means more hydrogen ions, so the solution is more acidic. To compare the strength of two acids, test them at the same concentration: the one with the lower pH is the stronger acid. Diluting an acid raises its pH but does not make it a weak acid.

Q: Do we need the formula pH = -log[H+] for Class 10?

No. The 2026-27 syllabus says the definition relating to logarithm is not required. Treat pH as a number that indicates acidic or basic nature.

Q: Why is pH important in everyday life?

Our body works only within pH 7.0 to 7.8, acid rain harms aquatic life, plants need a suitable soil pH, antacids relieve indigestion, and tooth decay begins below pH 5.5.

Q: Can a basic solution contain hydrogen ions?

Yes. It is basic because its hydroxide ions outnumber its hydrogen ions.

Updated 14 October 2026 · Fact-checked

The pH scale is a number from 0 to 14 that tells how acidic or basic a solution is. A neutral solution has pH 7, acids have pH below 7 and bases above 7. The more hydrogen ions a solution has, the lower its pH. pH matters for our body, digestion, teeth, soil and aquatic life.

Moving left from 7, H⁺ ions increase and pH falls, so acids get stronger; moving right, OH⁻ ions increase and bases get stronger.

Illustration drawn for QuestionBuddy.

Understand pH Scale and Its Importance

Litmus only tells you whether a solution is acidic or basic. It cannot tell you how strongly acidic or basic it is. For that we use a universal indicator, a mixture of several indicators that shows a different colour for each level of hydrogen ion concentration. Paper soaked in universal indicator (pH paper) is the usual way to measure pH in school.

What the numbers mean. The pH scale generally runs from 0 (very acidic) to 14 (very alkaline). Think of pH simply as a number that tells the acidic or basic nature of a solution. You do not need the logarithm definition for Class 10.

pH = 7: neutral (pure water, a solution of common salt).
pH below 7: acidic. The lower the number, the more hydrogen ions ( $H^{+} (a q)$ , also shown as $H_{3} O^{+}$ ) and the stronger the acidity.
pH above 7: basic. As pH rises from 7 to 14 the hydroxide ion ( $O H^{-}$ ) concentration rises, so the alkali is stronger.

The key rule: higher hydronium ion concentration, lower pH. A student who remembers it backwards loses marks on every ordering question.

Strong and weak acids. Take hydrochloric acid and acetic acid at the same concentration. Hydrochloric acid gives far more $H^{+}$ ions, so it is a strong acid with a lower pH; acetic acid gives fewer, so it is a weak acid. Bases are judged the same way by the $O H^{-}$ ions they give. Strength is not the same as concentration: diluting an acid with water lowers the concentration of $H_{3} O^{+}$ ions per unit volume, so its pH moves up towards 7, but the acid itself is still the same acid.

Why pH matters.

Our body works within a narrow pH range of 7.0 to 7.8, and living things survive only small pH changes.
Acid rain: rain with pH below 5.6. When it flows into rivers it lowers their pH and makes survival hard for aquatic life.
Soil: plants need a particular pH range. A farmer may treat soil that is too acidic with quick lime, slaked lime or chalk, which are basic.
Digestion: the stomach makes hydrochloric acid, which helps digest food. Too much acid in indigestion causes pain; an antacid, a mild base such as magnesium hydroxide (milk of magnesia), neutralises the excess.
Tooth decay starts when the pH in the mouth falls below 5.5. Bacteria break down sugar and food bits left after eating and make acids that corrode enamel (calcium hydroxyapatite, the hardest substance in the body). Cleaning the mouth after eating and using toothpaste, which is generally basic, helps.
Stings: a bee sting leaves an acid; rubbing a mild base such as baking soda on it gives relief.

Finally, salts have pH values too. A salt of a strong acid and a strong base (NaCl) is neutral, pH 7; a salt of a strong acid and a weak base is acidic (pH below 7); a salt of a strong base and a weak acid is basic (pH above 7).

pH Scale and Its Importance: formulas and key facts

Reading the pH scale: $p H < 7 acidic; p H = 7 neutral; p H > 7 basic$; Scale generally 0 to 14. Logarithm definition not required.
Ion concentration and pH: $[H^{+}] ↑\Rightarrow p H ↓$; More hydrogen (hydronium) ions, lower pH. More $O H^{-}$ ions, higher pH.
Important pH values (NCERT): $body 7.0 to 7.8; acid rain < 5.6; tooth decay below 5.5$; Use these numbers exactly as stated.
pH of salts: $strong acid + strong base \to p H = 7; strong acid + weak base \to p H < 7; strong base + weak acid \to p H > 7$; NaCl neutral; $N H_{4} C l$ acidic; $N a_{2} C O_{3}$ , $N a H C O_{3}$ basic.
Neutralisation at the ion level: $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$; The reason an antacid, toothpaste or lime raises pH.

How to solve pH Scale and Its Importance questions

Most pH questions are ordering, matching or 'explain the everyday effect' questions. Use this routine.

1Classify each value first: below 7 acidic, 7 neutral, above 7 basic.
2For 'strongly' and 'weakly', use distance from 7: pH 1 is strongly acidic, pH 6 weakly acidic; pH 13 strongly basic, pH 8 weakly basic.
3To order by $H^{+}$ concentration, sort by pH in the opposite direction: the highest pH has the least $H^{+}$ .
4For a change in pH (milk to curd, dilution, antacid), decide whether $H^{+}$ ions increase or decrease, then state the direction of pH change.
5For everyday questions, name the acid or base involved, the neutralisation, and the result (pain relieved, soil corrected, enamel protected).
6Quote NCERT values exactly: 7.0 to 7.8, 5.6 and 5.5.

pH Scale and Its Importance in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Change of pH and neutralisation

Suppose your grandfather eats a heavy meal at a wedding and complains of a burning feeling in his stomach. Your aunt offers him lemon juice, your cousin offers milk of magnesia, and you remember the antacid section of your Science book.

Which remedy should he take and why? Say what happens to the pH in his stomach in each case.

Try it first, then show the solutionSolution

The burning is caused by too much hydrochloric acid produced in the stomach during indigestion, so the pH there is lower than usual.
Lemon juice is acidic (contains citric acid); it would add more $H^{+}$ ions and lower the pH further, making the pain worse.
Milk of magnesia is magnesium hydroxide, a mild base (antacid); its $O H^{-}$ ions neutralise the extra $H^{+}$ ions: $H^{+} + O H^{-} \to H_{2} O$ .
With the excess acid neutralised, the pH rises back towards the usual stomach level and the pain eases.

Answer: Milk of magnesia. It is a mild base that neutralises excess stomach acid, raising the pH; lemon juice would lower the pH further.

Memory hook: Acid trouble? Send a mild base: milk of magnesia raises the pH.

Tricks to remember pH Scale and Its Importance

Short memory aids. Say each one aloud once, then use it on a question the same day.

Seven is the seesaw
pH 7 is the middle of the seesaw: tip down (below 7) towards acid, up (above 7) towards base.
Use it when: Classifying any pH value instantly.
Low pH, lots of H
Low number = lots of hydrogen ions.
Use it when: Ordering solutions by hydrogen ion concentration.
Five-five, five-six, seven-seven-eight
Teeth decay below 5.5, rain is acid below 5.6, body works at 7.0 to 7.8.
Use it when: Recall of the three NCERT numbers.

pH Scale and Its Importance: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Five solutions on universal indicator · 3 marks · Short answerBoard level

Five solutions P, Q, R, S and T show pH 4, 1, 11, 7 and 9 respectively when tested with universal indicator. Pick out (a) the neutral one, (b) the one that is a strong alkali, (c) the one that is a strong acid, (d) the one that is a weak acid and (e) the one that is a weak alkali. Then list the pH values in order of increasing hydrogen ion concentration.

Try it first, then show the solutionSolution

(a) S (pH 7); (b) R (pH 11); (c) Q (pH 1) (1 mark).
(d) P (pH 4); (e) T (pH 9) (1 mark).
Lower pH means more $H^{+}$ , so increasing $H^{+}$ concentration means decreasing pH: 11 < 9 < 7 < 4 < 1, i.e. R, T, S, P, Q (1 mark).

Answer: Neutral S; strongly alkaline R; strongly acidic Q; weakly acidic P; weakly alkaline T. Increasing [H⁺]: 11, 9, 7, 4, 1.

Example 2: Strong and weak acid with magnesium · 3 marks · Short answerHarder board level

Equal lengths of magnesium ribbon are placed in test tubes A and B. Hydrochloric acid is added to A and acetic acid to B, in the same amount and the same concentration. (a) In which tube is the fizzing more vigorous, and why? (b) Which acid has the lower pH? (c) Name the gas and write the equation for tube A.

Try it first, then show the solutionSolution

(a) Tube A. Hydrochloric acid is a strong acid and gives many more $H^{+}$ ions than acetic acid at the same concentration, so hydrogen is released faster (1 mark).
(b) Hydrochloric acid, since more $H^{+}$ ions means lower pH (1 mark).
(c) Hydrogen: $M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$ (1 mark).

Answer: (a) A, because HCl is a strong acid with more $H^{+}$ ions. (b) HCl. (c) Hydrogen; $M g + 2 H C l \to M g C l_{2} + H_{2}$ .

Example 3: Milk, curd and baking soda · 3 marks · Short answerHarder board level

Fresh milk has a pH of 6. (a) How will the pH change as it turns into curd? Explain. (b) A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH to slightly alkaline, and why does this milk take longer to set as curd?

Try it first, then show the solutionSolution

(a) Curd contains lactic acid, so as milk turns to curd the acid content rises and the pH falls below 6 (1 mark).
(b) A slightly alkaline milk does not turn sour quickly, so it keeps longer; the baking soda neutralises acid as it forms (1 mark).
(b) Before the milk can set, the acid produced must first neutralise the added base; only then can the pH fall enough, so curd takes longer to form (1 mark).

Answer: (a) pH decreases because lactic acid forms. (b) Alkaline milk resists souring and keeps longer; the acid formed is first used up neutralising the baking soda, so setting is delayed.

Example 4: Tooth decay · 4 marks · Short answerHarder board level

(a) At what pH does tooth decay start? (b) Explain how eating sweets can lead to decay, naming the substance that is attacked. (c) Why does brushing with toothpaste after meals help? (d) A student says 'Tooth enamel dissolves in water, so drinking a lot of water causes decay'. Correct this.

Try it first, then show the solutionSolution

(a) When the pH in the mouth falls below 5.5 (1 mark).
(b) Bacteria in the mouth break down sugar and food particles left after eating and produce acids; the pH falls below 5.5 and tooth enamel, calcium hydroxyapatite, is corroded (1 mark).
(c) Toothpastes are generally basic and neutralise the excess acid; brushing also removes the leftover sugar the bacteria use (1 mark).
(d) Enamel does not dissolve in water; it is corroded only when the pH in the mouth falls below 5.5 (1 mark).

Answer: (a) Below 5.5. (b) Bacteria make acids from sugar, corroding calcium hydroxyapatite enamel. (c) Basic toothpaste neutralises acid. (d) Enamel is insoluble in water; acid, not water, damages it.

Example 5: Dilution and pH · 3 marks · Short answerHOTS / competency

A student dilutes 10 mL of dilute hydrochloric acid to 100 mL with water. (a) What happens to the concentration of $H_{3} O^{+}$ ions per unit volume? (b) Will the pH rise or fall? Can it go above 7 by adding more water? (c) How must the acid and water be mixed safely if the acid were concentrated, and why?

Try it first, then show the solutionSolution

(a) The same ions are spread through a larger volume, so the $H_{3} O^{+}$ concentration per unit volume decreases (1 mark).
(b) Fewer $H^{+}$ ions per unit volume means a higher pH; it moves towards 7 but the solution stays acidic, so it does not go above 7 just by adding water (1 mark).
(c) Add the acid slowly to water with constant stirring. Mixing is highly exothermic; adding water to concentrated acid can make the mixture splash and cause burns or crack the glass from local heating (1 mark).

Answer: (a) Decreases. (b) pH rises towards 7 but stays acidic. (c) Acid to water slowly with stirring, because the process is highly exothermic.

Where students lose marks in pH Scale and Its Importance

The slips examiners see most often, and what to write instead.

Mistake: Reversing the link between pH and hydrogen ions
Students assume a bigger number means more of everything.
Wrong working
pH 11 has more $H^{+}$ ions than pH 4.
Right working
pH 4 has more $H^{+}$ ions; higher $H^{+}$ concentration means lower pH.
Do this: Write 'low pH = lots of $H^{+}$ ' at the top of your answer before ordering.
Usually costs: The ordering mark in NCERT-style questions.
Mistake: Confusing strong with concentrated
Dilution and strength both seem to be about 'how much acid'.
Wrong working
Diluting HCl makes it a weak acid.
Right working
Diluting HCl lowers the $H_{3} O^{+}$ concentration per unit volume; HCl is still a strong acid.
Do this: Strength depends on how many $H^{+}$ ions an acid gives at the same concentration; dilution only spreads ions out.
Usually costs: 1 mark in reasoning questions.
Mistake: Wrong NCERT values
Students mix up the three numbers or round them.
Wrong working
Acid rain is rain with pH below 7; tooth decay starts below pH 7.
Right working
Acid rain: pH below 5.6. Tooth decay: mouth pH below 5.5. Body: 7.0 to 7.8.
Do this: Learn the three values as one line using the memory trick.
Usually costs: 1 mark per value.
Mistake: Calling every salt neutral
The word 'salt' suggests common salt.
Wrong working
Sodium carbonate is a salt, so its pH is 7.
Right working
Sodium carbonate comes from a strong base and a weak acid, so its pH is above 7.
Do this: Always ask which acid and which base formed the salt.
Usually costs: 1 mark in salt pH questions.

Exam tips

In everyday-application answers, name the acid or base, say 'neutralises', and state the effect on pH.
Use 'hydronium ions' or ' $H^{+} (a q)$ ions' rather than just 'hydrogen' when explaining acidity.
Do not write a logarithm definition of pH; it is outside the 2026-27 syllabus line.
When a question gives pH values, label each one acidic, neutral or basic before answering what is asked.
For dilution questions, state both the change in ion concentration and the direction the pH moves.

Practice questions on pH Scale and Its Importance

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Red litmus paper dipped in a solution turns blue. Which pH value fits this solution best?
(a) 1 (b) 4 (c) 5 (d) 10
Hint
Red litmus turns blue in a base.
Answer
(d) 10.
Q2 · 2 marks · Very short answerBoard level
Solution A has pH 6 and solution B has pH 8. (a) Which has the higher hydrogen ion concentration? (b) Which is acidic and which is basic? (c) Does B contain any $H^{+} (a q)$ ions? Explain.
Hint
Lower pH, more $H^{+}$ . Basic solutions are basic because $O H^{-}$ outnumbers $H^{+}$ .
Answer
(a) A. (b) A acidic, B basic. (c) Yes, B has some $H^{+}$ ions, but the concentration of $O H^{-}$ ions is greater, so it is basic.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A farmer may spread quick lime on his field.
Reason (R): Plants grow well only within a particular pH range, and quick lime reduces the acidity of soil.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Is calcium oxide acidic or basic?
Answer
(a). Calcium oxide is basic, so it neutralises excess acid in soil and brings the pH into the range plants need.
Q4 · 3 marks · Short answerHarder board level
Predict whether each salt solution is acidic, basic or neutral and give the reason in terms of the acid and base it is formed from: (a) sodium chloride, (b) ammonium chloride, (c) sodium carbonate.
Hint
Identify the parent acid and base, and whether each is strong or weak.
Answer
(a) NaCl: strong acid (HCl) + strong base (NaOH), neutral, pH 7. (b) $N H_{4} C l$ : strong acid (HCl) + weak base ( $N H_{4} O H$ ), acidic, pH below 7. (c) $N a_{2} C O_{3}$ : strong base (NaOH) + weak acid (carbonic acid), basic, pH above 7.
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a lake near a factory town receives rain with a measured pH of 5.0 during one season, while rain in a nearby village measures 6.0. Fish in the lake become fewer over the season.
(a) Which rain is called acid rain according to NCERT, and why? (1 mark)
(b) Which sample has more hydrogen ions? (1 mark)
(c) Explain how acid rain affects fish in the lake, and suggest why the lake water's pH matters to its living things. (2 marks)
OR
(c) A student says 'rain at pH 6.0 is neutral because it is close to 7'. Evaluate the statement. (2 marks)
Hint
Acid rain is defined by a pH cut-off. Living things tolerate only a narrow pH range.
Answer
(a) The town rain (pH 5.0), because rain with pH below 5.6 is called acid rain. (b) The town sample (lower pH). (c) Acid rain flowing into the lake lowers the pH of its water; living organisms survive only within a narrow range of pH, so aquatic life such as fish finds survival difficult. OR (c) Wrong: any pH below 7 is acidic; pH 6.0 is weakly acidic, though it is not acid rain since it is above 5.6.
Q6 · 5 marks · Long answerHOTS / competency
Suppose you are given three unlabelled solutions: dilute HCl, dilute acetic acid of the same concentration, and distilled water, plus a box of pH paper. (a) Predict the order of pH readings from lowest to highest and explain. (b) Then 10 mL of the HCl is diluted to 50 mL. Where would its pH now lie compared with the acetic acid? Explain why you cannot be certain without measuring. (c) Why is pH paper better than litmus for this task?
Hint
Strength depends on how many $H^{+}$ ions are produced; dilution lowers concentration per unit volume.
Answer
(a) HCl < acetic acid < water (7). HCl is a strong acid giving more $H^{+}$ ions than acetic acid at the same concentration; distilled water is neutral (2 marks). (b) Dilution reduces the $H^{+}$ concentration of the HCl, so its pH rises towards 7 and moves closer to that of the acetic acid. Without the exact $H^{+}$ amounts we cannot tell whether it is now above or below the acetic acid; only a measurement can settle it (2 marks). (c) Litmus shows only acid or base; universal indicator paper shows different colours for different $H^{+}$ concentrations, so it can compare the strengths (1 mark).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.pH of a neutral solution?Show answer
7.
2.What does a universal indicator do?Show answer
Shows different colours at different hydrogen ion concentrations, so it gives the pH.
3.Range of pH in which our body works?Show answer
7.0 to 7.8.
4.Rain is called acid rain below which pH?Show answer
5.6.
5.Name the antacid that NCERT mentions.Show answer
Magnesium hydroxide (milk of magnesia).
6.Is acetic acid strong or weak?Show answer
Weak; it gives fewer $H^{+}$ ions than HCl at the same concentration.

pH Scale and Its Importance: frequently asked questions

What is the pH scale in simple words?

A number scale, generally from 0 to 14, showing how acidic or basic a solution is. 7 is neutral, below 7 is acidic and above 7 is basic.

Does a lower pH mean a stronger acid?

A lower pH means more hydrogen ions, so the solution is more acidic. To compare the strength of two acids, test them at the same concentration: the one with the lower pH is the stronger acid. Diluting an acid raises its pH but does not make it a weak acid.

Do we need the formula pH = -log[H+] for Class 10?

No. The 2026-27 syllabus says the definition relating to logarithm is not required. Treat pH as a number that indicates acidic or basic nature.

Why is pH important in everyday life?

Our body works only within pH 7.0 to 7.8, acid rain harms aquatic life, plants need a suitable soil pH, antacids relieve indigestion, and tooth decay begins below pH 5.5.

Can a basic solution contain hydrogen ions?

Yes. It is basic because its hydroxide ions outnumber its hydrogen ions.