Acids, Bases and Salts Class 10: pH Scale, Indicators & Solved Examples
Complete Class 10 Chemistry notes on acids, bases and salts with the pH scale, indicators, important reactions, pH numericals, neutralisation, common salts, solved examples, exam mistakes and FAQs.
Updated for 2026 Class 10 revision. This guide is designed as a companion resource to the Class 10 Chemistry Complete Guide and the broader Acids, Bases and Salts Class 10 notes .
1. What Are Acids, Bases and Salts?
Acids, bases and salts are among the most frequently tested concepts in Class 10 Chemistry. They appear not only as direct definition questions, but also in reaction equations, indicator questions, pH problems, application-based questions and case-based questions.
🧪 Acid
An acid is a substance that produces hydrogen ions in aqueous solution. At Class 10 level, common examples include HCl, H2SO4 and HNO3.
Litmus: Blue → Red
🧴 Base
A base is a substance that produces hydroxide ions in aqueous solution. Common examples include NaOH, KOH and Ca(OH)2.
Litmus: Red → Blue
🧂 Salt
Salts are ionic compounds commonly formed in acid-base reactions. Sodium chloride, sodium hydrogen carbonate and sodium carbonate are important Class 10 examples.
💧 Alkali
An alkali is a base that is soluble in water. Sodium hydroxide and potassium hydroxide are familiar examples.
2. Indicators: How Do We Identify Acids and Bases?
An indicator is a substance that gives a characteristic colour change in acidic or basic conditions. Indicators are especially useful when the solution itself is colourless.
| Indicator | Acidic Medium | Basic Medium | Exam Memory Tip |
|---|---|---|---|
| Blue litmus | Red | Blue | Acid turns blue litmus red |
| Red litmus | Red | Blue | Base turns red litmus blue |
| Phenolphthalein | Colourless | Pink | Basic solution → pink |
| Methyl orange | Red | Yellow | Acid → red; base → yellow |
| Universal indicator | Gives a range of colours corresponding approximately to different pH values. | Useful for estimating pH | |
Acid → Blue litmus becomes Red.
Base → Red litmus becomes Blue.
Phenolphthalein → Pink in a basic solution.
Methyl orange → Red in acid and Yellow in base.
3. pH Scale: The Most Important Concept
The pH scale is used to express how acidic or basic an aqueous solution is. In the familiar Class 10 representation, values are shown from approximately 0 to 14.
| pH Range | Nature | Interpretation |
|---|---|---|
| Below 7 | Acidic | Greater acidity as pH decreases |
| 7 | Neutral | Pure water is the standard school-level example |
| Above 7 | Basic / Alkaline | Greater basicity as pH increases |
A lower pH corresponds to a higher hydrogen-ion concentration under the usual conditions considered at this level. Therefore, a solution of pH 2 is more acidic than a solution of pH 5.
- pH 1 → strongly acidic
- pH 4 → acidic
- pH 7 → neutral
- pH 10 → basic
- pH 13 → strongly basic
4. pH Formula and Hydrogen-Ion Concentration
For aqueous solutions, the mathematical definition of pH is expressed in terms of hydrogen-ion or, more precisely, hydronium-ion activity. At the introductory Class 10 level, it is commonly represented using hydrogen-ion concentration.
The important idea is that the pH scale is logarithmic. A change of one pH unit corresponds to roughly a tenfold change in hydrogen-ion concentration under the simplified conditions used in school examples.
| [H+] mol/L | pH | Nature |
|---|---|---|
| 10−2 | 2 | Acidic |
| 10−5 | 5 | Acidic |
| 10−7 | 7 | Neutral reference at 25°C |
| 10−10 | 10 | Basic |
| 10−12 | 12 | Basic |
5. pH Solved Examples for Class 10
Example 1: Find the pH when [H+] = 10−3 mol/L.
Answer: The solution has pH 3 and is acidic.
Example 2: Find the pH when [H+] = 10−5 mol/L.
Answer: pH = 5, so the solution is acidic.
Example 3: Which is more acidic — pH 2 or pH 5?
The lower the pH, the greater the hydrogen-ion concentration in the simplified school-level interpretation.
Example 4: A solution has pH 11. Is it acidic or basic?
Answer: The solution is basic or alkaline.
Example 5: What happens to acidity when pH changes from 3 to 5?
A pH increase from 3 to 5 means the hydrogen-ion concentration becomes much lower in the simplified logarithmic model.
The difference is two pH units, corresponding to a factor of approximately 102 = 100 in hydrogen-ion concentration.
6. Strong vs Weak Acids and Bases
One of the most common Class 10 mistakes is treating strong/weak and concentrated/dilute as the same thing. They are different ideas.
| Term | Meaning |
|---|---|
| Strong acid | Ionises extensively in aqueous solution. |
| Weak acid | Ionises only partially in aqueous solution. |
| Concentrated solution | Contains a relatively large amount of solute per quantity of solution. |
| Dilute solution | Contains a relatively smaller amount of solute per quantity of solution. |
7. Important Reactions of Acids and Bases
7.1 Acid + Metal → Salt + Hydrogen
Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.
7.2 Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
7.3 Acid + Metal Hydrogen Carbonate → Salt + Water + Carbon Dioxide
The carbon dioxide evolved can turn limewater milky.
7.4 Base + Non-Metal Oxide → Salt + Water
This reaction is commonly used to demonstrate the acidic nature of carbon dioxide.
8. Neutralisation Reaction
A neutralisation reaction occurs when an acid reacts with a base to form salt and water.
Here hydrochloric acid reacts with sodium hydroxide. Sodium chloride and water are formed.
9. Important Salts for Class 10 Chemistry
| Common Name | Chemical Name | Formula | Important Point |
|---|---|---|---|
| Common salt | Sodium chloride | NaCl | Important starting material for several chemicals |
| Baking soda | Sodium hydrogen carbonate | NaHCO3 | Used in baking; releases CO2 on heating |
| Washing soda | Sodium carbonate decahydrate | Na2CO3·10H2O | Contains water of crystallisation |
| Bleaching powder | Calcium oxychloride | CaOCl2 | Associated with bleaching and disinfecting uses |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO4·½H2O | Sets to gypsum after adding water |
Baking Soda: NaHCO3
On heating, baking soda produces carbon dioxide. The gas helps dough and batter expand during baking.
Washing Soda: Na2CO3·10H2O
Washing soda is sodium carbonate containing ten molecules of water of crystallisation.
Plaster of Paris
Plaster of Paris is calcium sulphate hemihydrate. When water is added, it sets to form gypsum.
10. What Is Water of Crystallisation?
Water of crystallisation is the fixed number of water molecules associated with one formula unit of a crystalline salt.
This represents washing soda. The ten water molecules are part of the crystalline structure.
Another familiar example is blue copper sulphate crystals:
11. Acids, Bases and Salts in Everyday Life
🦷 Tooth Decay
Acidity in the mouth can contribute to the demineralisation of tooth enamel. Regular oral hygiene helps reduce the conditions associated with tooth decay.
🌱 Soil pH
Soil acidity or basicity affects plant growth. Different plants have different preferred soil conditions.
🍋 Food Acids
Many foods contain naturally occurring acids. Their acidic character can be discussed using indicators and pH.
💊 Antacids
Basic substances in antacid formulations can react with excess stomach acid through acid-base neutralisation.
12. Dilution of Acids and Bases: Important Safety Rule
Diluting concentrated acids can release considerable heat. This is why laboratory technique is important.
This is an important laboratory-safety concept and should not be treated merely as a memorisation trick.
13. Acids, Bases and Salts — Quick Reaction Sheet
| Reaction | Equation |
|---|---|
| Acid + metal | Zn + 2HCl → ZnCl2 + H2 |
| Acid + carbonate | Na2CO3 + 2HCl → 2NaCl + H2O + CO2 |
| Acid + hydrogen carbonate | NaHCO3 + HCl → NaCl + H2O + CO2 |
| Acid + base | HCl + NaOH → NaCl + H2O |
| Base + non-metal oxide | Ca(OH)2 + CO2 → CaCO3 + H2O |
| Heating baking soda | 2NaHCO3 → Na2CO3 + H2O + CO2 |
| Plaster of Paris + water | CaSO4·½H2O + 1½H2O → CaSO4·2H2O |
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14. Common Acids, Bases and Salts Exam Mistakes
- Confusing strong acids with concentrated acids.
- Writing Na2CO3 instead of NaHCO3 for baking soda.
- Forgetting that pH 7 is neutral in the standard school-level model.
- Mixing up red and blue litmus changes.
- Forgetting CO2 in acid-carbonate reactions.
- Failing to balance chemical equations.
- Confusing baking soda with washing soda.
- Forgetting the 10H2O in washing soda.
- Writing Plaster of Paris as CaSO4·2H2O.
- Learning formulas without understanding the reaction pattern.
- Assuming every salt solution is neutral.
- Ignoring laboratory safety in acid dilution questions.
15. Important Class 10 Questions with Answers
Q1. What is the pH scale?
The pH scale is used to express the acidity or basicity of an aqueous solution. In the familiar school-level range, pH below 7 is acidic, pH 7 is neutral and pH above 7 is basic.
Q2. Which is more acidic: pH 2 or pH 6?
pH 2 is more acidic because a lower pH corresponds to a greater hydrogen-ion concentration under the simplified model.
Q3. What is neutralisation?
Neutralisation is the reaction of an acid with a base to form salt and water.
Q4. What happens when an acid reacts with a metal?
A suitable acid can react with a metal to form a salt and hydrogen gas.
Q5. What happens when an acid reacts with a metal carbonate?
It generally produces salt, water and carbon dioxide.
Q6. What is baking soda?
Baking soda is sodium hydrogen carbonate, NaHCO3.
Q7. What is washing soda?
Washing soda is sodium carbonate decahydrate, Na2CO3·10H2O.
Q8. What is Plaster of Paris?
Plaster of Paris is calcium sulphate hemihydrate, CaSO4·½H2O.
Q9. What is water of crystallisation?
It is the fixed number of water molecules associated with one formula unit of a crystalline salt.
Q10. Why is concentrated acid added to water during dilution?
Dilution can release considerable heat. Adding acid slowly to water with stirring is the safer laboratory practice.
16. One-Minute Revision: Acids, Bases and Salts
| Topic | Remember This |
|---|---|
| Acid | Produces H+ in aqueous solution |
| Base | Produces OH− in aqueous solution |
| pH < 7 | Acidic |
| pH = 7 | Neutral reference at 25°C |
| pH > 7 | Basic / alkaline |
| Neutralisation | Acid + Base → Salt + Water |
| Acid + metal | Salt + Hydrogen |
| Acid + carbonate | Salt + Water + CO2 |
| Baking soda | NaHCO3 |
| Washing soda | Na2CO3·10H2O |
| Bleaching powder | CaOCl2 |
| Plaster of Paris | CaSO4·½H2O |
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Start 7-Day Demo →18. Frequently Asked Questions — Acids, Bases and Salts Class 10
What are acids and bases in Class 10 Chemistry?
What is the pH scale?
What is the formula for pH?
Which is more acidic, pH 2 or pH 5?
What happens when an acid reacts with a metal?
What happens when an acid reacts with a metal carbonate?
What is neutralisation?
What is baking soda?
What is washing soda?
What is Plaster of Paris?
What is water of crystallisation?
Why should acid be added to water during dilution?
19. How to Revise Acids, Bases and Salts Before an Exam
Use a four-step revision cycle instead of memorising the chapter as isolated facts.
1. Understand
Learn acids, bases, indicators and the meaning of pH.
2. Practise
Write and balance the important reactions without looking at the answer.
3. Memorise
Revise NaHCO3, Na2CO3·10H2O, CaOCl2 and CaSO4·½H2O.
4. Apply
Solve pH problems, indicator questions, reaction-based questions and case-based questions.
20. Trusted References for Class 10 Chemistry
For textbook and curriculum verification, students should use the current official sources alongside these revision notes.
- NCERT Science Textbook for Class X
- CBSE Academic — Curriculum 2026–27
- NCERT Science Laboratory Manual
The NCERT Class X Science textbook is the primary textbook reference, while the CBSE Academic curriculum page provides the current curriculum framework for the 2026–27 academic year. :contentReference[oaicite:3]{index=3}
21. Conclusion
Acids, Bases and Salts becomes much easier when the chapter is connected through a few central ideas: indicator → acidity/basicity → pH → reaction → salt → application.
For examination preparation, focus especially on indicator colour changes, the pH scale, strong versus weak acids, important reactions, neutralisation, baking soda, washing soda, bleaching powder, Plaster of Paris and water of crystallisation.
Do not rely only on memorisation. Write the equations, solve the pH examples and test yourself without looking at the answers. That approach helps convert the notes into actual examination performance.
