Premium Class 10 CBSE Science Study Notes
Acids, Bases and Salts
1. Chapter Overview
Many substances around us are either acidic, basic or neutral. Lemon juice, curd, vinegar and tamarind contain acids. Soap, baking soda and washing soda are basic in nature. When an acid reacts with a base, salt and water are formed.
This chapter explains the chemical nature of common substances, their reactions, uses in daily life, and importance in industries.
Why this chapter is important
- It explains why acids are sour and bases are bitter.
- It explains how acids and bases react with metals, oxides and carbonates.
- It helps students understand pH, tooth decay, digestion and soil treatment.
- It covers important salts like baking soda, washing soda, bleaching powder and Plaster of Paris.
| Concept | Real-life application |
|---|---|
| Acids | Lemon juice, vinegar, curd, digestion |
| Bases | Soap, toothpaste, antacids |
| pH | Soil testing, tooth decay, digestion |
| Neutralisation | Antacid tablets, treatment of acidic soil |
2. Important Definitions
Acid
An acid is a substance which produces hydrogen ions, H+, in aqueous solution.
Base
A base is a substance which produces hydroxide ions, OH−, in aqueous solution.
Alkali
A base which is soluble in water is called an alkali.
Important: All alkalis are bases, but all bases are not alkalis.
Salt
A salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion.
Neutralisation Reaction
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
3. Acids
Physical properties of acids
| Property | Explanation |
|---|---|
| Taste | Sour |
| Effect on litmus | Turns blue litmus red |
| pH | Less than 7 |
| Conductivity | Conduct electricity in aqueous solution |
Common acids
| Acid | Found in |
|---|---|
| Acetic acid | Vinegar |
| Citric acid | Lemon and orange |
| Lactic acid | Curd |
| Hydrochloric acid | Stomach |
Chemical properties of acids
1. Acid reacts with metal
Test for hydrogen gas: Hydrogen gas burns with a pop sound.
2. Acid reacts with metal carbonate
3. Acid reacts with metal hydrogen carbonate
4. Acid reacts with base
This is a neutralisation reaction.
5. Acid reacts with metal oxide
Metal oxides are basic in nature.
4. Bases
Physical properties of bases
| Property | Explanation |
|---|---|
| Taste | Bitter |
| Touch | Soapy or slippery |
| Effect on litmus | Turns red litmus blue |
| pH | More than 7 |
Chemical properties of bases
1. Base reacts with acid
2. Base reacts with non-metal oxide
This shows that carbon dioxide is acidic in nature.
3. Strong base reacts with some metals
5. Indicators and Olfactory Indicators
| Indicator | Acidic medium | Basic medium |
|---|---|---|
| Litmus | Blue litmus turns red | Red litmus turns blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
| Turmeric | Yellow | Reddish-brown |
Olfactory Indicator
An olfactory indicator is a substance whose smell changes in acidic or basic medium.
Examples: Onion, vanilla essence and clove oil.
6. pH Scale
pH is a scale used to express the acidic or basic strength of a solution.
| Substance | Approximate pH | Nature |
|---|---|---|
| Hydrochloric acid | 1 | Strongly acidic |
| Lemon juice | 2 | Acidic |
| Pure water | 7 | Neutral |
| Baking soda solution | 8 to 9 | Basic |
| Sodium hydroxide | 14 | Strongly basic |
7. Importance of pH in Daily Life
| Situation | Concept |
|---|---|
| Stomach acidity | Excess HCl is neutralised by antacids like milk of magnesia. |
| Tooth decay | Tooth decay starts when pH of mouth falls below 5.5. |
| Acidic soil | Quicklime or slaked lime is added. |
| Basic soil | Organic matter or compost is added. |
| Ant sting | Methanoic acid is neutralised using baking soda. |
8. Neutralisation Reaction in Real Life
| Example | Neutralisation used |
|---|---|
| Acidity in stomach | Antacid neutralises excess acid. |
| Ant sting | Baking soda neutralises methanoic acid. |
| Acidic soil | Lime neutralises soil acidity. |
| Factory waste | Waste is neutralised before discharge. |
9. Important Salts
| Salt | Chemical name | Formula | Important use |
|---|---|---|---|
| Common salt | Sodium chloride | NaCl | Food and raw material |
| Caustic soda | Sodium hydroxide | NaOH | Soap and paper industry |
| Bleaching powder | Calcium oxychloride | CaOCl2 | Disinfection and bleaching |
| Baking soda | Sodium hydrogen carbonate | NaHCO3 | Baking and antacid |
| Washing soda | Sodium carbonate decahydrate | Na2CO3·10H2O | Cleaning and softening hard water |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO4·½H2O | Fracture support and decorative items |
10. Caustic Soda
Chemical name: Sodium hydroxide
Formula: NaOH
Preparation
Sodium hydroxide is prepared by passing electricity through an aqueous solution of sodium chloride, called brine. This process is called the chlor-alkali process.
| Product | Formed at |
|---|---|
| Chlorine gas | Anode |
| Hydrogen gas | Cathode |
| Sodium hydroxide | Near cathode |
Uses
- Manufacture of soaps and detergents
- Manufacture of paper
- Degreasing metals
- Artificial fibres
11. Bleaching Powder
Chemical name: Calcium oxychloride
Formula: CaOCl2
Preparation
Bleaching powder is prepared by passing chlorine gas over dry slaked lime.
Uses
- Bleaching cotton and linen in textile industry
- Bleaching wood pulp in paper industry
- Disinfecting drinking water
- Used as an oxidising agent
12. Baking Soda
Chemical name: Sodium hydrogen carbonate
Formula: NaHCO3
Preparation
Action of heat
Uses
- Used in baking powder
- Used as an antacid
- Used in soda-acid fire extinguishers
- Makes cakes and bread soft and spongy
13. Washing Soda
Chemical name: Sodium carbonate decahydrate
Formula: Na2CO3·10H2O
Preparation
Uses
- Used as a cleaning agent
- Used for softening hard water
- Used in glass, soap and paper industries
14. Water of Crystallisation
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.
| Salt | Formula | Water molecules |
|---|---|---|
| Copper sulphate crystals | CuSO4·5H2O | 5 |
| Washing soda | Na2CO3·10H2O | 10 |
| Gypsum | CaSO4·2H2O | 2 |
15. Plaster of Paris
Chemical name: Calcium sulphate hemihydrate
Formula: CaSO4·½H2O
Preparation
Plaster of Paris is prepared by heating gypsum at 373 K.
Setting of Plaster of Paris
Uses
- Making support for fractured bones
- Making decorative items
- Making statues and toys
- Making smooth surfaces
16. Important Flowcharts
Flowchart 1: Classification of substances
Flowchart 2: Acid reactions
Acid → Carbonate → Salt + Water + CO2
Acid → Base → Salt + Water
Flowchart 3: Common salt and its products
17. NCERT-Based Important Points
- Acids produce H+ ions in aqueous solution.
- Bases produce OH− ions in aqueous solution.
- Hydrogen ions combine with water to form H3O+ ions.
- Dry HCl gas does not affect dry litmus paper.
- Hydrogen gas burns with a pop sound.
- Carbon dioxide turns lime water milky.
- Metal oxides are basic in nature.
- Non-metal oxides are acidic in nature.
- Tooth decay starts when mouth pH falls below 5.5.
- Bleaching powder is prepared by passing chlorine over dry slaked lime.
18. Common Mistakes by Students
| Mistake | Correct concept |
|---|---|
| Writing acid + metal gives oxygen | Acid + metal gives hydrogen gas |
| Writing CO2 turns lime water blue | CO2 turns lime water milky |
| Confusing baking soda and washing soda | Baking soda: NaHCO3; Washing soda: Na2CO3·10H2O |
| Writing POP formula as CaSO4·2H2O | POP: CaSO4·½H2O |
| Saying all bases are alkalis | Only water-soluble bases are alkalis |
19. Exam-Oriented Questions
A. Very Short Answer Questions
- What is an acid?
- What is a base?
- What is an alkali?
- Name the acid present in lemon juice.
- Name the acid present in curd.
- What is the chemical formula of baking soda?
- What is the chemical formula of washing soda?
- What is the chemical name of Plaster of Paris?
- What is the pH of a neutral solution?
- What gas is evolved when acids react with metals?
B. Short Answer Questions
- Why do acids not show acidic behaviour in the absence of water?
- Why does dry HCl gas not change the colour of dry litmus paper?
- Write the reaction between zinc and dilute hydrochloric acid.
- Write the test for hydrogen gas.
- Write the test for carbon dioxide gas.
- Why are metal oxides called basic oxides?
- What is neutralisation? Give one example.
- Why is baking soda used in baking?
C. Long Answer Questions
- Explain the chemical properties of acids with suitable reactions.
- Explain the chemical properties of bases with suitable reactions.
- Describe the chlor-alkali process. Write the products formed and their uses.
- Explain the preparation, properties and uses of baking soda.
- Explain the importance of pH in everyday life.
D. Assertion-Reason Questions
Options: A. Both true and Reason explains Assertion. B. Both true but Reason does not explain Assertion. C. Assertion true, Reason false. D. Assertion false, Reason true.
- Assertion: Acids conduct electricity in aqueous solution.
Reason: Acids produce ions in water. - Assertion: Dry HCl gas does not turn dry blue litmus red.
Reason: HCl needs water to produce hydrogen ions. - Assertion: Tooth decay starts when pH of mouth falls below 5.5.
Reason: Acids produced by bacteria damage tooth enamel.
E. Case-Study Based Question
A student eats chocolates frequently and does not brush properly. Bacteria present in the mouth break down sugar and produce acids. These acids lower the pH of the mouth. When pH falls below 5.5, tooth enamel starts getting damaged. Toothpaste helps prevent tooth decay because it is basic in nature.
- What happens to the pH of the mouth after eating sugary food?
- Why does tooth decay start below pH 5.5?
- Why is toothpaste basic in nature?
- Suggest two ways to prevent tooth decay.
F. Diagram-Based Questions
- Draw a labelled diagram to show the reaction of zinc granules with dilute hydrochloric acid.
- Draw a labelled diagram to show the test for carbon dioxide using lime water.
- Draw a pH scale and mark acidic, neutral and basic regions.
- Draw a flowchart showing preparation of useful chemicals from common salt.
20. Quick Revision Sheet
Acids
Sour taste Blue litmus → Red pH less than 7 Produces H+ ionsBases
Bitter taste Soapy touch Red litmus → Blue pH more than 7 Produces OH− ionsImportant formulas
| Substance | Formula |
|---|---|
| Hydrochloric acid | HCl |
| Sodium hydroxide | NaOH |
| Common salt | NaCl |
| Bleaching powder | CaOCl2 |
| Baking soda | NaHCO3 |
| Washing soda | Na2CO3·10H2O |
| Gypsum | CaSO4·2H2O |
| Plaster of Paris | CaSO4·½H2O |
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