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Chemical Properties of Acids and Bases
Introduction to Acids and Bases
- Sour Taste: Due to acids.
- Bitter Taste: Due to bases.
- Remedy for Acidity: Baking soda solution (neutralizes acid).
- Properties: Acids and bases can cancel each other out.
Identifying Acids and Bases
- Acids: Sour, turn blue litmus red.
- Bases: Bitter, turn red litmus blue.
- Natural Indicators:
- Litmus: Purple dye from lichen, turns red in acid, blue in base.
- Turmeric: Turns reddish-brown with base, yellow with water.
- Other Natural Indicators: Red cabbage, hydrangea, petunia, geranium petals.
Chemical Properties of Acids and Bases
Laboratory Activity with Indicators
Activity : (Click here)
- Collect Solutions:
- Acids: HCl, H2SO4, HNO3, CH3COOH.
- Bases: NaOH, Ca(OH)2, KOH, Mg(OH)2, NH4OH.
- Test with Indicators:
- Red litmus, blue litmus, phenolphthalein, methyl orange.
- Observe color changes.
- Record observations in a table.
Olfactory Indicators
Activity with Onion:
- Activity with Onion:
- Soak cloth strips with onion overnight.
- Test with HCl and NaOH.
- Observe odour changes.
- Other Olfactory Indicators:
- Vanilla essence and clove oil:
- Test with HCl and NaOH.
- Observe and record odour changes.
- Vanilla essence and clove oil:
Summary
- Natural and Synthetic Indicators: Used to identify acids and bases.
- Olfactory Indicators: Change odour in the presence of acids or bases.
- Important Concepts: Neutralization, identification using indicators.
How Do Acids and Bases React with Metals?
Activity 2.3: Reaction of Acids with Metals
- Setup: With teacher’s help, set up the apparatus as shown in Fig. 2.1.
- Procedure:
- Add 5 mL of dilute sulphuric acid to a test tube.
- Add a few pieces of zinc granules.
- Observe bubbles on zinc surface (hydrogen gas).
- Pass the gas through soap solution to form bubbles.
- Bring a burning candle near a bubble to see it pop (hydrogen gas burns).
- Repeat with HCl, HNO3, and CH3COOH.
- Observe if reactions are similar.
- Reaction Summary:
- General Reaction: Acid + Metal → Salt + Hydrogen gas
- Example Equation: HCl+Zn→ZnCl2+H2HCl+Zn→ZnCl2+H2
Activity 2.4: Reaction of Bases with Metals
- Procedure:
- Place zinc pieces in a test tube.
- Add 2 mL of sodium hydroxide solution and warm it.
- Follow steps from Activity 2.3 and record observations.
- Reaction Summary:
- Example Equation: 2NaOH+Zn→Na2ZnO2+H22NaOH+Zn→Na2ZnO2+H2
- Hydrogen gas is formed.
How do Metal Carbonates and Metal Hydrogen Carbonates React with Acids?
Activity 2.5: Reaction with Metal Carbonates and Bicarbonates
- Procedure:
- Label two test tubes as A and B.
- Add 0.5 g of sodium carbonate (Na2CO3) in A and 0.5 g of sodium bicarbonate (NaHCO3) in B.
- Add 2 mL of dilute HCl to each.
- Observe and pass the gas through lime water.
- Reactions:
- Test Tube A: Na2CO3+HCl→2NaCl+H2O+CO2Na2CO3+HCl→2NaCl+H2O+CO2
- Test Tube B: NaHCO3+HCl→NaCl+H2O+CO2NaHCO3+HCl→NaCl+H2O+CO2
- Observation: Lime water turns milky (CO2 forms white precipitate of CaCO3).
How do Acids and Bases React with Each Other?
Activity 2.6: Neutralization Reaction
- Procedure:
- Add 2 mL of NaOH to a test tube.
- Add two drops of phenolphthalein (solution turns pink).
- Add HCl drop by drop (solution becomes colorless).
- Add NaOH again (pink color returns).
- Reaction Summary:
- Equation: NaOH+HCl→NaCl+H2ONaOH+HCl→NaCl+H2O
- Acid and base neutralize each other to form salt and water.
Reaction of Metallic Oxides with Acids
Activity 2.7: Reaction of Copper Oxide with Acid
- Procedure:
- Add copper oxide to a beaker.
- Slowly add dilute hydrochloric acid while stirring.
- Observe the blue-green color (formation of copper(II) chloride).
- Reaction Summary:
- Equation: CuO+HCl→CuCl2+H2OCuO+HCl→CuCl2+H2O
- Metal oxides react with acids to form salts and water (basic oxides).
Reaction of Non-metallic Oxides with Bases
- Example:
- Reaction: Calcium hydroxide (base) reacts with carbon dioxide (non-metallic oxide) to form calcium carbonate and water.
- Conclusion:
- Non-metallic oxides are acidic because they react with bases to form salts and water.
- Equation: Ca(OH)2+CO2→CaCO3+H2OCa(OH)2+CO2→CaCO3+H2O
What Do All Acids and Bases Have in Common?
Common Properties of Acids
- All acids produce hydrogen gas when reacting with metals.
- Hydrogen is common in all acids.
Activity 2.8: Investigating Acids and Hydrogen
- Setup:
- Take solutions of glucose, alcohol, HCl, and H2SO4.
- Fix two nails on a cork in a beaker, connect to a 6V battery, bulb, and switch.
- Pour dilute HCl and switch on the current.
- Repeat with dilute H2SO4.
- Repeat with glucose and alcohol solutions.
- Observations:
- Bulb glows with acids (HCl, H2SO4) indicating electricity conduction.
- Glucose and alcohol do not conduct electricity.
- Acids contain H+ ions, which conduct electricity.
- Conclusion: Acids produce hydrogen ions (H+) in solution, responsible for acidic properties.
Activity with Alkalis
- Repeat Activity 2.8 using sodium hydroxide, calcium hydroxide, etc.
- Observation: Bases also conduct electricity, indicating they produce ions.
What Happens to an Acid or Base in Water Solution?
Activity 2.9: Testing HCl in Different Conditions
- Setup:
- Take 1g solid NaCl in a test tube.
- Add concentrated sulphuric acid.
- Test gas with dry and wet blue litmus paper.
- Observations:
- Litmus paper changes color only when wet.
- Conclusion: HCl gas produces H+ ions only in the presence of water.
- Reaction: HCl+H2O→H3O++Cl−HCl+H2O→H3O++Cl−
- Hydronium Ion Formation: H++H2O→H3O+H++H2O→H3O+
Bases in Water
- Dissolution of Bases:
- NaOH: NaOH→Na++OH−NaOH→Na++OH−
- KOH: KOH→K++OH−KOH→K++OH−
- Mg(OH)_2: Mg(OH)2→Mg2++2OH−Mg(OH)2→Mg2++2OH−
- Alkalis: Bases that dissolve in water.
Neutralization Reaction
- Reaction: Acid + Base → Salt + Water
- Ions Involved: H++OH−→H2OH++OH−→H2O
Dilution of Acids and Bases
Activity 2.10: Mixing Acids/Bases with Water
- Procedure:
- Add a few drops of concentrated H2SO4 to 10 mL water.
- Touch the base of the beaker to feel temperature change.
- Repeat with sodium hydroxide pellets.
- Observations:
- Temperature increases (exothermic process).
- Always add acid to water, not water to acid, to prevent splashing and burns.
- Conclusion: Dilution decreases the concentration of ions (H3O+/OH–), making the solution less concentrated.
How Strong Are Acid or Base Solutions?
Understanding Acidity and Alkalinity
- Acid-Base Indicators: Help distinguish between acids and bases.
- Universal Indicator: Mixture of indicators showing different colors at different hydrogen ion concentrations.
- pH Scale: Measures hydrogen ion concentration.
- Range: 0 (very acidic) to 14 (very alkaline).
- Neutral pH: 7.
- Acidic Solutions: pH < 7.
- Alkaline Solutions: pH > 7.
- Stronger Acids: Lower pH.
- Stronger Bases: Higher pH.
Strong vs. Weak Acids and Bases
- Strong Acids: Produce more H+ ions (e.g., HCl).
- Weak Acids: Produce fewer H+ ions (e.g., acetic acid).
- Strong Bases: Produce more OH- ions.
- Weak Bases: Produce fewer OH- ions.
Importance of pH in Everyday Life
pH and Living Organisms
- Body pH Range: 7.0 to 7.8.
- Acid Rain: pH < 5.6, harmful to aquatic life.
pH and Soil for Plants
Activity 2.12: Check soil pH to determine the best conditions for plant growth.
Steps:
- Add 5 mL water to 2 g soil in a test tube.
- Shake and filter the mixture.
- Test the filtrate’s pH with universal indicator paper.
pH in the Digestive System
- Stomach Acid: Hydrochloric acid helps digestion.
- Indigestion: Excess acid causes pain; antacids neutralize it (e.g., magnesium hydroxide).
pH and Tooth Decay
- Tooth Decay: Starts when pH < 5.5.
- Bacterial Acid: Produced by bacteria from food residues.
- Prevention: Clean mouth after eating and use basic toothpaste to neutralize acid.
Self-Defense in Nature
- Bee Stings: Inject acid causing pain; treated with mild base like baking soda.
- Nettle Plant: Stinging hairs inject methanoic acid; traditional remedy is dock plant leaves, which neutralize the acid.
Practical Tips
- Mixing Acids and Bases with Water: Always add acid to water to prevent splashing and burns. Mixing decreases ion concentration (dilution).
| Natural Source | Acid | Natural Source | Acid |
|---|---|---|---|
| Vinegar | Acetic acid | Tamarind | Tartaric acid |
| Sour milk (Curd) | Lactic acid | Ant sting | Methanoic acid |
| Orange | Citric acid | Tomato | Oxalic acid |
| Lemon | Citric acid | Nettle sting | Methanoic acid |
More About Salts
Family of Salts
Activity 2.13: Understanding Salt Families
- Chemical Formulae of Salts:
- Potassium sulphate: K₂SO₄
- Sodium sulphate: Na₂SO₄
- Calcium sulphate: CaSO₄
- Magnesium sulphate: MgSO₄
- Copper sulphate: CuSO₄
- Sodium chloride: NaCl
- Sodium nitrate: NaNO₃
- Sodium carbonate: Na₂CO₃
- Ammonium chloride: NH₄Cl
- Identifying Acids and Bases:
- Acids and bases from which these salts are obtained.
- Salt Families:
- Salts with the same positive or negative radicals belong to the same family.
- Example: NaCl and Na₂SO₄ (Sodium salts), NaCl and KCl (Chloride salts).
pH of Salts
Activity 2.14: Testing Salt pH
- Salt Samples:
- Sodium chloride, potassium nitrate, aluminium chloride, zinc sulphate, copper sulphate, sodium acetate, sodium carbonate, sodium hydrogencarbonate.
- Steps:
- Check solubility in distilled water.
- Test with litmus paper.
- Find pH using pH paper.
- Observations:
- Determine if salts are acidic, basic, or neutral.
- Identify the acid or base used to form the salt.
General Rules:
- Salts of strong acid and strong base: Neutral (pH = 7).
- Salts of strong acid and weak base: Acidic (pH < 7).
- Salts of strong base and weak acid: Basic (pH > 7).
Chemicals from Common Salt
Sodium Chloride (Common Salt)
- Source:
- Found in seawater and rock salt deposits.
- Rock salt is mined and purified.
- Historical Significance:
- Mahatma Gandhi’s Dandi March highlighted the importance of salt in India’s freedom struggle.
Common Salt as a Raw Material
- Uses:
- Sodium hydroxide, baking soda, washing soda, bleaching powder, etc.
Sodium Hydroxide Production
- Chlor-alkali Process:
- Electricity is passed through an aqueous solution of sodium chloride (brine).
- Reaction: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
- Products: Sodium hydroxide, chlorine gas, and hydrogen gas.
- Uses of Products:
- Sodium hydroxide: Soap making, paper manufacturing.
- Chlorine: Disinfecting water, making PVC.
- Hydrogen: Fuel, making margarine.
Bleaching Powder
- Production:
- Chlorine gas (from electrolysis of brine) reacts with dry slaked lime (Ca(OH)₂).
- Formula: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
- Uses:
- Bleaching cotton and linen in the textile industry.
- Bleaching wood pulp in paper factories.
- Disinfecting washed clothes in laundry.
- As an oxidizing agent in chemical industries.
- Making drinking water germ-free.
Baking Soda
- Chemical Name:
- Sodium hydrogencarbonate (NaHCO₃).
- Production:
- NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
- Properties:
- Mild, non-corrosive basic salt.
- Can neutralize acids.
- Uses:
- Making baking powder (NaHCO₃ + mild edible acid).
- When heated, it produces CO₂, making bread/cakes rise.
- Ingredient in antacids (neutralizes stomach acid).
- Used in soda-acid fire extinguishers.
- Making baking powder (NaHCO₃ + mild edible acid).
Washing Soda
- Chemical Name:
- Sodium carbonate decahydrate (Na₂CO₃.10H₂O).
- Production:
- Recrystallization of sodium carbonate after heating baking soda.
- Properties:
- Basic salt.
- Uses:
- Glass, soap, and paper industries.
- Manufacture of sodium compounds (e.g., borax).
- Domestic cleaning agent.
- Removing permanent hardness of water.
Are the Crystals of Salts Really Dry?
Activity 2.15: Heating Copper Sulphate Crystals
- Steps:
- Heat copper sulphate crystals in a dry boiling tube.
- Observe color change and water droplets formation.
- Add water to heated crystals and observe color restoration.
- Explanation:
- Copper sulphate crystals contain water of crystallization.
- Heating removes water, turning crystals white.
- Adding water restores blue color.
- Concept:
- Water of crystallization: Fixed number of water molecules in a salt’s formula unit.
- Example: CuSO₄.5H₂O (copper sulphate with five water molecules).
Concept:
- Water of crystallization: Fixed number of water molecules in a salt’s formula unit.
- Example: CuSO₄.5H₂O (copper sulphate with five water molecules).
Plaster of Paris
- Production:
- Heating gypsum (CaSO₄.2H₂O) at 373K.
- Forms calcium sulphate hemihydrate (CaSO₄.1/2H₂O).
- Properties:
- White powder.
- Mixes with water to form hard gypsum again.
- Uses:
- Supporting fractured bones.
- Making toys, decorative materials.
- Smoothing surfaces.
Chapter Summary:
- Acid-base indicators are dyes or mixtures of dyes that show the presence of acids and bases.
- A substance is acidic due to the formation of H⁺(aq) ions in solution.
- The formation of OH⁻(aq) ions in solution makes a substance basic.
- When an acid reacts with a metal, hydrogen gas and a corresponding salt are formed.
- When a base reacts with a metal, hydrogen gas and a salt with a negative ion composed of the metal and oxygen are formed.
- An acid reacting with a metal carbonate or metal hydrogencarbonate produces a corresponding salt, carbon dioxide gas, and water.
- Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions, respectively.
- The strength of an acid or an alkali is measured using the pH scale (0-14), which indicates hydrogen ion concentration in a solution.
- A neutral solution has a pH of exactly 7, an acidic solution has a pH less than 7, and a basic solution has a pH more than 7.
- Living beings carry out their metabolic activities within an optimal pH range.
- Mixing concentrated acids or bases with water is a highly exothermic process.
- Acids and bases neutralize each other to form corresponding salts and water.
- Water of crystallization is the fixed number of water molecules present in one formula unit of a salt.
- Salts have various uses in everyday life and industries.
