Notes on Acids, Bases and Salts

Acids, Bases and Salts

Introduction to Acids and Bases

  • Sour and Bitter Tastes:

    • Foods may have sour or bitter tastes, attributed to the presence of acids and bases, respectively.

  • Common Remedial Solutions for Acidity:

    • Options: lemon juice, vinegar, or baking soda solution.

    • The choice of remedy hinges on an understanding of the neutralizing ability of acids and bases.

  • Indicators for Testing Acids and Bases:

    • Litmus:

    • Natural indicator; turns blue litmus red in the presence of acids and red litmus blue in the presence of bases.

    • Turmeric:

    • Changes from yellow to reddish-brown when it comes in contact with soap, a basic solution.

  • Synthetic Indicators:

    • Methyl orange and phenolphthalein can also be used to detect acids and bases.

Understanding Chemical Properties of Acids and Bases

Acids and Bases in the Laboratory

  • Indicators: Change in Color:

    • Indicators are substances that change color in acidic or basic media.

    • Some indicators are olfactory—their smell can change based on the pH level.

Activity 2.1

  • Testing Acidity/Bases Using Indicators:

    • Obtain three test tubes: one with distilled water, one with an acidic solution, one with a basic solution.

    • With red litmus paper only, identify the contents of each test tube.

  • Required Solutions:

    • Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), acetic acid (CH3COOH), sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)2], ammonium hydroxide (NH4OH).

    • Test each solution with given indicators and note color changes.

Activity 2.2

  • Olfactory Indicators:

    • Prepare cloth strips with finely chopped onions for testing odors in acidic and basic media.

    • Note odor changes when dilute HCl and NaOH are added to the strips.

    • Reaction of Acids and Bases with Metals

Activity 2.3

  • Metal Reaction with Acid:

    • Set up apparatus with dilute sulphuric acid and zinc granules.

    • Observe hydrogen gas evolution.

    • Pass gas through soap solution to see bubbles, indicating hydrogen.

    • Conduct a flame test for gas; observe reactions with different acids.

  • General Reaction Summary:

    • Acid + Metal → Salt + Hydrogen gas

Activity 2.4

  • Specific Reactions:

    • Test tube A:

    • Na2CO3 + HCl → NaCl + H2O + CO2 (gas evolves)

    • Test tube B:

- NaHCO3 + HCl → NaCl + H2O + CO2 (gas evolves)

  • Notes on Carbon Dioxide:

    • Gas produces a milky white precipitate in limewater (Ca(OH)₂).

Reactivity of Metal Carbonates and Metal Hydrogencarbonates with Acids
Activity 2.5

  • Take Na₂CO₃ and NaHCO₃ in separate test tubes; add dilute HCl.

  • Note gas production; pass through limewater.

Activity 2.6

  • Neutralization Tests:

    • Use phenolphthalein; observe color changes during NaOH and HCl neutralization.

Reaction of Metallic Oxides with Acids
Activity 2.7

  • Observe the reaction between copper oxide and dilute HCl.

    • Formation of blue-green copper(II) chloride noted.

Non-Metallic Oxide Reactions with Bases

  • Carbon dioxide reacts with calcium hydroxide (lime water) similarly to acid-base reactions.

Common Characteristics of Acids and Bases

2.2 Similarities Among Acids and Bases

  • All acids share common properties; primarily they generate H+ ions in solution, which gives them acidic properties.

Activity 2.8

  • Electric Conductivity of Solutions:

    • Test various substances: glucose, alcohol, HCl, H2SO4.

    • Only acids will cause a bulb connected in series to glow, indicating conductivity due to the presence of ions.

Behavior in Water

  • Discuss observations of adding solid NaCl to concentrated H2SO4.

    • Gas evolution and its test with litmus paper.

Ionization in Solutions

  • Reaction of acids and bases dissolved in water:

  • Example:

  • HCl + H2O → H3O+ + Cl−

  • Bases dissociating to give hydroxide ions.

Strength of Acid and Base Solutions

pH Scale

  • Universal indicator concepts to quantify acidity/basicity.

  • pH measured from 0 (very acidic) to 14 (very alkaline).

  • Neutralization:

  • H+(aq) + OH−(aq) → H2O(l)

Assessing pH in Everyday Life

  • Influence of pH on plant growth, safety of aquatic life.

  • pH in digestive health; the role of antacids for excess acidity.

Salts

More about Salts

  • Preparation and Classification:

    • Salts: Formed during reactions between acids and bases.

  • Family of Salts:

    • Salts can belong to families based on cation/anion composition.
      e.g. Sodium salts like NaCl, Na2SO4.

pH of Salts

  • Salts of strong/weak acids and bases have varying pH (neutral, acidic, basic) effects.

Chemical Production From Common Salt

  • Uses of sodium chloride across industries.

The Chlor-alkali Process:

  • 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)

  • Bleaching Powder and its Uses:

  • Produced via chlorine on slaked lime; used generally for disinfection, bleaching, and chemical industry.

Water of Crystallization

  • Crystallized salts contain water which influences physical properties.

  • Plaster of Paris: Chemistry and applications in medicine; one example being (1/2)CaSO4.2H2O.

Summary of Learning

  • Acid-base reactions, properties of acids/bases, implications of pH in biological systems, safety measures involving acids and bases.

Exercises

1. A solution turns red litmus blue; its pH is likely to be

(a) 1

(b) 4

(c) 5

(d) 10 ✔

2. A solution reacts with crushed eggshells to give a gas that turns limewater milky. The solution contains

(a) NaCl

(b) HCl ✔

(c) LiCl

(d) KCl

3. 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be

(a) 4 mL

(b) 8 mL

(c) 12 mL

(d) 16 mL ✔

4. Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(c) Antacid ✔

(d) Antiseptic

5. Write word equations and then balanced equations for the reaction taking place when –

(a) Dilute sulfuric acid reacts with zinc granules.

H₂SO₄+Zn —> ZnSO₄+H₂


(b) Dilute hydrochloric acid reacts with magnesium ribbon.

Balanced Equation:

(c) Dilute sulfuric acid reacts with aluminum powder.

(d) dilute hydrochloric acid reacts with iron filings.

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

7. Why does distilled water not conduct electricity, whereas rainwater does?

8. Why does dry HCL gas not show acidic behaviour in the absence of water?

9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4,1,11,7 and 9, respectively. Which solution is

(a) neutral?

(b) strongly alkaline?

(c) strongly acidic?

(d) weakly acidic?

(e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration.

10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

12. A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?

13. Plaster of Paris should be stored in a moisture-proof container. Explain why? 14. What is a neutralisation reaction? Give two examples.

15. Give two important uses of washing soda and baking soda.