Science Chapter 2: Acids, Bases and Salts

Fundamental Characteristics of Acids and Bases

Acids and bases are defined by distinct physical and chemical properties. These substances are pervasive in both natural environments and industrial applications.

• Acids

  • Taste: Acids are characteristically sour in taste.

  • Indicator Change: They change the color of blue litmus paper to red.

  • Examples:

    • Hydrochloric Acid: $HCl$

    • Sulphuric Acid: $H_2SO_4$

    • Nitric Acid: $HNO_3$

    • Acetic Acid: $CH_3COOH$

• Bases

  • Taste: Bases are characteristically bitter in taste.

  • Indicator Change: They change the color of red litmus paper to blue.

  • Examples:

    • Sodium hydroxide: $NaOH$

    • Potassium hydroxide: $KOH$

    • Calcium hydroxide: $Ca(OH)_2$

    • Ammonium hydroxide: $NH_4OH$

Naturally Occurring Acids

Many organic substances contain acids that occur naturally. The following is a list of common sources and their corresponding acids:

• Vinegar: Acetic Acid

• Orange: Citric Acid

• Lemon: Citric Acid

• Tamarind: Tartaric Acid

• Tomato: Oxalic Acid

• Sour milk (Curd): Lactic Acid

• Ant and Nettle sting: Methanoic Acid

Acid-Base Indicators

Indicators are substances that signal the presence of an acid or a base in a solution through changes in color or odor.

• Natural Indicators:

  • Litmus solution: This is a natural indicator extracted from Lichens. It appears as a purple dye in its neutral state.

  • Other Examples: Red Cabbage, turmeric, and the colored petals of certain flowers like Petunia.

• Olfactory Indicators:

  • These indicators undergo a change in odor when placed in acidic or basic media.

  • Examples: Onion and clove.

• Indicator Color Change Reference Table:

  • Blue litmus solution: Turns red with acid; no change with base.

  • Red litmus solution: No change with acid; turns blue with base.

  • Turmeric: No change with acid; turns red with base.

  • Methyl orange: Turns red with acid; turns yellow with base.

  • Phenolphthalein: Remains colourless with acid; turns pink with base.

Concentration and Dilution of Acids

• Dilute Acid: This solution contains a small amount of acid relative to a large volume of water.

• Concentrated Acid: This solution contains a high concentration of acid with a very small amount of water.

Chemical Properties of Acids and Bases

Reaction with Metals

Both acids and bases react with certain metals to produce a salt and hydrogen gas.

• General Formula: $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}$

• Specific Reactions:

  • $2HCl + Zn \rightarrow ZnCl_2 + H_2$

  • $2HNO_3 + Zn \rightarrow Zn(NO_3)_2 + H_2$

  • $H_2SO_4 + Zn \rightarrow ZnSO_4 + H_2$

  • $2CH_3COOH + Zn \rightarrow (CH_3COO)_2Zn + H_2$

• Base Reaction: $\text{Base} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}$

  • Example: $2NaOH + Zn \rightarrow Na_2ZnO_2 + H_2$ (Producing Sodium Zincate).

  • Note: These reactions between bases and metals are not possible with all metals.

• The Pop Test: This is a diagnostic test for the presence of hydrogen gas. When a burning candle is brought near the mouth of a test tube containing hydrogen, the gas burns with a characteristic "pop" sound.

Reaction with Metal Carbonates and Bicarbonates

Acids react with metal carbonates and metal hydrogencarbonates (bicarbonates) to yield salt, carbon dioxide ($CO_2$), and water.

• Metal Carbonate + Acid:

  • $Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$

• Metal Bicarbonate + Acid:

  • $NaHCO_3 + HCl \rightarrow NaCl + CO_2 + H_2O$

• Lime Water Test for $CO_2$:

  • When the evolved $CO_2$ is passed through lime water ($Ca(OH)_2$), the solution turns milky due to the formation of a white precipitate of calcium carbonate:

  • $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$

  • Excess $CO_2$ Reaction: If excess carbon dioxide is passed through the solution, the white precipitate dissolves to form calcium hydrogencarbonate, which is soluble in water:

  • $CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)$

Neutralization Reactions

A neutralization reaction occurs when an acid and a base react with each other to nullify their respective effects, resulting in the formation of salt and water.

• General Formula: $\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$

• Example: $NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$

Reactions with Metallic and Non-Metallic Oxides

• Metal Oxides with Acids: Metal oxides react with acids to produce salt and water. Because they mimic the behavior of bases in neutralization, metallic oxides are classified as Basic Oxides.

  • $CuO + 2HCl \rightarrow CuCl_2 + H_2O$

  • Observation: The solution turns blue-green due to the formation of Copper(II) chloride ($CuCl_2$).

• Non-Metallic Oxides with Bases: Non-metallic oxides react with bases to produce salt and water. Consequently, they are classified as Acidic Oxides.

  • $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$

Acids and Bases in Aqueous Solutions

All acidic solutions conduct electricity because they produce ions in water. The presence of these moving ions allows electric current to flow through the solution.

• Acids in Water: Acids generate hydrogen ions ($H^+$) in the presence of water. $H^+$ ions cannot exist alone; they associate with water molecules to form hydronium ions ($H_3O^+$).

  • $HCl + H_2O \rightarrow H_3O^+ + Cl^-$

  • $H^+ + H_2O \rightarrow H_3O^+$

• Bases in Water: Bases dissociate to provide hydroxide ions ($OH^-$) in water.

  • $NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$

  • $KOH(s) \xrightarrow{H_2O} K^+(aq) + OH^-(aq)$

  • $Mg(OH)_2(s) \xrightarrow{H_2O} Mg^{2+}(aq) + 2OH^-(aq)$

• Alkalis: An alkali is a specific type of base that is soluble in water.

  • Examples: $NaOH$, $KOH$, $Ca(OH)_2$, and $NH_4OH$.

  • Key Principle: All alkalis are bases, but all bases are not alkalis.

Safety and Dilution

Mixing an acid or base with water is a highly exothermic reaction. Due to the significant release of heat, the following precautions must be observed:

• The acid must always be added to the water slowly.

• The mixture must be stirred constantly.

• Reasoning: Adding water to a concentrated acid can cause the mixture to splash out and cause burns, or it may even break the glass container.

• Effect of Dilution: Diluting an acid or base decreases the concentration of ($H_3O^+$) or ($OH^-$) ions per unit volume.

The pH Scale: Measuring Strength

The strength of an acid or base is determined by the number of $H^+$ or $OH^-$ ions it produces. This is measured using a Universal Indicator or the pH Scale.

• pH Definition: "pH" comes from the German word Potenz, meaning power. It measures the hydrogen ion concentration.

• The Scale: It ranges from $0$ (very acidic) to $14$ (very alkaline).

  • Neutral: $pH = 7$ (e.g., pure water).

  • Acidic: pH < 7

  • Basic: pH > 7

• pH Values and Colors:

  1. $pH = 0$: Dark red; Highly acidic; Very high $H^+$ conc.

  2. $pH = 4$: Orange or yellow; Acidic; High $H^+$ conc.

  3. $pH = 7$: Green; Neutral; Equal $H^+$ and $OH^-$ conc.

  4. $pH = 10$: Bluish green or blue; Alkaline; Low $H^+$ conc.

  5. $pH = 14$: Dark blue or violet; Highly basic; Very low $H^+$ conc.

• Categorization of Strength:

  • Strong Acids: Ionize completely to give more $H^+$ ions (e.g., $HCl$, $H_2SO_4$, $HNO_3$).

  • Weak Acids: Give rise to fewer $H^+$ ions (e.g., $CH_3COOH$, $H_2CO_3$).

  • Strong Bases: Give rise to more $OH^-$ ions (e.g., $NaOH$, $KOH$, $Ca(OH)_2$).

  • Weak Bases: Give rise to fewer $OH^-$ ions (e.g., $NH_4OH$).

Importance of pH in Daily Life

• Digestive System: The stomach produces hydrochloric acid to help digest food. Indigestion leads to excess acid production, causing pain and irritation. Antacids (mild bases) are used to neutralize this excess acid.

• Acid Rain: When the pH of rainwater falls below $5.6$, it is classified as acid rain. This lowers the pH of river water, threatening aquatic life survival.

• Soil pH: Plants require a specific pH range for healthy growth. Farmers add fertilizers to adjust the soil pH if it becomes too acidic or basic.

• Human Body: Our body functions within a narrow pH range of $7.0$ to $7.8$.

• Tooth Decay: Bacteria in the mouth degrade sugar and food particles into acids. If the mouth's pH drops, it causes tooth decay. Basic toothpastes neutralize this acid to prevent damage.

• Stings: Bee and nettle stings contain methanoic acid. Applying a weak base, such as baking soda, provides relief by neutralizing the acid.

Classification and Derivation of Salts

Salts are formed by the reaction of acids and bases. They can be categorized into families based on their radicals (e.g., Sodium salts, Chloride salts).

• Salt Derivation Examples:

  • Potassium Sulphate ($K_2SO_4$): Derived from $KOH$ and $H_2SO_4$.

  • Sodium Sulphate ($Na_2SO_4$): Derived from $NaOH$ and $H_2SO_4$.

  • Sodium Chloride ($NaCl$): Derived from $NaOH$ and $HCl$.

  • Ammonium Chloride ($NH_4Cl$): Derived from $NH_4OH$ and $HCl$.

• pH of Salts:

  • Neutral Salts: Strong Acid + Strong Base ($pH = 7$). Examples: $NaCl$, $CaSO_4$.

  • Acidic Salts: Strong Acid + Weak Base (pH < 7). Examples: $NH_4Cl$, $NH_4NO_3$.

  • Basic Salts: Strong Base + Weak Acid (pH > 7). Examples: $CaCO_3$, $CH_3COONa$.

Chemicals Derived from Common Salt (Sodium Chloride)

Sodium Chloride ($NaCl$) is obtained from seawater and as brown crystals called "Rock Salt" (mined like coal). It serves as a raw material for various industrial chemicals.

1. Sodium Hydroxide ($NaOH$)

• Preparation: Prepared via the Chlor-alkali process.

• Reaction: $2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$

• Note: It is named "Chlor-alkali" because it produces chlorine and an alkali ($NaOH$).

2. Bleaching Powder ($CaOCl_2$)

• Preparation: Produced by the action of chlorine on dry slaked lime.

• Reaction: $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$

• Uses: Textile industry, factories, laundry, and as a disinfectant for water.

3. Baking Soda ($NaHCO_3$)

• Chemical Name: Sodium Hydrogen Carbonate.

• Preparation: $NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$

• Effect of Heating: When heated during cooking, it produces sodium carbonate, water, and carbon dioxide.

  • $2NaHCO_3 \xrightarrow{\text{Heat}} Na_2CO_3 + H_2O + CO_2$

• Uses: Antacid ingredient; making baking powder. The $CO_2$ released makes cakes and pastries spongy.

• Baking Powder Reaction: $NaHCO_3 + H^+ \rightarrow CO_2 + H_2O + \text{Sodium Salt of acid}$

4. Washing Soda ($Na_2CO_3 \cdot 10H_2O$)

• Preparation: Recrystallization of sodium carbonate.

• Reaction: $Na_2CO_3 + 10H_2O \xrightarrow{\text{Heat}} Na_2CO_3 \cdot 10H_2O$

• Uses: Glass, soap, and paper industries; domestic cleaning agent; removal of permanent water hardness; manufacture of borax.

Water of Crystallization

Water of crystallization refers to the fixed number of water molecules present in one formula unit of a salt.

• Copper Sulphate: Hydrated crystals formula is $CuSO_4 \cdot 5H_2O$. When heated, it loses water droplets.

• Gypsum: Formula is $CaSO_4 \cdot 2H_2O$.

• Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$):

  • Formed by heating Gypsum at $373\,K$.

  • Reaction with Water: Mixing Plaster of Paris with water converts it back into hard solid Gypsum.

  • $CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$

  • Uses: Supporting fractured bones (plaster); making toys; decorative materials; creating smooth surfaces.

Exercise / Question Bank

Very Short Answer Questions (1 Mark)

1. Two solutions have pH values of $4$ and $9$; which has the higher $H^+$ concentration?

2. Why should curd and sour substances not be stored in brass or copper vessels?

3. State the chemical name of bleaching powder.

4. Write the molecular formula for one strong acid and one weak acid.

5. Why must Plaster of Paris be stored in moisture-proof containers?

6. Identify the gas evolved when dilute sulphuric acid reacts with sodium carbonate.

7. What is the role of common salt in the soap industry?

8. Describe the observation when a burning candle meets hydrogen gas.

9. Identify the indicator used to measure pH values across the entire range.

10. Write the chemical formula for washing powder.

Short Answer Questions (2-3 Marks)

1. List two physical properties of an acid.

2. Complete the reaction: $CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow \text{ ?}$

3. Why does the color of a solution containing $NaOH$ and phenolphthalein change when $HCl$ is added?

4. Define why metallic oxides are basic and non-metallic oxides are acidic.

5. Explain the result of placing a bulb circuit in an $HCl$ solution versus an $NaOH$ solution.

6. Identify the reaction type: $HX + MOH \rightarrow MX + HOH$.

7. Explain why all alkalis are bases but not all bases are alkalis.

8. Define acid rain, specify its pH, and explain its impact on aquatic life.

9. Describe the reaction of a metal with dilute hydrochloric acid. Fill in the blank: $NaOH + Zn \rightarrow \dots + \dots$.

10. Explain the exothermic nature of adding acid to water and the correct procedure for mixing.

Long Answer Questions (5 Marks)

1. (a) Name five industrial products formed using common salt. (b) Provide the chemical name and one unique use for each.

2. Complete the following:

   • (a) $\text{Acid} + \dots \rightarrow \text{Salt} + \text{Water}$.

   • (b) $\dots + \text{Metal} \rightarrow \text{Salt} + \dots$.

   • (c) $\text{Metal carbonate/bicarbonate} + \text{acid} \rightarrow \dots + \dots + \dots$.

   • (d) $NaOH \xrightarrow{H_2O} \dots + \dots$.

   • (e) $Na_2CO_3 + 10H_2O \rightarrow \dots$.