Comprehensive Study Notes on Acids, Bases, and Salts

Introduction to Acids, Bases, and Indicators

• The sour and bitter tastes of food are attributed to the presence of acids and bases, respectively.

• Acids are sour in taste and possess the ability to change the color of blue litmus to red.

• Bases are bitter in taste and change the color of red litmus to blue.

• Neutralization: Acids and bases have the ability to nullify each other's effects.

• Natural Indicators:

  • Litmus: A purple dye extracted from lichen, a plant belonging to the division Thallophyta. It is purple when the solution is neither acidic nor basic.

  • Turmeric: Turns reddish-brown when in contact with basic substances (like soap) and returns to yellow when washed with plenty of water.

  • Others: Red cabbage leaves and the colored petals of flowers such as Hydrangea, Petunia, and Geranium.

• Synthetic Indicators: Methyl orange and phenolphthalein are commonly used laboratory substitutes for natural indicators.

• Olfactory Indicators: Substances whose odor changes in acidic or basic media. Examples include onion, vanilla essence, and clove oil.

  • Finely chopped onions kept with cloth strips overnight can be used to test media; the odor persists in acid but may change or disappear in base.

  • Vanilla and clove oil also exhibit distinct odor changes when treated with dilute $NaOH$.

Chemical Properties of Acids and Bases

2.1.2 Reaction of Acids and Bases with Metals

• When an acid reacts with a metal, the metal displaces hydrogen from the acid as hydrogen gas and forms a compound called a salt.

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

• Activity Instance: Zinc granules reacting with dilute sulphuric acid.

  • Observation: Bubbles of hydrogen gas form on the surface of the zinc.

  • Testing Hydrogen: When passed through a soap solution, bubbles form. A burning candle brought near a hydrogen-filled bubble results in a pop sound.

  • Reaction: $H_2SO_4(aq) + Zn(s) \rightarrow ZnSO_4(aq) + H_2(g)$

• Bases and Metals: Certain bases also react with metals to produce hydrogen gas, though this is not possible with all metals.

  • Reaction with Sodium Hydroxide: $2NaOH(aq) + Zn(s) \rightarrow Na_2ZnO_2(s) + H_2(g)$

  • The product $Na_2ZnO_2$ is identified as Sodium zincate.

2.1.3 Reaction of Metal Carbonates and Metal Hydrogencarbonates with Acids

• All metal carbonates and hydrogencarbonates react with acids to produce a salt, carbon dioxide, and water.

• General Formula: $\text{Metal Carbonate/Metal Hydrogencarbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Carbon dioxide} + \text{Water}$

• Experimental Observations (Activity 2.5):

  • Test tube A with sodium carbonate ($Na_2CO_3$) and dilute $HCl$:     $Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$

  • Test tube B with sodium hydrogencarbonate ($NaHCO_3$) and dilute $HCl$:     $NaHCO_3(s) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$

• Testing for Carbon Dioxide: Passing the gas through lime water ($Ca(OH)_2$) produces a white precipitate of calcium carbonate ($CaCO_3$).

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

  • Excess Carbon Dioxide: If excess $CO_2$ is passed, the precipitate dissolves to form soluble calcium hydrogencarbonate.

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

• Limestone, chalk, and marble are distinct forms of calcium carbonate.

2.1.4 Interaction Between Acids and Bases

• The effect of a base is nullified by an acid, and vice versa. This is termed a neutralization reaction.

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

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

• Indicators like phenolphthalein change color during this process (pink in base, colorless when neutralized by acid).

2.1.5 Reaction of Metallic and Non-metallic Oxides

• Metallic Oxides + Acid: Metallic oxides react with acids to produce salt and water. Therefore, metallic oxides are categorized as basic oxides.

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

  • The solution becomes blue-green due to the formation of copper(II) chloride.

• Non-metallic Oxides + Base: Non-metallic oxides (like $CO_2$) react with bases (like $Ca(OH)_2$) to produce salt and water, indicating that non-metallic oxides are acidic oxides.

Common Characteristics: Hydronium and Hydroxide Ions

• All acids generate hydrogen gas when reacting with metals, suggesting hydrogen is common to all acids.

• Electrical Conductivity: Aqueous solutions of acids conduct electricity due to the presence of ions.

  • Acids produce $H^+(aq)$ ions in solution, which are responsible for acidic properties.

  • Compounds like glucose and alcohol contain hydrogen but do not ionize in water, thus their solutions do not conduct electricity.

• Necessity of Water: Hydrogen ions in $HCl$ are only produced in the presence of water. Dry $HCl$ gas does not show acidic behavior.

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

  • Hydrogen ions cannot exist alone; they exist as hydronium ions ($H_3O^+$) or $H^+(aq)$.

• Bases in Water: Bases generate hydroxide ($OH^-$) ions in water.

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

  • Alkalis: Bases that are soluble in water. They are soapy to touch, bitter, and corrosive.

• Dilution: The process of mixing an acid or base with water results in a decrease in the concentration of ions ($H_3O^+ / OH^-$) per unit volume.

  • This process is highly exothermic.

  • Safety Warning: Concentration acid must always be added slowly to water with constant stirring. Adding water to concentrated acid can cause the mixture to splash out or the glass container to break.

The pH Scale and Acid-Base Strength

• pH Scale: A scale developed to measure hydrogen ion concentration. The 'p' stands for potenz (German for power).

  • Range: $0$ (very acidic) to $14$ (very alkaline).

  • Neutral solution: $pH = 7$.

  • Acidic solution: pH < 7.

  • Basic solution: pH > 7.

• Universal Indicator: A mixture of several indicators that shows different colors at different concentrations of hydrogen ions.

• Strength:

  • Strong Acids: Give rise to more $H^+$ ions (e.g., $HCl$).

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

Importance of pH in Everyday Life

• Biological Systems: The human body works within a pH range of $7.0 \text{ to } 7.8$.

• Acid Rain: Rainwater with a pH less than $5.6$. It makes the survival of aquatic life difficult.

• Venus: The atmosphere of Venus is composed of thick white and yellowish clouds of sulphuric acid, making life impossible.

• Stomach pH: The stomach produces hydrochloric acid ($HCl$) for digestion. Overproduction causes indigestion and pain, treatable with antacids like magnesium hydroxide (Milk of Magnesia).

• Tooth Decay: Starts when the mouth's pH falls below $5.5$. Bacteria degrade sugar into acids that corrode tooth enamel (calcium hydroxyapatite). Basic toothpaste helps neutralize these acids.

• Defense Mechanisms:

  • Honey-bee stings inject an acid; baking soda (mild base) provides relief.

  • Nettle leaves have stinging hair that injects methanoic acid. The dock plant, which often grows nearby, acts as a traditional basic remedy.

Chemicals from Common Salt (NaCl)

1. Sodium Hydroxide (Chlor-alkali Process)

• Electricity passed through brine (aqueous $NaCl$) produces sodium hydroxide, chlorine gas, and hydrogen gas.

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

• Chlorine is released at the anode; Hydrogen is released at the cathode.

2. Bleaching Powder ($Ca(ClO)_2$)

• Produced by the action of chlorine on dry slaked lime ($Ca(OH)_2$).

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

• Uses: Bleaching textiles and wood pulp, oxidising agent, and disinfecting drinking water.

3. Baking Soda ($NaHCO_3$)

• Chemical Name: Sodium hydrogencarbonate.

• Solvay Process Reaction: $NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$

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

• Baking Powder: A mixture of baking soda and a weak edible acid like tartaric acid. It produces $CO_2$ which makes bread/cake spongy.

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

• Obtained by recrystallization of sodium carbonate.

• Reaction: $Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O$

• Uses: Glass, soap, and paper industries; manufacturing borax; cleaning agent; removing permanent hardness of water.

Water of Crystallization

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

• Hydrated Copper Sulphate: $CuSO_4 \cdot 5H_2O$ (Blue color). Heating removes water and turns it white.

• Gypsum: $CaSO_4 \cdot 2H_2O$.

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

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

  • Setting: On mixing with water, it turns back into gypsum.

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

  • Two formula units of $CaSO_4$ share one molecule of water.

Questions & Discussion

• Q: How to identify contents of three test tubes (distilled water, acid, base) using only red litmus?

  • A: Dip red litmus in all. The one that turns blue is the base. Use that blue litmus for the other two; the one that turns red is acidic, and the one that remains blue is water.

• Q: Why should curd/sour substances not be kept in brass/copper?

  • A: Acids in these substances react with the metal to form toxic metallic salts.

• Q: Which gas is liberated when acid reacts with metal?

  • A: Hydrogen ($H_2$). Tested with a burning splinter (pop sound).

• Q: Why does an aqueous solution of acid conduct electricity?

  • A: Because it dissociates into ions that carry the electric current.

• Q: Why does dry $HCl$ gas not change the color of dry litmus?

  • A: Absence of water means no $H^+$ ions are produced to show acidic properties.

• Q: Effect of dilution on $H_3O^+ / OH^-$ concentration?

  • A: Concentration per unit volume decreases.

• Q: Under what conditions does a farmer use lime ($CaO$/$Ca(OH)_2$)?

  • A: When the soil is too acidic for healthy plant growth.

• Q: Why store Plaster of Paris in moisture-proof containers?

  • A: It reacts with moisture to set into hard gypsum, becoming useless.