Acids, Bases, and Salts Notes

Acids, Bases, and Salts

Introduction

• Acids and bases are commonly encountered in everyday life, primarily through the sour and bitter tastes of foods.

• Acidity problems, often resulting from overeating, can be addressed using substances that neutralize the acid's effect. Baking soda solution is suggested as a suitable remedy due to its ability to counteract acidity.

• Acids are known for their sour taste and their ability to turn blue litmus paper red. Bases, conversely, taste bitter and turn red litmus paper blue.

• Litmus is a natural indicator derived from lichen, displaying a purple color when neither in acidic nor basic conditions.

• Turmeric is another natural indicator; curry stains on white cloth turn reddish-brown when scrubbed with soap (basic) and revert to yellow upon rinsing with water.

• Synthetic indicators like methyl orange and phenolphthalein can also be employed to test for acids and bases.

Chemical Properties of Acids and Bases

Indicators

• Indicators signal whether a substance is acidic or basic through color changes.

• Olfactory indicators are substances that exhibit a change in odor depending on the acidity or basicity of the medium.

Activity 2.1: Identifying Solutions

• This activity involves differentiating between distilled water, an acidic solution, and a basic solution using only red litmus paper.

• Hydrochloric acid $(\text{HCl})$, sulfuric acid $(\text{H}<em>2\text{SO}</em>4)$, nitric acid $(\text{HNO}<em>3)$, acetic acid $(\text{CH}</em>3\text{COOH})$, sodium hydroxide $(\text{NaOH})$, calcium hydroxide $[\text{Ca(OH)}<em>2]$, potassium hydroxide $(\text{KOH})$, magnesium hydroxide $[\text{Mg(OH)}</em>2]$, and ammonium hydroxide $(\text{NH}_4\text{OH})$ are tested with red litmus, blue litmus, phenolphthalein, and methyl orange.

Activity 2.2: Olfactory Indicators

• Finely chopped onions and cloth strips are stored in a plastic bag overnight to infuse the cloth with onion odor, which then can be used to test for acids and bases.

• Vanilla essence and clove oil are also tested with dilute $(\text{HCl})$ and dilute $(\text{NaOH})$ to check for changes in odor, determining their effectiveness as olfactory indicators.

Activity 2.3: Reaction of Acids and Bases with Metals

• Dilute sulfuric acid reacts with zinc granules to produce hydrogen gas, which is then tested by burning a gas-filled bubble with a burning candle.

• The general reaction is: $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas}$

Activity 2.4: Reaction with Sodium Hydroxide

• Zinc metal reacts with sodium hydroxide to form sodium zincate and hydrogen gas:
  $2\text{NaOH(aq)} + \text{Zn(s)} \rightarrow \text{Na}<em>2\text{ZnO}</em>2\text{(s)} + \text{H}_2\text{(g)}$

• Hydrogen gas is produced, but such reactions are not possible with all metals.

Activity 2.5: Metal Carbonates and Hydrogencarbonates Reacting with Acids

• Sodium carbonate $(\text{Na}<em>2\text{CO}</em>3)$ and sodium hydrogencarbonate $(\text{NaHCO}<em>3)$ react with dilute $(\text{HCl})$ to produce carbon dioxide gas, which is passed through lime water $[\text{Ca(OH)}</em>2]$. The reaction produces a white precipitate of calcium carbonate.

  • Test tube A: $\text{Na}<em>2\text{CO}</em>3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}<em>2\text{O(l)} + \text{CO}</em>2\text{(g)}$

  • Test tube B: $\text{NaHCO}<em>3\text{(s)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}</em>2\text{O(l)} + \text{CO}_2\text{(g)}$

  • Lime water reaction: $\text{Ca(OH)}<em>2\text{(aq)} + \text{CO}</em>2\text{(g)} \rightarrow \text{CaCO}<em>3\text{(s)} + \text{H}</em>2\text{O(l)}$

Activity 2.6: Neutralization Reaction

• Dilute $(\text{NaOH})$ solution with phenolphthalein turns colorless after adding dilute $(\text{HCl})$. The pink color reappears upon adding $(\text{NaOH})$.

• The reaction can be written as:
  $\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$

• The general form is: $\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$

Activity 2.7: Reaction of Metallic Oxides with Acids

• Copper oxide reacts with dilute hydrochloric acid to form copper(II) chloride, and the solution turns blue-green.

• $\text{Metal oxide} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$
  The metal oxides are considered basic oxides based on this reaction.

Reaction of Non-metallic Oxide with Base

• Non-metallic oxides react with bases to produce salt and water so they are acidic.

Common Characteristics of Acids and Bases

Activity 2.8: Conductivity of Acidic Solutions

• Solutions of glucose, alcohol, hydrochloric acid, and sulfuric acid are tested for electrical conductivity using a 6-volt battery, a bulb, and a switch.

• Acids conduct electricity due to the presence of ions, while glucose and alcohol solutions do not.
  $(\text{H}^+)$ ions are responsible for acidic properties.

Activity 2.9: Production of Ions in Aqueous Solution

• Solid $(\text{NaCl})$ reacts with concentrated sulfuric acid to produce $(\text{HCl})$ gas, which is then tested with dry and wet blue litmus paper.

• The experiment suggests that hydrogen ions in $(\text{HCl})$ are produced in the presence of water.

• Hydrogen ions cannot exist alone and combine with water molecules:
  $\text{HCl} + \text{H}<em>2\text{O} \rightarrow \text{H}</em>3\text{O}^+ + \text{Cl}^-$
  $\text{H}^+ + \text{H}<em>2\text{O} \rightarrow \text{H}</em>3\text{O}^+$

• Bases generate hydroxide \text{OH}^-$ ions in water:
  \text{NaOH(s)} \xrightarrow{\text{H}2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}$$\text{KOH(s)} \xrightarrow{\text{H}2\text{O}} \text{K}^+\text{(aq)} + \text{OH}^-\text{(aq)}$<br>$\text{Mg(OH)}2\text{(s)} \xrightarrow{\text{H}2\text{O}} \text{Mg}^{2+}\text{(aq)} + 2\text{OH}^-\text{(aq)}$</p></li><li><p>Soluble bases are called alkalis.</p></li></ul><h5 id="0f5dfcc5-ddd1-4732-8108-6bf6ced29272" data-toc-id="0f5dfcc5-ddd1-4732-8108-6bf6ced29272" collapsed="false" seolevelmigrated="true">Neutralization Reaction as Combination of$(\text{H}^+)$and$(\text{OH}^-)$</h5><ul><li><p>The neutralization reaction can be seen as:<br>$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$</p></li></ul><h5 id="732a6260-3326-4746-aec6-c7bd8552dfe3" data-toc-id="732a6260-3326-4746-aec6-c7bd8552dfe3" collapsed="false" seolevelmigrated="true">Activity 2.10: Dilution of Acids and Bases</h5><ul><li><p>Concentrated$(\text{H}2\text{SO}4)$is added to water, and the temperature change is observed.</p></li><li><p>The process is highly exothermic.</p></li><li><p>Mixing acid or base with water decreases the concentration of ions$\text{(H}_3\text{O}^+/\text{OH}^-)$per unit volume, called dilution.</p></li></ul><h4 id="5fc51236-9cd2-41f5-b248-7df7c46f69f8" data-toc-id="5fc51236-9cd2-41f5-b248-7df7c46f69f8" collapsed="false" seolevelmigrated="true">Strength of Acid or Base Solutions</h4><ul><li><p>A universal indicator, a mixture of several indicators, is used to quantitatively find the amount of ions present in a solution.</p></li><li><p>The pH scale measures hydrogen ion concentration.</p></li><li><p>pH is a number indicating the acidic or basic nature of a solution.</p><ul><li><p>pH$\text{7}$: neutral</p></li><li><p>pH$\text{< 7}$: acidic</p></li><li><p>pH$\text{> 7}$: basic. As the pH value increases from 7 to 14, it represents an increase in$\text{OH}^-$ ion concentration in the solution

Activity 2.11: Testing pH Values

• The pH values of different solutions are tested to determine their nature.

• The strength of acids and bases depends on the number of $\text{H}^+$ ions and $\text{OH}^-$ ions produced, respectively.

• Acids producing more $\text{H}^+$ ions are strong, while those producing less are weak.

Importance of pH in Everyday Life

• Living organisms can survive only in a narrow range of pH change (7.0 to 7.8 for the human body).

• Acid rain has a pH less than 5.6 which poses a threat to aquatic life.

Activity 2.12: Soil pH

• Soil samples are collected, and their pH is tested using universal indicator paper to determine the ideal soil pH for plant growth in the region.

pH in the Digestive System

• The stomach produces hydrochloric acid to aid digestion.

• During indigestion, excess acid is produced, causing pain and irritation.

• Antacids, like magnesium hydroxide (Milk of magnesia), are used to neutralize the excess acid.

pH Change as the Cause of Tooth Decay

• Tooth decay starts when the pH of the mouth is lower than 5.5.

• Tooth enamel, made up of calcium hydroxyapatite, corrodes at this pH.

• Bacteria in the mouth produce acids by degrading sugar and food particles so cleaning the mouth and using toothpastes are recommended to prevent tooth decay.

Self-Defense by Animals and Plants

• Bee stings inject acid, causing pain, which can be relieved by applying a mild base like baking soda.

• Nettle plant's stinging hair injects methanoic acid, causing burning pain.

Salts

Family of Salts

• Salts with the same positive or negative radicals belong to a family.

• For example, $(\text{NaCl})$ and $(\text{Na}<em>2\text{SO}</em>4)$ belong to the family of sodium salts.

Activity 2.14: pH of Salts

• Salt samples, such as sodium chloride, potassium nitrate, and others, are tested for solubility and their effect on litmus paper to determine their pH.

• Salts of strong acid and strong base are neutral with a pH of 7.

• Salts of strong acid and weak base are acidic with a pH less than 7.

• Salts of strong base and weak acid are basic with a pH more than 7.

Chemicals from Common Salt

• Common salt (sodium chloride) is a raw material for various materials such as sodium hydroxide, baking soda, washing soda, and bleaching powder.

Sodium Hydroxide

• When electricity is passed, brine (aqueous solution of sodium chloride) produce sodium hydroxide. * The process is known as the chlor-alkali process.
  \2\text{NaCl(aq)} + 2\text{H}2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}2\text{(g)} + \text{H}2\text{(g)} $(\text{Cl}</em>2)$ gas is produced at the anode and $(\text{H}_2)$ gas at the cathode. $(\text{NaOH})$ is formed near the cathode.

Bleaching Powder

• $(\text{Cl}<em>2)$ gas reacted with dry slaked lime to produce bleaching powder. $2\text{Ca(OH)}</em>2 + 2\text{Cl}<em>2 \rightarrow \text{Ca(ClO)}</em>2 + \text{CaCl}<em>2 + 2\text{H}</em>2\text{O}$

Baking Soda

• Baking soda is sodium hydrogencarbonate $(\text{NaHCO}_3)$, produced using sodium chloride.

• $\text{NaCl} + \text{H}<em>2\text{O} + \text{CO}</em>2 + \text{NH}<em>3 \rightarrow \text{NH}</em>4\text{Cl} + \text{NaHCO}_3$

• When heated, it decomposes:
  $2\text{NaHCO}<em>3 \xrightarrow{\text{Heat}} \text{Na}</em>2\text{CO}<em>3 + \text{H}</em>2\text{O} + \text{CO}_2$

• Baking powder contains sodium hydrogencarbonate and a mild edible acid such as tartaric acid
  $\text{NaHCO}<em>3 + \text{H}^+ \rightarrow \text{CO}</em>2 + \text{H}_2\text{O} + \text{Sodium salt of acid}$

Washing Soda

• Washing soda is $(\text{Na}<em>2\text{CO}</em>3.10\text{H}_2\text{O})$, obtained by recrystallization of sodium carbonate.

• $\text{Na}<em>2\text{CO}</em>3 + 10\text{H}<em>2\text{O} \rightarrow \text{Na}</em>2\text{CO}<em>3.10\text{H}</em>2\text{O}$

Water of Crystallization

Activity 2.15: Copper Sulphate Crystals

• Copper sulphate crystals contain water of crystallization. Heating removes the water, turning the salt white. Adding water restores the blue color.

• $(\text{CuSO}<em>4.5\text{H}</em>2\text{O})$ is the chemical formula for hydrated copper sulphate.

Plaster of Paris

• Gypsum $(\text{CaSO}<em>4.2\text{H}</em>2\text{O})$ loses water when heated at 373 K, forming Plaster of Paris $(\text{CaSO}<em>4.\frac{1}{2}\text{H}</em>2\text{O})$ (calcium sulphate hemihydrate).

• $\text{CaSO}<em>4.\frac{1}{2}\text{H}</em>2\text{O} + 1\frac{1}{2} \text{H}<em>2\text{O} \rightarrow \text{CaSO}</em>4.2\text{H}_2\text{O}$