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}2\text{SO}4), nitric acid (\text{HNO}3), acetic acid (\text{CH}3\text{COOH}), sodium hydroxide (\text{NaOH}), calcium hydroxide [\text{Ca(OH)}2], potassium hydroxide (\text{KOH}), magnesium hydroxide [\text{Mg(OH)}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}2\text{ZnO}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}2\text{CO}3) and sodium hydrogencarbonate (\text{NaHCO}3) react with dilute (\text{HCl}) to produce carbon dioxide gas, which is passed through lime water [\text{Ca(OH)}2]. The reaction produces a white precipitate of calcium carbonate.
Test tube A: \text{Na}2\text{CO}3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}2\text{O(l)} + \text{CO}2\text{(g)}
Test tube B: \text{NaHCO}3\text{(s)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}2\text{O(l)} + \text{CO}_2\text{(g)}
Lime water reaction: \text{Ca(OH)}2\text{(aq)} + \text{CO}2\text{(g)} \rightarrow \text{CaCO}3\text{(s)} + \text{H}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}2\text{O} \rightarrow \text{H}3\text{O}^+ + \text{Cl}^-
\text{H}^+ + \text{H}2\text{O} \rightarrow \text{H}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)}
\text{Mg(OH)}2\text{(s)} \xrightarrow{\text{H}2\text{O}} \text{Mg}^{2+}\text{(aq)} + 2\text{OH}^-\text{(aq)}Soluble bases are called alkalis.
Neutralization Reaction as Combination of (\text{H}^+) and (\text{OH}^-)
The neutralization reaction can be seen as:
\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}
Activity 2.10: Dilution of Acids and Bases
Concentrated (\text{H}2\text{SO}4) is added to water, and the temperature change is observed.
The process is highly exothermic.
Mixing acid or base with water decreases the concentration of ions \text{(H}_3\text{O}^+/\text{OH}^-) per unit volume, called dilution.
Strength of Acid or Base Solutions
A universal indicator, a mixture of several indicators, is used to quantitatively find the amount of ions present in a solution.
The pH scale measures hydrogen ion concentration.
pH is a number indicating the acidic or basic nature of a solution.
pH \text{7}: neutral
pH \text{< 7}: acidic
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}2\text{SO}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}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}2) gas reacted with dry slaked lime to produce bleaching powder. 2\text{Ca(OH)}2 + 2\text{Cl}2 \rightarrow \text{Ca(ClO)}2 + \text{CaCl}2 + 2\text{H}2\text{O}
Baking Soda
Baking soda is sodium hydrogencarbonate (\text{NaHCO}_3), produced using sodium chloride.
\text{NaCl} + \text{H}2\text{O} + \text{CO}2 + \text{NH}3 \rightarrow \text{NH}4\text{Cl} + \text{NaHCO}_3
When heated, it decomposes:
2\text{NaHCO}3 \xrightarrow{\text{Heat}} \text{Na}2\text{CO}3 + \text{H}2\text{O} + \text{CO}_2Baking powder contains sodium hydrogencarbonate and a mild edible acid such as tartaric acid
\text{NaHCO}3 + \text{H}^+ \rightarrow \text{CO}2 + \text{H}_2\text{O} + \text{Sodium salt of acid}
Washing Soda
Washing soda is (\text{Na}2\text{CO}3.10\text{H}_2\text{O}), obtained by recrystallization of sodium carbonate.
\text{Na}2\text{CO}3 + 10\text{H}2\text{O} \rightarrow \text{Na}2\text{CO}3.10\text{H}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}4.5\text{H}2\text{O}) is the chemical formula for hydrated copper sulphate.
Plaster of Paris
Gypsum (\text{CaSO}4.2\text{H}2\text{O}) loses water when heated at 373 K, forming Plaster of Paris (\text{CaSO}4.\frac{1}{2}\text{H}2\text{O}) (calcium sulphate hemihydrate).
\text{CaSO}4.\frac{1}{2}\text{H}2\text{O} + 1\frac{1}{2} \text{H}2\text{O} \rightarrow \text{CaSO}4.2\text{H}_2\text{O}