Chapter Notes

UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS AND BASES

You've probably noticed that some foods, like lemons, taste sour, while others, like baking soda, have a bitter taste. These tastes are due to chemicals called acids and bases. Acids are responsible for the sour taste, and bases for the bitter taste.

But tasting chemicals is dangerous! Instead, we use substances called indicators to identify whether something is an acid or a base. Indicators change their properties, like colour or smell, when they come into contact with acids or bases.

Acids and Bases in the Laboratory

There are several types of indicators we can use to test for acids and bases.

• Natural Indicators: These are found in nature.

  • Litmus: A natural dye that is red in acidic solutions and blue in basic solutions.
  • Turmeric: The yellow spice in curry. It stays yellow in acids but turns reddish-brown in bases. [!example] Have you ever spilled curry on a white shirt? When you scrub it with soap (which is a base), the yellow stain turns reddish-brown. When you rinse it with lots of water, it turns yellow again.

• Synthetic Indicators: These are man-made chemicals.

  • Methyl orange: Turns red in acidic solutions and yellow in basic solutions.
  • Phenolphthalein: Is colourless in acidic solutions but turns pink in basic solutions.

• Olfactory Indicators: These are substances whose smell changes depending on whether they are mixed with an acid or a base.

  • Examples include onion, vanilla essence, and clove oil. Typically, they lose their characteristic smell in the presence of a base, but their smell remains in an acid.

How do Acids and Bases React with Metals?

When an acid reacts with a metal, it produces two things: a salt and hydrogen gas. We can see the hydrogen gas as bubbles fizzing from the metal.

The general rule for this reaction is: Acid + Metal → Salt + Hydrogen gas

Some bases also react with certain metals to produce hydrogen gas. For example, sodium hydroxide reacts with zinc metal to form sodium zincate and hydrogen gas. $2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{Zn}(\mathrm{~s}) \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2}(\mathrm{~s})+\mathrm{H}_{2}(\mathrm{~g})$ However, this type of reaction is not possible with all metals.

How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

Acids react with metal carbonates (like washing soda) and metal hydrogencarbonates (like baking soda) to produce a salt, carbon dioxide gas, and water.

The general rule is: Metal carbonate / Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

The carbon dioxide gas produced can be tested by passing it through lime water (calcium hydroxide solution). The lime water will turn milky or form a white precipitate of calcium carbonate.

How do Acids and Bases React with each other?

When an acid and a base react, they cancel out, or nullify, each other's effects. This is a very important reaction that produces a salt and water. This specific type of reaction is called a neutralisation reaction.

The general rule is: Base + Acid → Salt + Water

This is the principle behind taking an antacid for indigestion. The antacid is a base that neutralizes the excess acid in your stomach.

Reaction of Metallic Oxides with Acids

Metallic oxides (compounds of a metal and oxygen) are generally basic in nature. Because they are basic, they react with acids in a way that is very similar to a neutralization reaction, producing a salt and water.

The general rule is: Metal oxide + Acid → Salt + Water

Reaction of a Non-metallic Oxide with Base

In contrast to metallic oxides, non-metallic oxides (compounds of a non-metal and oxygen) are acidic in nature. Therefore, they react with bases to form a salt and water.

WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON?

All acids share similar chemical properties because they all have something in common. When dissolved in water, all acids produce hydrogen ions (H+). It is the presence of these H+ ions that gives acids their characteristic properties.

Similarly, all bases, when dissolved in water, produce hydroxide ions (OH⁻). The presence of these OH⁻ ions is responsible for the properties of bases.

What Happens to an Acid or a Base in a Water Solution?

Water plays a crucial role for acids and bases. An acid will only show its acidic properties when it is dissolved in water.

Hydrogen ions (H+) cannot exist by themselves. They immediately combine with a water molecule ($\mathrm{H}_{2}\mathrm{O}$) to form a hydronium ion (H₃O⁺). So, in a solution, hydrogen ions are always found as H⁺(aq) or H₃O⁺. $\mathrm{HCl}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{Cl}^{-}$

Bases also generate ions in water. For example, sodium hydroxide dissolves in water to produce sodium ions (Na⁺) and hydroxide ions (OH⁻). $\mathrm{NaOH}(\mathrm{~s}) \xrightarrow{\mathrm{H}_{2} \mathrm{O}} \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$

• Alkalis: Bases that are soluble in water are called alkalis. All alkalis are bases, but not all bases are alkalis (because not all bases dissolve in water).

The Dilution Process Mixing a concentrated acid or base with water is a highly exothermic process, meaning it releases a lot of heat.

When you mix an acid or base with water, you are performing dilution. This process decreases the concentration of H₃O⁺ or OH⁻ ions per unit volume, making the solution less acidic or less basic.

HOW STRONG ARE ACID OR BASE SOLUTIONS?

Not all acids and bases are equally strong. The strength of an acid depends on the number of H⁺ ions it produces in solution, while the strength of a base depends on the number of OH⁻ ions it produces.

• Strong Acids: Acids that produce a large number of H⁺ ions in solution (e.g., hydrochloric acid, HCl).
• Weak Acids: Acids that produce a smaller number of H⁺ ions (e.g., acetic acid, CH₃COOH).
• Strong and Weak Bases: The same logic applies to bases based on the amount of OH⁻ ions they produce.

To measure the strength of an acid or base, we use the pH scale. The 'p' in pH stands for 'potenz,' which is German for 'power.' The scale generally runs from 0 to 14.

• A pH of 7 is neutral (like pure water).
• A pH less than 7 is acidic. The lower the number, the stronger the acid.
• A pH greater than 7 is basic (or alkaline). The higher the number, the stronger the base.

Importance of pH in Everyday Life

The pH level is incredibly important in many aspects of our lives and the world around us.

• Living Organisms: Our bodies function best within a very narrow pH range of 7.0 to 7.8. If this balance is disturbed, it can be very harmful. Aquatic life is also sensitive to pH. When the pH of rainwater drops below 5.6, it's called acid rain, which can lower the pH of rivers and harm fish and other organisms.
• Soil pH: Plants require a specific pH range in the soil to grow well. Farmers often test the soil's pH and may add substances like quick lime or slaked lime (which are basic) to neutralize acidic soil.
• Digestive System: Our stomach produces hydrochloric acid to help digest food. Sometimes, it produces too much acid, causing pain and irritation (indigestion). To fix this, people take antacids, which are mild bases like Magnesium hydroxide (Milk of magnesia), to neutralize the excess stomach acid.
• Tooth Decay: Tooth decay begins when the pH in our mouth drops below 5.5. Bacteria in the mouth break down sugar and food particles, producing acids that corrode tooth enamel. Using toothpaste, which is generally basic, helps neutralize this acid and prevent decay.
• Self-defense in Nature: A bee sting is painful because the bee injects an acid. Applying a mild base like baking soda can provide relief. Similarly, the stinging hair of a nettle plant injects methanoic acid. Nature often provides a remedy nearby; the leaf of the dock plant, which is basic, can be rubbed on the sting to neutralize the acid.

MORE ABOUT SALTS

A salt is a compound formed from the reaction between an acid and a base. Common table salt, sodium chloride (NaCl), is just one of many different types of salts.

Family of Salts

Salts that share the same positive ion (cation) or negative ion (anion) are said to belong to the same family.

• Example: Sodium chloride (NaCl) and sodium sulphate (Na₂SO₄) both belong to the family of sodium salts.
• Example: Sodium chloride (NaCl) and potassium chloride (KCl) both belong to the family of chloride salts.

pH of Salts

The solution of a salt in water can be acidic, basic, or neutral. This depends on the strength of the acid and base that were used to create the salt.

• Neutral Salts (pH = 7): Formed from a strong acid and a strong base. (e.g., NaCl)
• Acidic Salts (pH < 7): Formed from a strong acid and a weak base.
• Basic Salts (pH > 7): Formed from a strong base and a weak acid.

Chemicals from Common Salt

Common salt (sodium chloride) is an important raw material for making many other useful chemicals.

Sodium hydroxide (NaOH)

When electricity is passed through an aqueous solution of NaCl (called brine), it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas. This process is called the chlor-alkali process. $2 \mathrm{NaCl}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{Cl}_{2}(\mathrm{~g})+\mathrm{H}_{2}(\mathrm{~g})$

Bleaching powder (CaOCl₂)

The chlorine gas produced from the chlor-alkali process is used to manufacture bleaching powder. It is made by reacting chlorine with dry slaked lime [Ca(OH)₂].

• Uses: Bleaching cotton and linen in the textile industry, bleaching wood pulp in paper factories, and disinfecting drinking water.

Baking soda (NaHCO₃)

The chemical name for baking soda is sodium hydrogencarbonate. It is a mild, non-corrosive basic salt. When heated, baking soda produces sodium carbonate, water, and carbon dioxide. $2 \mathrm{NaHCO}_{3} \xrightarrow{\text { Heat }} \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}$

• Uses:
  • As an ingredient in baking powder (a mix of baking soda and a mild acid). The CO₂ produced makes bread and cake rise.
  • As an antacid to relieve indigestion.
  • In soda-acid fire extinguishers.

Washing soda (Na₂CO₃·10H₂O)

Washing soda is sodium carbonate with ten molecules of water attached. It is formed by recrystallizing sodium carbonate (which can be obtained by heating baking soda).

• Uses:
  • In the glass, soap, and paper industries.
  • As a cleaning agent for domestic purposes.
  • For removing the permanent hardness of water.

Are the Crystals of Salts really Dry?

Many salt crystals appear to be dry but actually contain a fixed number of water molecules within their structure. This trapped water is called the water of crystallisation.

Another important salt with water of crystallisation is gypsum ($\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}$).

Plaster of Paris (CaSO₄·½H₂O)

When gypsum is carefully heated to 373 K, it loses some of its water molecules and becomes calcium sulphate hemihydrate, commonly known as Plaster of Paris.

• It is a white powder that, when mixed with water, sets into a hard solid mass, turning back into gypsum. $\underset{\text { (Plaster of Paris) }}{\mathrm{CaSO}_{4} \cdot \frac{1}{2} \mathrm{H}_{2} \mathrm{O}}+1 \frac{1}{2} \mathrm{H}_{2} \mathrm{O} \rightarrow \underset{\text { (Gypsum) }}{\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}}$
• Uses: By doctors for setting fractured bones, for making toys and decorative materials, and for making surfaces smooth.