Acids
Definition: Acids are special types of substances that, when mixed with water, release hydrogen ions (H⁺). These hydrogen ions are what give acids their sour taste and chemical strength.
Ionisation in Water: When acids dissolve in water, their molecules split apart (this is called ionisation). The bond holding the hydrogen in the acid molecule breaks, releasing hydrogen ions (H⁺) and leaving behind negatively charged ions.
Examples of Ionisation: For example, when hydrochloric acid (HCl) is added to water, it breaks apart into hydrogen ions (H⁺) and chloride ions (Cl⁻). So we write: HCl → H⁺ + Cl⁻.
Strength of Acids: The strength of an acid depends on how completely it ionises in water. Strong acids break apart fully and release lots of H⁺ ions. Weak acids only break apart a little and release fewer H⁺ ions.
Examples of Strong Acids: Strong acids include hydrochloric acid (HCl), nitric acid (HNO₃), and sulfuric acid (H₂SO₄). These acids ionise fully in water and release many hydrogen ions.
Examples of Weak Acids: Weak acids don’t ionise fully in water. Some common weak acids are ethanoic acid (CH₃COOH), methanoic acid (HCOOH), and phosphoric acid (H₃PO₄).
Role of Water: Water helps acids release hydrogen ions. It also helps form a special kind of ion called the hydronium ion (H₃O⁺), which is made when a hydrogen ion joins with a water molecule.
Alkalis (Bases)
Definition: Alkalis are substances that produce hydroxide ions (OH⁻) when they are dissolved in water. These OH⁻ ions are what make a solution feel slippery and give it basic or alkaline properties.
Ionisation in Water: When alkalis dissolve in water, they break apart (dissociate) into positively charged metal ions and negatively charged hydroxide ions (OH⁻).
Examples of Ionisation: For example, when sodium hydroxide (NaOH) dissolves in water, it breaks into sodium ions (Na⁺) and hydroxide ions (OH⁻): NaOH → Na⁺ + OH⁻.
Strength of Alkalis: Just like acids, alkalis can be strong or weak. Strong alkalis completely break apart in water, while weak alkalis only partially break apart.
Examples of Strong Alkalis: Some examples of strong alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH), and barium hydroxide (Ba(OH)₂).
Example of Weak Alkali: A well-known weak alkali is ammonia (NH₃). It doesn’t break apart completely in water, so it releases fewer OH⁻ ions.
Role of Water: Water is very important for alkalis because it allows the hydroxide ions (OH⁻) to separate and move freely. This is what gives the solution its alkaline properties.
Water’s Role in Acidity and Alkalinity
Medium for Ionisation: Water is the key ingredient that allows acids and alkalis to show their special properties. Without water, they can’t split into ions and wouldn’t behave as acids or alkalis.
In Acids: In acidic solutions, water helps the acid molecules break up and release H⁺ ions. These H⁺ ions often combine with water molecules to make hydronium ions (H₃O⁺), which are very important in acid chemistry.
In Alkalis: In basic or alkaline solutions, water helps alkali compounds break apart into hydroxide ions (OH⁻), which give the solution its basic nature.
Without Water: If there is no water, the acid or alkali cannot break into ions. That means they won’t show any acidic or alkaline behavior—they will just stay as dry substances with no special effect.
pH Scale
What it Measures: The pH scale is a tool we use to tell how acidic or alkaline a solution is. It does this by measuring how many hydrogen ions (H⁺) or hydroxide ions (OH⁻) are present in the water.
Acidic Solutions: If a solution has a pH less than 7, it’s considered acidic. This means it has more hydrogen ions (H⁺) than hydroxide ions.
Neutral Solution: If a solution has a pH of exactly 7, it’s neutral. That means the number of H⁺ ions and OH⁻ ions are equal. Pure water is a good example.
Alkaline Solutions: If a solution has a pH greater than 7, it is alkaline. That means it has more hydroxide ions (OH⁻) than hydrogen ions.
Key Concepts and Ions
Ion Released by Acids: The main ion that acids release when they dissolve in water is the hydrogen ion (H⁺).
Ion Released by Alkalis: The main ion that alkalis release when they dissolve in water is the hydroxide ion (OH⁻).
Hydronium Ion Formula: When a hydrogen ion joins with a water molecule, it forms a hydronium ion. The formula for this ion is H₃O⁺.
Arrhenius Theory (Acids): According to Arrhenius, acids are substances that have hydrogen in their formula and release H⁺ ions when placed in water.
Arrhenius Theory (Alkalis): According to Arrhenius, alkalis are substances that release OH⁻ ions in water, which makes the solution alkaline.
Summary
Why Water is Crucial: Water is extremely important because it allows acids and alkalis to split into ions. These ions are what give the substances their acidic or alkaline properties.
Strength Determinants: The more completely a substance breaks into ions in water, the stronger it is as an acid or alkali. So strength depends on how well it ionises in water.
Function of the pH Scale: The pH scale helps us understand how acidic or alkaline a solution is. It does this by showing how many H⁺ or OH⁻ ions are in the solution.