What Are Acids?
Acids are a fascinating and important group of chemicals that you encounter every day — from the tangy taste of lemon juice to the battery acid in a car. Understanding their properties helps us explain countless phenomena in chemistry and biology.
A substance that releases hydrogen ions (H⁺) when dissolved in water. Acids have a characteristic set of physical and chemical properties that distinguish them from other substances.
Strictly speaking, H⁺ ions (bare protons) don't exist freely in water — they immediately attach to water molecules to form hydronium ions (H₃O⁺). At MYP level, writing H⁺(aq) is perfectly acceptable and is the convention we use throughout these notes, but it's good to know that H⁺(aq) and H₃O⁺(aq) mean the same thing in practice.
Acids can be found in both natural and manufactured contexts:
- Natural acids: citric acid (lemons, oranges), acetic acid (vinegar), lactic acid (yoghurt, muscles), carbonic acid (fizzy drinks)
- Laboratory/industrial acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃)
Think of an acid like a generous donor at a party — it always brings something to give away. In chemistry, that "something" is hydrogen ions (H⁺). The more H⁺ ions an acid releases, the stronger (and more reactive) it is.
The study of acids sits at the heart of the unit theme "Action Triggers Reaction" — acids are classic triggers of chemical reactions, and understanding their properties tells us how and why those reactions occur.
MYP Framework Connection
In MYP Sciences, the Key Concept of Change underpins this subtopic: acids drive chemical changes in materials around us. The Related Concept of Transformation captures how substances are chemically transformed when they react with acids — new substances with new properties are formed. As you work through these notes, consider the Global Context of Scientific and Technical Innovation: how has our understanding of acid chemistry driven advances in medicine, industry, and environmental science?
Physical Properties of Acids
Physical properties are those that can be observed or measured without changing the chemical identity of the substance. Acids share several characteristic physical properties.
1. State at room temperature
Most common laboratory acids are aqueous solutions (dissolved in water). They appear as clear, colourless liquids, though some (like concentrated sulfuric acid) are viscous and oily.
2. Taste
Acids have a characteristic sour taste. This is why vinegar and citrus fruits taste sharp and tangy.
Never taste laboratory acids. The sour taste description applies only to dilute, food-safe acids like those in everyday foods. Concentrated laboratory acids are extremely corrosive and dangerous.
3. Odour
Some acids have a sharp, pungent smell. For example, concentrated hydrochloric acid produces hydrogen chloride gas which has a sharp, choking smell. Acetic acid (vinegar) has its recognisable sharp odour.
4. Electrical conductivity
Acidic solutions conduct electricity because they contain free-moving ions (H⁺ and the associated anion). This makes them electrolytes.
5. Effect on indicators
One of the most useful physical properties is the effect on acid-base indicators — substances that change colour depending on the pH of a solution.
| Indicator | Colour in Acid | Colour in Alkali |
|---|---|---|
| Litmus | Red | Blue |
| Universal indicator | Red/orange/yellow | Green/blue/purple |
| Phenolphthalein | Colourless | Pink/magenta |
| Methyl orange | Red | Yellow |
Notice that phenolphthalein is colourless in acid but turns pink/magenta in alkaline conditions. This makes it very useful — if you add a base to an acid and the solution suddenly turns pink, you know you've gone past neutral. Its colourless appearance in acid is only significant when you know what it looks like in alkali!
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