6.1. Acids

Learning check

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pH and pH indicators 

How do you know if something is acid?

• It has a sour taste.
• A pH indicator may show it.

Lemon has a sour taste because it contains citric acid.

Tea contains colors that become lighter in acidic solutions.

BTB (Bromothymol blue):

• Yellow in acidic solutions.
• Green in neutral solutions.
• Blue in basic (alkaline) solutions.

BTB in acidic, neutral, and basic solutions.

Phenolphthalein in acidic, neutral, and basic solutions.

Phenolphthalein.

• Colorless in acidic and neutral solutions.
• Pink in basic solutions.

Acidic and basic (alkaline) solutions

• Acidic solution: pH < 7
• Neutral solution: pH = 7
• Basic solution: pH > 7

Acidic solutions conduct electricity

Acidic solutions conduct electricity because there are ions in the solution.

• The more ions, the higher the conductivity.

Acidic solutions conduct electricity.

Why are there ions in an acidic solution?

• The hydrolysis (reaction with water) of hydrochloric acid:

Hydrochloric acid reacts with water. A chloride ion and a hydronium ion form.

• Acidic substances donate protons (hydrogen ions, H⁺).
• Acidic solutions contain hydronium ions (H₃O⁺).

Acetic acid

Protolysis of acetic acid:

Acetic acid reacts with water.

• With structural formulas:
  
  When acetic acid reacts with water, the indicated hydrogen ion is donated to the water molecule.

Acids easily react with base metals

Example: Magnesium in hydrochloric acid

• Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
• Mg(s) + 2H⁺(aq) + 2Cl^–(aq) → Mg²⁺(aq) + H₂(g) + 2Cl^–(aq)

Note: Hydrogen gas is formed.

• Base metals are sometimes called hydrogen-expelling.