Acids, Bases and Salts · Lesson 7 of 12
How Strong Are Acid Or Base Solutions?
“The pH scale turns acidity into a number so the colours finally have some paperwork.”
• Explain the purpose of the universal indicator and pH scale. • Interpret acidic, neutral and basic pH values. • Relate hydrogen ion concentration to pH. • Distinguish strong and weak acids and bases. • Distinguish strength from concentration.
Litmus can tell us whether a solution is acidic or basic, but it cannot tell us how strongly acidic or basic that solution is. To compare solutions more precisely, a numerical scale is used.
Universal Indicator
A Universal Indicator is a mixture of indicators that produces different colours at different hydrogen ion concentrations.
The pH Scale
pH is a numerical measure used to describe the acidic or basic nature of an aqueous solution in relation to hydrogen ion concentration.
The scale commonly extends from very acidic values at the low end to strongly alkaline values at the high end. A neutral solution lies at the middle value. Lower pH corresponds to greater hydronium ion concentration, while increasing pH indicates decreasing hydrogen ion concentration and increasing basic character.
| pH Region | Nature |
|---|---|
| Below neutral | Acidic |
| Neutral point | Neutral |
| Above neutral | Basic |
As hydronium ion concentration increases, pH decreases. As hydronium ion concentration decreases, pH increases.
pH Of Common Substances
Common substances occupy different regions of the pH scale. Gastric juice is strongly acidic. Lemon juice is acidic. Pure water is neutral, while blood is close to neutral but slightly basic. Milk of magnesia is basic and sodium hydroxide solution is strongly basic.
Strong And Weak Acids
A Strong Acid produces a relatively large amount of H⁺ or H₃O⁺ ions in water at a given concentration.
A Weak Acid produces a smaller amount of H⁺ or H₃O⁺ ions in water at the same concentration.
Hydrochloric acid and acetic acid can have the same overall concentration but produce different numbers of hydrogen ions. Hydrochloric acid ionises much more extensively, whereas acetic acid produces fewer hydrogen ions.
Strong And Weak Bases
The same idea applies to bases. A strong base produces a larger amount of hydroxide ions in water at a given concentration, while a weak base produces fewer hydroxide ions.
Strength And Concentration Are Different
Strength describes the extent to which an acid or base forms ions. Concentration describes how much solute is present in a given volume of solution. Diluting a strong acid reduces its concentration but does not transform it chemically into a weak acid.
Problem
Solution A has lower pH than solution B. Which has the greater hydrogen ion concentration?
- 1.Recall the inverse relationship between pH and hydrogen ion concentration.
- 2.Lower pH means greater hydrogen ion concentration.
- 3.Therefore solution A has the greater hydrogen ion concentration.
Quiz
If solution A has a lower pH than solution B, which statement is correct?
Which pH represents a neutral aqueous solution?
What happens to pH when hydrogen ion concentration increases?
What distinguishes a strong acid from a weak acid at the same concentration?
Practice Problems
- Explain what a universal indicator shows that ordinary litmus does not.
- Classify solutions with pH below seven, equal to seven and above seven as acidic, neutral or basic.
- Explain the relationship between hydrogen ion concentration and pH.
- Compare a strong acid and a weak acid when both have the same concentration.
- Explain why a dilute strong acid is not the same thing as a weak acid.
Key Takeaways
• Universal indicator gives a range of colours corresponding to different pH values. • Acidic solutions have pH below seven, neutral solutions have pH seven, and basic solutions have pH above seven. • Lower pH corresponds to greater hydrogen ion concentration. • Strong acids produce more H⁺ or H₃O⁺ ions than weak acids at the same concentration. • Strong bases produce more OH⁻ ions than weak bases at the same concentration. • Strength and concentration describe different properties of a solution.