A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Theory and principles
CSCA Chemistry · Lesson 16 of 21Acids and bases
Acides et bases · الأحماض والقواعد
▶ Watch the video lesson (A7LAM students)
Acids release H⁺ ions, bases accept them. The pH scale measures how acidic a solution is.
Key ideas
- pH < 7 acidic, pH = 7 neutral, pH > 7 basic (at 25 °C).
- pH = −log[H⁺]. Each pH unit is a factor of 10.
- Acid + base → salt + water (neutralisation).
- Strong acids (HCl, H₂SO₄, HNO₃) ionise completely in water.
Formulas to know
- pH = −log[H⁺]
- HCl + NaOH → NaCl + H₂O
- [H⁺][OH⁻] = 10⁻¹⁴ at 25 °C
Worked example
A solution has [H⁺] = 10⁻³ mol/L. Find its pH and say if it is acidic.
- pH = −log(10⁻³) = 3.
- 3 < 7.
Answer: pH = 3, acidic
Practice questions
1. pH of a solution with [H⁺] = 10⁻⁹ mol/L?
9, basic ✓ · 9, acidic · −9 · 5, acidic
pH = 9 > 7, so the solution is basic.
2. Products of HCl + NaOH?
NaH + ClOH · NaCl + H₂O ✓ · Na + Cl₂ + H₂O · NaOH + HCl
Neutralisation gives a salt and water.
3. A pH 2 solution is how many times more acidic than a pH 4 solution?
2 · 20 · 100 ✓ · 1,000
Two pH units = 10 × 10 = 100 times more H⁺.