Acids, Bases, and pH

Grade 10 · Science · Study Guide

Explore the properties of acids and bases, the pH scale, and neutralization reactions using Arrhenius and Brønsted-Lowry theories.

Acids and bases are two fundamental classes of chemical compounds that play critical roles in chemistry and biology. The Arrhenius definition describes acids as substances that produce hydrogen ions (H⁺) in water and bases as substances that produce hydroxide ions (OH⁻). The broader Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors, which explains reactions that occur without water.

The pH scale measures the acidity or basicity of a solution on a scale from 0 to 14. pH is defined as the negative logarithm of the hydrogen ion concentration: pH = -log[H⁺]. Pure water at 25°C has a pH of 7 (neutral). Solutions with pH below 7 are acidic, and those above 7 are basic (alkaline). Because the scale is logarithmic, each unit change represents a tenfold change in H⁺ concentration.

Strong acids (like HCl and H₂SO₄) and strong bases (like NaOH) dissociate completely in water. Weak acids and bases only partially dissociate, establishing equilibria. Acid-base neutralization reactions produce water and a salt: HCl + NaOH → NaCl + H₂O. Indicators are substances that change color at specific pH values, allowing visual identification of acidic or basic conditions.

Key terms

pH: A logarithmic scale measuring hydrogen ion concentration; pH = -log[H⁺]; ranges from 0 (most acidic) to 14 (most basic)
Arrhenius acid: A substance that produces H⁺ ions when dissolved in water
Brønsted-Lowry base: Any species that accepts a proton (H⁺) from another substance in a chemical reaction
Neutralization: A reaction between an acid and a base that produces water and a salt, resulting in a more neutral pH
Strong acid: An acid that completely dissociates into ions in aqueous solution (e.g., HCl, HNO₃, H₂SO₄)
Buffer: A solution that resists changes in pH when small amounts of acid or base are added; contains a weak acid and its conjugate base

Flashcards

pH formula: pH = -log[H⁺] where [H⁺] is the molar concentration of hydrogen ions
Neutral pH: pH = 7 at 25°C; [H⁺] = [OH⁻] = 1 × 10⁻⁷ mol/L
Conjugate acid-base pair: Two species that differ by one proton (H⁺); the acid donates H⁺ to form its conjugate base
Litmus indicator: Turns red in acidic solutions (pH < 7) and blue in basic solutions (pH > 7)
Dissociation constant (Ka): Equilibrium constant for the dissociation of a weak acid; larger Ka = stronger acid
Amphoteric substance: A substance that can act as both an acid and a base (e.g., water, HCO₃⁻)
pOH: pOH = -log[OH⁻]; related to pH by: pH + pOH = 14 at 25°C
Equivalence point: The point in a titration where the moles of acid equal the moles of base; solution may be neutral, acidic, or basic depending on the salt formed

Questions

A solution has a hydrogen ion concentration of 1 × 10⁻³ mol/L. What is its pH?
1. 3
2. 7
3. 11
4. -3
Show answer
Answer: 3
pH = -log(1 × 10⁻³) = -(-3) = 3
According to the Brønsted-Lowry theory, an acid is defined as a proton donor.
Show answer
Answer: True
The Brønsted-Lowry definition extends Arrhenius by defining acids as proton donors and bases as proton acceptors
Which of the following is a strong base?
1. NH₃
2. Ca(OH)₂ (slightly soluble)
3. NaOH
4. CH₃COOH
Show answer
Answer: NaOH
NaOH is a strong base that completely dissociates to Na⁺ and OH⁻ in water; NH₃ is a weak base; CH₃COOH is an acid
When an acid and a base react in a neutralization reaction, the products are water and a ________.
Show answer
Answer: salt
Neutralization: acid + base → salt + water. The salt is formed from the cation of the base and the anion of the acid
If the pH of solution A is 4 and the pH of solution B is 6, solution A has how many times more H⁺ ions than solution B?
1. 2 times
2. 10 times
3. 100 times
4. 1000 times
Show answer
Answer: 100 times
pH 4 has [H⁺] = 10⁻⁴; pH 6 has [H⁺] = 10⁻⁶. Ratio = 10⁻⁴ / 10⁻⁶ = 10² = 100 times more

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