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Acids, Bases, and the pH Scale

with Atlas

Atlas stands at a high-school chemistry lab bench, carefully dipping a red litmus strip into a beaker of clear solution, while a second beaker of amber liquid and a digital pH meter sit beside a glowing 0-to-14 pH wall chart.

Define acids and bases using the Bronsted-Lowry model as proton donors and proton acceptors
Distinguish the Bronsted-Lowry model from the Arrhenius model using at least one example
Explain what the pH scale measures and how its logarithmic nature relates to hydrogen-ion concentration
Predict whether a solution is acidic, neutral, or basic from its pH value
Describe what happens to pH during a neutralization reaction between an acid and a base

Key terms

Bronsted-Lowry acid: Any species that donates a proton (H+) to another species in a reaction.
Bronsted-Lowry base: Any species that accepts a proton (H+) from another species in a reaction.
pH: The negative base-10 logarithm of the hydrogen-ion concentration, pH = -log10[H+].
Neutralization: A reaction in which an acid and a base combine, driving the pH toward 7.
Amphoteric species: A substance such as water that can act as either an acid or a base depending on its partner.

From Arrhenius to Bronsted-Lowry

The Arrhenius model defines acids as substances that release H+ in water and bases as substances that release OH-. This works for HCl and NaOH but cannot explain ammonia, which is basic yet contains no hydroxide. Bronsted-Lowry generalizes the idea to proton transfer: an acid donates H+ and a base accepts it. Ammonia (NH3) accepts a proton from water to form NH4+ and OH-, so it qualifies as a base without ever containing OH- itself. This proton-centered view covers far more chemistry and is the model used throughout academy-level work.

Why the pH scale is logarithmic

Hydrogen-ion concentrations in real solutions span an enormous range, from roughly 1 mol/L in strong acid down to 10^-14 mol/L in strong base. Writing those tiny exponential numbers is awkward, so chemists take the negative base-10 logarithm. Because the scale is logarithmic, each whole pH unit corresponds to a tenfold change in [H+]. A drop from pH 6 to pH 3 is not 'twice as acidic' but 10^3 = 1000 times more acidic. This compression is why a single pH unit matters so much in blood chemistry, soil science, and industrial processes.

Worked examples

Calculate the pH of a solution with [H+] = 1 x 10^-4 mol/L.

Write the definition: pH = -log10[H+].
Substitute the concentration: pH = -log10(1 x 10^-4).
The base-10 log of 10^-4 is -4, so -log10(10^-4) = 4.

Answer: pH = 4 (acidic, since it is below 7).

A solution moves from pH 9 to pH 6. How does its acidity change?

Find the change in pH units: 9 - 6 = 3 units.
Each unit is a tenfold change in [H+], and pH dropping means [H+] rising.
Three units rising = 10 x 10 x 10 = 10^3 = 1000 times more H+.

Answer: The solution becomes 1000 times more acidic.

Hi, I'm Atlas. Let's build a clear picture of acids and bases — starting with the right model. In the Bronsted-Lowry model, an acid is any substance that donates a hydrogen ion (a proton, written H+ — shorthand for the hydronium ion H3O+ that actually forms in water), and a base is any substance that accepts that proton. Notice this says nothing about hydroxide ions. For example, ammonia (NH3) is a classic Bronsted-Lowry base: it accepts a proton from water to form the ammonium ion (NH4+), with no free OH- produced. The older Arrhenius model defines bases as OH- producers, which works for compounds like NaOH but misses bases like NH3. At this level we use Bronsted-Lowry because it covers far more substances. A word about litmus: it is a natural dye that turns red in acidic solutions and blue in basic solutions. It tells you which side of neutral you are on, but not how acidic or basic — for that, we use the pH scale. The pH scale runs from 0 to 14 (and a little beyond at extremes). It is defined as the negative logarithm of the H+ (H3O+) concentration, so every single unit represents a tenfold change in acidity. A solution at pH 3 is ten times more acidic than one at pH 4, and one hundred times more acidic than one at pH 5. Pure water at room temperature is neutral at pH 7, where H+ and OH- concentrations are exactly equal. Below 7 is acidic; above 7 is basic (alkaline). When an acid and a base react in a neutralization reaction, the donated protons are accepted, effectively reducing the excess H+, and the pH moves toward 7. If you get stuck, picture the scale: smaller number means more acid, larger number means more base, and 7 is the balance point.

Activity

Drag these solutions into order from MOST acidic to MOST basic using their pH values.

Practice

Find the pH of a solution whose hydrogen-ion concentration is 1 x 10^-2 mol/L, then state whether it is acidic or basic.

Explain why ammonia is classified as a base in the Bronsted-Lowry model even though it contains no hydroxide ion.

Common mistakes to avoid

pH 4 is twice as acidic as pH 8The scale is logarithmic, so pH 4 is 10^4 = 10000 times more acidic than pH 8, not twice.
All bases must contain hydroxide ionsBronsted-Lowry bases simply accept protons, so ammonia and carbonate are bases without containing OH-.

Check your understanding

In the Bronsted-Lowry model, what does an acid do?

A solution changes from pH 5 to pH 3. How has its acidity changed?

What happens to the pH when you add a base to an acidic solution until they fully neutralize?

Recap

Bronsted-Lowry defines acids as proton donors and bases as proton acceptors, a model broad enough to include ammonia. pH measures hydrogen-ion concentration on a logarithmic scale where each unit is a tenfold change, with 7 neutral, below 7 acidic, and above 7 basic. Neutralization moves pH toward 7.

Reflect

Where in everyday life have you noticed a small pH change producing a surprisingly large effect?