How to Determine If a Substance Is Acidic: A Practical Guide

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By Philip J. Carlson, Updated Aug 30, 2022

Acidity is a strictly defined chemical property, not a measure of how destructive a substance appears. While common imagery links acids with corrosion and bases with rust, chemists assess acidity by the behavior of hydrogen ions in solution.

Defining Acids and Bases

Arrhenius: Acids increase the concentration of ∼+ ions (protons) when dissolved in water; bases increase ∻- (hydroxide) ions.
Brønsted–Lowry: An acid donates a proton to another species; a base accepts a proton.
Lewis: An acid is an electron‑pair acceptor; a base is an electron‑pair donor.

For most everyday applications, the Arrhenius and Brønsted–Lowry definitions are sufficient.

Practical Test: Baking Soda Reaction

One of the simplest ways to check if a liquid is acidic is to add a small amount of baking soda (sodium bicarbonate). If the solution contains an acid, it will react, producing visible bubbles of carbon dioxide:

Acid + NaHCO₃ → Na⁺ + CO₂ + H₂O + OH^-

No bubbling indicates the absence of significant acidity.

Acid Strengths: From Strong to Weak

A strong acid fully dissociates in water, releasing all its protons. A weak acid only partially dissociates, leaving a mixture of intact molecules and ions. Substances that contain hydrogen but show no dissociation in water are considered non‑acidic.

The pH Scale

The pH scale quantitatively expresses acidity:

pH < 7: Acidic
pH = 7: Neutral
pH > 7: Basic (alkaline)

Measuring pH

Several reliable methods exist:

Litmus paper – Color change indicates approximate pH when matched to a standard chart.
Solution indicators – Adding a color‑changing reagent directly to the liquid shows its pH class.
Titration – A precise laboratory technique that determines pH by neutralizing the sample with a base or acid of known concentration.
pH meter – The most accurate and user‑friendly method; an electrode measures the electrical potential related to hydrogen ion concentration.