What defines an acid and a base on the Regents, and what does the pH scale tell you?
Acids, bases and the pH scale: identify Arrhenius acids and bases, interpret the pH scale, and relate a change in pH to a change in hydrogen ion concentration.
A focused Regents Chemistry answer on Arrhenius acids and bases, the pH scale, and how each pH unit means a tenfold change in hydrogen ion concentration, using Table K and Table L of the Reference Tables.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The Core Curriculum asks you to identify Arrhenius acids and bases, to interpret the pH scale, and to relate a change in pH to a change in hydrogen ion concentration. The Regents uses the Arrhenius definitions and lists common acids and bases on Table K and Table L. This page sets up neutralization and titration.
Arrhenius acids and bases
Common acids on Table K include hydrochloric acid (), sulfuric acid () and nitric acid (); common bases on Table L include sodium hydroxide () and potassium hydroxide (). You can often recognize an acid because its formula starts with hydrogen, and a base because it contains hydroxide. The Regents uses these Arrhenius definitions, not the broader Bronsted-Lowry definition (which is honors or AP content).
Strong and weak
The Regents treats strength qualitatively: you should know that strong acids and bases ionize more fully than weak ones, but you are not asked to calculate ionization constants. Strength (how fully it ionizes) is a different idea from concentration (how much is dissolved).
The pH scale
So a substance with pH is strongly acidic, pH is neutral, and pH is strongly basic. Indicators and pH meters are used to measure pH in the laboratory, and the Reference Tables include indicator information for choosing a suitable one.
Each pH unit is a factor of ten
This logarithmic relationship is a favorite Part B-2 question: "how many times more acidic" means count the pH units and raise ten to that power. From pH to pH is four units, so times more hydrogen ions.
Try this
Q1. State the only negative ion produced by an Arrhenius base in solution. [1 point]
- Cue. The hydroxide ion, .
Q2. State whether a solution of pH is acidic, neutral or basic, and how its compares with pH . [1 point]
- Cue. Acidic; it has ten times the hydrogen ion concentration of pH .
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part A style)1 marksAccording to the Arrhenius theory, an acid is a substance that produces, as the only positive ion in an aqueous solution, (1) (hydrogen ion) (2) (3) (4) Show worked answer →
A 1-point Part A item on the Arrhenius definition. The answer is (1) (hydrogen ion).
An Arrhenius acid is a substance that produces hydrogen ions (, equivalently the hydronium ion ) as the only positive ion in aqueous solution. An Arrhenius base produces hydroxide ions () as the only negative ion. Choices 3 and 4 are spectator ions, not the defining ion.
Markers reward identifying the hydrogen ion as the defining positive ion of an Arrhenius acid.
Regents (Part B-2 style)3 marksTwo solutions are compared: solution X has a pH of 2 and solution Y has a pH of 5. (a) State which solution is more acidic. (b) State how many times greater the hydrogen ion concentration of the more acidic solution is. (c) State whether a solution of pH 9 is acidic, basic or neutral.Show worked answer →
A 3-point constructed-response item on the pH scale.
(a) More acidic (1 point): solution X (pH 2) is more acidic, because a lower pH means a higher hydrogen ion concentration.
(b) Factor (1 point): each pH unit is a tenfold change, so a difference of pH units (from 5 to 2) is times greater hydrogen ion concentration.
(c) pH 9 (1 point): pH greater than 7 is basic, so a solution of pH 9 is basic.
Markers reward identifying the lower pH as more acidic, applying the tenfold-per-unit rule (so times for three units), and classifying pH 9 as basic.
Related dot points
- Neutralization and salts: write neutralization reactions of an acid with a base to form a salt and water, and identify the salt produced.
A focused Regents Chemistry answer on neutralization: how an acid and a base react to form a salt and water, how to predict the salt from the acid's anion and the base's cation, and the role of the hydrogen and hydroxide ions.
- Titration: use titration data and the Table T titration relationship to calculate the unknown concentration of an acid or base.
A focused Regents Chemistry answer on titration: the laboratory procedure, the role of the indicator and the endpoint, and the Table T relationship M_A V_A = M_B V_B for a one-to-one acid-base reaction, with a worked calculation.
- Equilibrium and Le Chatelier's principle: describe dynamic equilibrium and predict the shift in a system when concentration, temperature or pressure is changed.
A focused Regents Chemistry answer on dynamic equilibrium and Le Chatelier's principle: equal forward and reverse rates, and how a change in concentration, temperature or pressure shifts the equilibrium to relieve the stress.
- Reaction rates and collision theory: use collision theory to explain how concentration, temperature, surface area and a catalyst affect the rate of a reaction.
A focused Regents Chemistry answer on reaction rates and collision theory: why effective collisions need enough energy and the right orientation, and how concentration, temperature, surface area, the nature of the reactants and a catalyst change the rate.
- Oxidation numbers and redox reactions: assign oxidation numbers using the standard rules, and identify oxidation, reduction, and the oxidizing and reducing agents in a reaction.
A focused Regents Chemistry answer on oxidation numbers and redox: the rules for assigning oxidation states, the meaning of oxidation (loss of electrons) and reduction (gain of electrons), and how to identify the oxidizing and reducing agents.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)