What makes a substance an acid or a base, and what does the pH scale measure?
Acids, bases and the pH scale: describe the properties and definitions of acids and bases, the pH scale, and the relationship between pH and hydrogen ion concentration.
A focused Virginia SOL Chemistry answer on acids and bases under CH.5: the Arrhenius and Bronsted-Lowry definitions, the properties of acids and bases, the pH scale from 0 to 14, and how pH relates to hydrogen ion concentration and strength.
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What this topic is asking
Standard CH.5 asks you to define acids and bases, to know their properties, and to use the pH scale and its link to hydrogen ion concentration. Virginia expects the Arrhenius definition (and an introduction to Bronsted-Lowry), the range of the pH scale, and the idea that each pH unit is a factor of ten in hydrogen ion concentration.
Defining acids and bases
So hydrochloric acid () is an acid because it releases in water, and sodium hydroxide () is a base because it releases . The Bronsted-Lowry view is more general because it focuses on the transfer of a proton, which covers reactions that do not happen in water.
Properties of acids and bases
Indicators such as litmus give a quick qualitative test; a pH meter or universal indicator gives a numerical value. Strong acids and bases ionize completely in water, producing many ions, while weak acids and bases ionize only partly.
The pH scale
Because the scale is logarithmic, small differences in pH mean large differences in concentration: a pH of is ten times more acidic than pH and a hundred times more acidic than pH . The lower the pH, the higher the hydrogen ion concentration; the higher the pH, the higher the hydroxide ion concentration. Knowing this factor-of-ten relationship is the most commonly tested quantitative point.
Try this
Q1. State whether a solution with a pH of is acidic, neutral or basic. [1 point]
- Cue. Basic; a pH above is basic.
Q2. Name one property that distinguishes a base from an acid. [1 point]
- Cue. Any one: bases feel slippery, taste bitter, turn red litmus blue, or have a pH above (acids do the opposite).
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksA solution has a pH of . This solution is (A) strongly basic (B) weakly basic (C) acidic (D) neutralShow worked answer →
The answer is (C) acidic.
On the pH scale, values below are acidic, is neutral, and above is basic. A pH of is below , so the solution is acidic. The lower the pH, the more acidic, so a pH of is a fairly strong acid.
The trap is reading the scale backward; a low pH means more acidic (more hydrogen ions), not basic.
SOL (tech-enhanced, fill in the blank)2 marks(a) State how the hydrogen ion concentration of a solution at pH compares with one at pH . (b) Name a property shared by acids.Show worked answer →
A 2-point item on the pH scale and acid properties.
(a) Comparison (1 point): the pH solution has a higher hydrogen ion concentration; each pH unit is a factor of , so pH is times more concentrated in hydrogen ions than pH (two units, ).
(b) Property (1 point): any one of: tastes sour, turns blue litmus red, reacts with active metals to release hydrogen gas, or has a pH below .
Markers reward recognizing the factor-of-ten nature of the pH scale and naming a valid acid property.
Related dot points
- Neutralization and titration: write neutralization reactions that form a salt and water, and use titration data to find an unknown concentration.
A focused Virginia SOL Chemistry answer on neutralization under CH.5: the acid plus base gives salt plus water reaction, the role of indicators and the equivalence point, and using titration data with M1V1 = M2V2 to find an unknown concentration.
- Solutions, solubility and concentration: describe solutes, solvents and the dissolving process, the factors that affect rate of dissolving and solubility, and how to read a solubility curve.
A focused Virginia SOL Chemistry answer on solutions under CH.5: solute and solvent, the dissolving process and like dissolves like, the factors that change the rate of dissolving and solubility, saturated and unsaturated solutions, and reading a solubility curve.
- Molarity and solution stoichiometry: calculate molarity, prepare and dilute solutions, and use molarity in solution stoichiometry.
A focused Virginia SOL Chemistry answer on concentration under CH.5: molarity as moles per liter, calculating molarity, the dilution equation M1V1 = M2V2, and using molarity to find moles in solution stoichiometry.
- Types of chemical reactions: classify reactions as synthesis, decomposition, single replacement, double replacement or combustion, and predict their products.
A focused Virginia SOL Chemistry answer on reaction types under CH.3: the five categories (synthesis, decomposition, single replacement, double replacement, combustion), how to recognize each, and how to predict the products including using an activity series.
- Naming compounds and writing formulas: name and write formulas for ionic compounds (including polyatomic ions), binary molecular compounds and simple acids.
A focused Virginia SOL Chemistry answer on nomenclature under CH.3: writing formulas for ionic compounds by balancing charges (the crossover method), using polyatomic ions and roman numerals, and naming binary molecular compounds with prefixes and simple acids.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)