Acids, Bases, and pH

Defining Acids and Bases

The Regents expects two definitions. An Arrhenius acid is a substance that produces hydrogen ions (H+) — effectively hydronium ions (H3O+) — when dissolved in water. An Arrhenius base produces hydroxide ions (OH-) in water. The broader Bronsted-Lowry model defines an acid as a proton (H+) donor and a base as a proton acceptor.

The Bronsted-Lowry view matters because it explains reactions in which no OH- is present — for example ammonia (NH3) accepting a proton to become NH4+. Knowing both models is a common 1-credit question.

Properties You Can Test

Acids and bases have distinct, testable properties used in the required Investigation on acids and bases:

• Acids taste sour, turn blue litmus red, react with active metals to release H2 gas, and conduct electricity (they are electrolytes).
• Bases taste bitter, feel slippery, turn red litmus blue, and also conduct electricity.
• Both produce ions in solution, so both are electrolytes; pure water and most molecular compounds are not.

Acid-Base Indicators table (legacy Table M)

The 2025 NYS Chemistry Reference Tables include the Acid-Base Indicators section (the section the legacy edition labeled Table M), listing indicators and their color-change pH ranges. For example, litmus changes around pH 5.5-8.0 and bromthymol blue changes around pH 6.0-7.6. On a cluster, match the observed color to the table to estimate pH — do not guess.

The pH Scale

pH measures how acidic or basic a solution is on a scale from 0 to 14:

The scale is logarithmic: each change of one pH unit equals a tenfold change in hydrogen-ion concentration. A solution of pH 3 has ten times more H+ than pH 4 and one hundred times more H+ than pH 5. As pH falls, [H+] rises and [OH-] falls. This single relationship is one of the most-tested acid-base facts.

Worked pH Example

A solution changes from pH 6 to pH 4. The pH dropped 2 units, so [H+] increased by a factor of 10 x 10 = 100. The solution became more acidic.

Neutralization

Neutralization is the reaction of an acid with a base to form a salt and water:

acid + base → salt + water

The driving reaction is always H+ + OH- → H2O. The salt is the ionic compound formed from the acid's negative ion and the base's positive ion. For example:

HCl + NaOH → NaCl + H2O

Here the H+ from HCl and OH- from NaOH combine to make water, leaving Na+ and Cl- to form the salt sodium chloride.

Titration Calculations

Titration is a lab procedure that finds an unknown concentration by adding a measured volume of a known solution until neutralization. The Regents working equation is:

Ma x Va / na = Mb x Vb / nb

where M is molarity, V is volume, and n is the number of H+ or OH- supplied per formula unit. For a single-proton (monoprotic) acid and a single-OH base, n values are 1 and the relationship simplifies to Ma x Va = Mb x Vb.

Worked Titration

25.0 mL of HCl is neutralized by 20.0 mL of 0.50 M NaOH. Find the HCl molarity.

1. Both are monoprotic/single-OH, so use Ma x Va = Mb x Vb.
2. Ma x 25.0 = 0.50 x 20.0.
3. Ma = 10.0 / 25.0 = 0.40 M HCl.

Diprotic-Acid Trap

A diprotic acid such as H2SO4 supplies two H+ per formula unit, so it needs twice the moles of base to neutralize. Forgetting the factor of 2 (using n = 1 instead of n = 2) is a classic error that halves or doubles your answer.

Strong Versus Weak Acids and Bases

The Regents distinguishes strong from weak by how completely a substance ionizes, not by concentration. A strong acid (HCl, HNO3, H2SO4) ionizes almost completely in water, releasing nearly all of its H+. A weak acid (such as acetic acid, CH3COOH) ionizes only slightly, so most molecules stay intact.

The Common Acids table (legacy Table K) lists common acids and the Common Bases table (legacy Table L) lists common bases. Strong acids and bases are strong electrolytes — they conduct electricity well because they produce many ions. A weak acid is a weak electrolyte even at high concentration.

Do not confuse "strong" with "concentrated." A dilute solution of a strong acid is still a strong acid; a concentrated solution of a weak acid is still weak. This is a frequent multiple-choice distractor.

The Acids-and-Bases Investigation

One of the three required NYSED Investigations focuses on properties of acids and bases. Expect a cluster built around lab data: a student tests household substances with indicators, records colors, and estimates pH using the Acid-Base Indicators table (legacy Table M). You may be asked to write a claim-evidence-reasoning response — state which substance is most acidic, cite the indicator color or pH, and reason from the data. Practice this format because constructed-response items are about 40% of the exam.

Common Mistakes to Avoid

• Reversing the pH direction — remember lower pH means MORE acidic and MORE H+.
• Saying a tenfold concentration change equals a 10-unit pH change (it is a 1-unit change).
• Forgetting units (mL versus L) in titration setups — volumes can stay in mL as long as both sides match.
• Calling water or sugar solution an electrolyte; they do not ionize.
• Mislabeling the salt: it forms from the spectator ions, not from H+ and OH-.