Chemistry Regents review
Acids, bases, and pH: a night-before review
We wrote this for the new Regents exam, Physical Science: Chemistry, and its 2025 Reference Tables, the pages of charts and data you get to use during the exam. The next exam date is Tuesday, January 26, 2027.
On the June and August 2026 exams, 10 of the 98 questions were about acids, bases, and pH, about one question in ten. The 2025 Reference Tables have no acid, base, or indicator table and no pH formula, so you need to know what pH means without looking it up. On the exams it has released, New York printed an indicator chart beside every question about indicators.
Taking the legacy Physical Setting/Chemistry exam instead? Your reference tables still list common acids, bases, and indicators in Tables K, L, and M.
Answer each question from memory, out loud or on paper, before you open its answer. In the study behind this advice (Karpicke and Roediger, 2007), three groups of college students had the same number of turns with a list of words. The group that spent more of those turns recalling the words remembered more a week later than the group that spent more of its turns studying the list again.
The six kinds of exam question
The New York State Education Department (NYSED) asked about acids, bases, and pH in these six ways on its 2026 exams and sample clusters.
A lake sample has a pH of 4, and pure water has a pH of 7. Which has the greater [H₃O⁺], and how many times greater?
Check your answer to question 1
The lake sample, with 1,000 times the [H₃O⁺] of pure water. Each pH unit is a factor of 10, so 3 units is 10 × 10 × 10, not 3 or 30.
Methyl orange is red below pH 3.2 and yellow above 4.4. It turns a sample yellow. What do you know about the sample’s pH?
Check your answer to question 2
Only that it’s above 4.4. The sample could still be acidic.
Sea water has a pH of about 8. Is it acidic, basic, or neutral?
Check your answer to question 3
Basic, because 8 is above 7.
In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, which particle is the base, and what does it do?
Check your answer to question 4
NH₃ is the base. It accepts a proton (H⁺) from water and becomes NH₄⁺, so here water is the acid.
You add baking soda, a source of HCO₃⁻, to water where CO₂(aq) + 2H₂O(l) ⇌ H₃O⁺(aq) + HCO₃⁻(aq). Which way does the equilibrium shift, and what happens to the pH?
Check your answer to question 5
It shifts to the left, toward CO₂. The reverse reaction uses up H₃O⁺, so [H₃O⁺] falls and the pH rises.
The water in an aquarium is at pH 6, and the fish do best at pH 7. Should you raise or lower [H₃O⁺], and by what factor?
Check your answer to question 6
Lower it to a tenth of what it is now, since pH 7 has one tenth the [H₃O⁺] of pH 6. Adding a base does that.
Know these without a table
What does a pH of 7 mean, and which side of 7 is acidic?
Check your answer to question 7
A pH of 7 is neutral. Below 7 is acidic, and above 7 is basic.
What range of pH values do you work with in class?
Check your answer to question 8
In class you work with pH values from 0 to 14, starting at 0, not 1. A very concentrated acid can have a pH below 0, and a very concentrated base a pH above 14.
When a solution’s pH drops by 1, what happens to its [H₃O⁺]?
Check your answer to question 9
It goes up by a factor of 10. At each step down the pH scale, [H₃O⁺] is ten times greater.
Write the hydronium ion concentration at pH 3 in the exam’s notation.
Check your answer to question 10
[H₃O⁺] = 1 × 10⁻³ mol/L.
Read the indicator chart
Published ranges differ slightly between sources; these come from the University of British Columbia.
| Indicator | Changes between pH | Color below | Color above |
|---|---|---|---|
| Methyl orange | 3.2–4.4 | red | yellow |
| Bromcresol green | 3.8–5.4 | yellow | blue |
| Bromthymol blue | 6.0–7.6 | yellow | blue |
| Phenolphthalein | 8.2–10.0 | colorless | pink |
A sample turns bromcresol green blue and bromthymol blue yellow. Using the chart, what range is its pH in?
Check your answer to question 11
Between 5.4 and 6.0. Blue bromcresol green means it’s above 5.4, and yellow bromthymol blue means it’s below 6.0.
Using the chart, give the color of each of the four indicators in a sample at pH 5.7.
Check your answer to question 12
Methyl orange: yellow, since 5.7 is above its range. Bromcresol green: blue, above its range. Bromthymol blue: yellow, below its range. Phenolphthalein: colorless, below its range.
Which indicator on the chart tells a sample at pH 7.8 apart from one at pH 10.5?
Check your answer to question 13
Phenolphthalein. It is colorless at 7.8 and pink at 10.5, and each of the other three shows the same color at both.
Traps that cost credit
NYSED hadn’t asked about the two marked “From the standard” on a released exam by August 2026.
What is the name of the ion OH⁻?
Check your answer to question 14
Hydroxide. Hydroxyl is the name of the –OH group bonded inside a molecule, not of the ion. NYSED has told graders, in the rating guides it has released, to give no credit for “hydroxyl” in place of “hydroxide.”
Write the formula for the particle an acid donates.
Check your answer to question 15
H⁺, with its charge. H is a neutral hydrogen atom and H₂ is a hydrogen molecule. NYSED has told graders, in the rating guides it has released, to give no credit for H or H₂ in place of H⁺.
Does adding a catalyst shift an equilibrium?
Check your answer to question 16
No. A catalyst speeds up the forward and reverse reactions equally, so the balance point doesn’t move. Regents graders have given no credit for naming a catalyst as the change that shifts an equilibrium.
Methane, CH₄, has four hydrogen atoms. Is it an acid?
Check your answer to question 17
No. An acid donates a proton, and methane’s hydrogens stay bonded to carbon.
Is unpolluted rain neutral?
Check your answer to question 18
No. Its pH is about 5.6, slightly acidic, because carbon dioxide from the air dissolves in it.
Does “strong acid” mean the same thing as “concentrated acid”? From the standard
Check your answer to question 19
No. A strong acid ionizes completely in water, breaking apart into ions, even when it’s dilute. Concentrated means there’s a lot of acid in each liter.
You add sodium hydroxide to acetic acid until all the acid has reacted. Is the solution at pH 7? From the standard
Check your answer to question 20
No. It is above 7, about 8.7 when the acid and the base are both 0.100 mol/L, because the acetate ion left behind is a weak base.
You write a claim but leave out its evidence. How much credit does that written answer earn?
Check your answer to question 21
None. Each written answer is worth one credit, all or nothing, and you earn it only when your answer has every part you’re asked for.
Missed some? Work through the free pH module.