Task Verbs and Free-Response Technique: Exam Skills Guide (Chemistry)

How to use this guide

This guide is for students in the final months before the exam who know the chemistry but lose points in the free-response section. It covers the whole course, not one unit. Read the verb table first, then work through the three worked examples, then try the six tasks in “Practise it” without looking at the answers.

Every question on this page is original Marlbridge practice, not past exam material, and the substances HA, X, Y and Z are fictional. The marks shown are a suggested Marlbridge rubric to help you judge your own answers. They are not official scoring, and no total on this page predicts a score.

The free-response section at a glance

Feature	What to expect
Whole exam	3 hours 15 minutes; 60 multiple-choice questions, then 7 free-response questions
Free-response weight	50% of the exam
Free-response time	105 minutes
Question types	3 long questions (10 points each) and 4 short questions (4 points each), so 46 points in total
Format	Questions are shown in the Bluebook app; you handwrite answers in a paper booklet
Tools	A scientific or graphing calculator is recommended; a periodic table and a formula sheet are provided
Practices	All six science practices appear; Practice 5 (mathematical routines) carries the most weight, and Practice 3 (representing data and phenomena) is assessed only here

Long questions usually have many lettered parts that mix calculation, explanation and drawing. Short questions are focused: often one context and two to four parts.

What each task verb asks you to do

The course publishes a short list of verbs used in free-response questions. The table below gives each one in our own words, with what a full answer needs and a rough guide to length.

Verb	What a complete answer contains	Typical length
Identify / Indicate / Circle (also “What is…?”, “Which…?”)	The requested item only: a species, a value, a direction, a mark on a diagram	A word, a formula or a mark
Calculate	An equation, the numbers put into it, a final value with its unit and sensible significant figures	Two to five lines of working
Determine	A decision or value reached by reasoning, an observation or a calculation; show whichever route you used	One line of reasoning or a short calculation
Estimate	A rough value, or whether a quantity is greater than, less than or equal to another, or its sign, based on given data	One line, plus the data it comes from
Describe	The relevant features of the thing named, without needing to say why	One or two sentences
Explain (also “Give one reason”)	How or why something happens, built from evidence and a chemical principle	Two or three linked sentences
Justify	Evidence that supports the claim and the reasoning that connects that evidence to the claim	One to three sentences, often with a number
Make a claim	A clear statement (increases, decreases, stays the same; A or B) based on evidence or knowledge	One short sentence; usually followed by Justify
Predict	The effect of a change on a system, or the cause of an effect	One sentence, usually followed by a reason
Represent / Draw / Write an equation / Complete a diagram	A correct symbolic, graphical or particle model: balanced equation, Lewis diagram, sketched graph, labelled particle drawing	A drawing or one equation

Two rules sit behind the table. First, answer exactly what the verb asks: an Identify part does not reward a paragraph, and a Justify part does not reward a bare answer. Second, “Explain how” and “Explain why” differ. “How” asks for the steps of the process or relationship; “why” asks for the cause, usually a particle-level reason such as attraction between charges or collisions between particles.

One short example per verb group

Short-answer verbs (Identify, Indicate, Circle). Identify the oxidizing agent in Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Answer: Cu²⁺(aq). That is enough. Adding why it is the oxidizing agent costs time and earns nothing extra.

Number verbs (Calculate, Determine, Estimate). Estimate whether the pH of a buffer with [A⁻] = 3[HA] is above or below the pK_a of HA. Answer: above, because the base form is in excess, so the log term in the Henderson–Hasselbalch equation is positive. An Estimate needs a direction or rough size, not a full calculation, but it still needs the data it rests on.

Reasoning verbs (Explain, Justify, Make a claim, Predict). Predict how the rate of a reaction that is first order in B changes if [B] is halved, and justify. Answer: the rate halves, because rate = k[B] and k does not change at a fixed temperature.

Showing verbs (Describe, Represent, Draw, Write an equation). Write the net ionic equation for the precipitation of silver chloride. Answer: Ag⁺(aq) + Cl⁻(aq) → AgCl(s), with state symbols and no spectator ions.

Reading a multi-part question

1. Read the stem and every part before writing anything. A later part often tells you what the earlier part was aiming at.
2. Underline the verb and the object of each part. “Calculate the mass of …” and “Calculate the number of moles of …” need different final units.
3. Note which data each part uses. New data often appear in the middle of a long question. They apply to the parts after them, not to the parts before.
4. Track the chain. If (c) uses your answer to (b), label your (b) answer clearly so you can find it and reuse it.
5. If you are stuck on a part that later parts need, assume a value. Write “Assume n = 0.0100 mol” and carry on. You keep the chance to earn points for method in every later part.
6. Answer in the right place. Put each part’s answer next to its letter so a reader does not need to hunt for it.

Planning time across the section

The section gives 105 minutes for 46 points, about 2.3 minutes per point. That makes a long question worth roughly 23 minutes and a short one roughly 9 minutes. A practical Marlbridge plan:

Block	Suggested time
3 long questions	about 20 minutes each (60 minutes)
4 short questions	about 9 minutes each (36 minutes)
Final check of units, signs and blank parts	about 9 minutes

Skim all seven questions in the first minute or two. Start with one you feel sure of to build momentum, but do not leave the long questions until last: they carry about 65% of the section’s points. If a part stalls you for more than two or three minutes, write what you can, mark it and move on.

Showing work on calculations

The course defines Calculate as working that includes the expression, the substituted values, and correct units and significant figures. Set out every calculation the same way:

1. Write the equation in symbols (q = mcΔT, PV = nRT, K = [products]/[reactants]).
2. Substitute numbers with units.
3. Give the final answer with a unit, and a sign where direction matters (ΔH, E°, q).
4. Round at the end only, to the significant figures of the least precise data.
5. Check sense: is the value positive when it must be? Is a percent between 0 and 100? Is the pH of an acidic solution below 7?

What earns and loses credit

Usually earns credit	Usually loses credit
A claim plus the specific evidence that supports it	A correct claim with no reason when the verb is Justify
Particle-level language: “attraction between molecules”, “collisions per second”	Vague words: “it is more stable”, “the bonds are stronger” for intermolecular forces
Naming both substances in a comparison	Describing only one side of a comparison
A value computed from the data given (Q, ΔT, ratio of rates)	Quoting a rule (“Le Châtelier”) without applying it to the data
Units, sign and state symbols where asked	Dropping the minus sign on an exothermic ΔH
One clear answer	Two contradictory answers, hoping one is right

Worked example 1: a comparison explanation (Unit 3)

Question. Methanol, CH₃OH, is a liquid at room temperature. Ethane, C₂H₆, is a gas. Explain this difference in terms of intermolecular forces. (2 points)

Weak answer. “Methanol has hydrogen bonds, so its bonds are stronger and harder to break.”

What it misses. It says “bonds”, which suggests covalent bonds inside molecules are broken on boiling; they are not. It does not deal with London dispersion forces, so it never shows why hydrogen bonding decides the comparison. It says nothing about ethane.

Strong answer. Both molecules have 18 electrons and similar molar masses (32.04 and 30.07 g mol⁻¹), so their London dispersion forces are similar. Methanol also has an O–H group, so its molecules form hydrogen bonds with each other; ethane molecules cannot. The total attraction between methanol molecules is therefore greater, more energy is needed to separate them, and methanol stays liquid at room temperature.

Suggested Marlbridge rubric	Point
Says dispersion forces are similar (similar electron count or size) and both substances are discussed	1
Identifies hydrogen bonding between methanol molecules (O–H) as the extra, stronger attraction that needs more energy to overcome	1

Worked example 2: a calculation with sign and units (Unit 6)

Question. Excess zinc powder is added to 50.0 mL of 0.400 M CuSO₄(aq) in an insulated cup. The temperature rises from 21.3 °C to 39.8 °C. Calculate ΔH for Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), in kJ per mole of Cu²⁺. Use the data in the calculator note. (3 points; the data are invented)

Weak answer. “q = 50 × 4.18 × 18.5 = 3866. 3866 ÷ 0.02 = 193 300. ΔH = 193.”

What it misses. There is no unit anywhere. The value 193 300 is in J mol⁻¹ but is then quoted as 193 with no conversion stated. The sign is wrong: the temperature rose, so the reaction released energy and ΔH must be negative. The moles of Cu²⁺ are not shown being calculated.

Strong answer.

1. ΔT = 39.8 − 21.3 = 18.5 °C. Mass of solution = 50.0 mL × 1.00 g mL⁻¹ = 50.0 g.
2. q(solution) = mcΔT = 50.0 g × 4.18 J g⁻¹ °C⁻¹ × 18.5 °C = 3866.5 J = 3.87 kJ absorbed by the solution.
3. n(Cu²⁺) = 0.0500 L × 0.400 mol L⁻¹ = 0.0200 mol. Zinc is in excess, so all the Cu²⁺ reacts.
4. q(reaction) = −3866.5 J. ΔH = −3866.5 J ÷ 0.0200 mol = −193 000 J mol⁻¹ = −193 kJ mol⁻¹ (3 significant figures).

Suggested Marlbridge rubric	Point
Correct q for the solution with q = mcΔT shown	1
Correct moles of the limiting reactant, Cu²⁺	1
ΔH = −193 kJ mol⁻¹, with negative sign and unit	1

Worked example 3: predict and justify with data (Unit 7)

Question. For the fictional reaction 2X(g) + Y(g) ⇌ 2Z(g), K = 25 at a certain temperature. A mixture at that temperature has [X] = 0.20 M, [Y] = 0.10 M and [Z] = 0.50 M. Predict whether [Z] increases, decreases or stays the same as the system moves to equilibrium. Justify your prediction. (2 points)

Weak answer. “[Z] decreases because of Le Châtelier’s principle: there is too much product.”

What it misses. “Too much product” is a guess until you compare it with K. The verb Justify needs evidence, and the evidence here is a calculated Q.

Strong answer. Q = [Z]² ÷ ([X]²[Y]) = (0.50)² ÷ ((0.20)² × 0.10) = 0.25 ÷ 0.0040 = 62.5. Because Q (62.5) > K (25), the ratio of products to reactants is too large. The net reaction runs in reverse until Q falls to K, so [Z] decreases.

Suggested Marlbridge rubric	Point
Correct Q expression and value (62.5)	1
Compares Q with K and concludes the net reaction goes to the left, so [Z] decreases	1

Common mistakes

• Writing a paragraph for an Identify part, then running out of time on a Calculate part worth more.
• Giving a claim with no evidence when the verb is Justify or Explain.
• Calling intermolecular forces “bonds”, or saying covalent bonds break when a molecular substance boils.
• Leaving out units, or the minus sign on an exothermic ΔH or a negative ΔS.
• Rounding in the middle of a calculation, which shifts the final digit.
• Using data from later in a long question to answer an earlier part that did not supply it.
• Naming a rule (Le Châtelier, Coulomb’s law) without applying it to the particles or numbers in front of you.
• Comparing two substances but describing only one of them.
• Writing two different answers and leaving both visible.

Practise it

Task 1 (Unit 1). Identify which atom, Na or Cl, has the smaller atomic radius. Justify your answer.

Model answer

Cl. Both atoms have valence electrons in the n = 3 shell and both have 10 core electrons. Cl has 17 protons and Na has 11, so Cl’s valence electrons feel a larger effective nuclear charge and are pulled closer to the nucleus. (A claim plus a Coulomb-style reason: 2 points in a suggested Marlbridge rubric.)

Task 2 (Unit 2). Draw the Lewis diagram for formaldehyde, CH₂O (C is central). Determine the H–C–H bond angle to the nearest common value.

Model answer

12 valence electrons (4 + 2 + 6). C is single-bonded to each H and double-bonded to O; O carries two lone pairs; every atom except H has an octet. C has three electron domains and no lone pairs, so the geometry is trigonal planar and the angle is about 120°.

Task 3 (Unit 4). Solutions of barium nitrate and sodium sulfate are mixed and a white solid forms. Write the net ionic equation and identify the spectator ions.

Model answer

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The spectator ions are Na⁺(aq) and NO₃⁻(aq). State symbols matter: without (s) the equation does not show that a precipitate forms.

Task 4 (Unit 5). For the fictional reaction A → products, doubling [A] from 0.10 M to 0.20 M increases the initial rate from 2.0 × 10⁻³ M s⁻¹ to 8.0 × 10⁻³ M s⁻¹. Determine the order with respect to A, then calculate k with units.

Model answer

Doubling [A] multiplies the rate by 8.0 × 10⁻³ ÷ 2.0 × 10⁻³ = 4 = 2², so the reaction is second order in A. Rate = k[A]², so k = 2.0 × 10⁻³ M s⁻¹ ÷ (0.10 M)² = 0.20 M⁻¹ s⁻¹. Check with the second run: 0.20 × (0.20)² = 8.0 × 10⁻³ M s⁻¹.

Task 5 (Unit 8). A buffer contains 0.10 M of the fictional weak acid HA (pK_a = 4.20) and 0.30 M of its conjugate base A⁻. Estimate whether the pH is above or below 4.20, then calculate it.

Model answer

Estimate: above 4.20, because [A⁻] > [HA]. Calculate: pH = pK_a + log([A⁻]/[HA]) = 4.20 + log(0.30/0.10) = 4.20 + 0.48 = 4.68. The calculated value agrees with the estimate.

Task 6 (Unit 9). Make a claim about the sign of ΔS° for N₂(g) + 3H₂(g) → 2NH₃(g). Justify your claim.

Model answer

ΔS° is negative. Four moles of gas become two moles of gas. Fewer gas particles have fewer ways to arrange themselves and their energy, so the entropy of the system decreases. (Claim with sign: 1 point; reasoning from moles of gas: 1 point, suggested Marlbridge rubric.)

Where to practise next

• Intermolecular and interparticle forces study guide for more comparison explanations.
• Heat capacity and calorimetry practice for calculations that need sign and units.
• Reaction quotient and Le Châtelier’s principle study guide for predict-and-justify parts.
• Introduction to rate law practice for Determine parts with data tables.
• Net ionic equations study guide for Write an equation parts.
• Properties of buffers practice for Estimate and Calculate together.
• Unit 7 review for multi-part questions that mix verbs.