TASC Chemistry Level 4 common mistakes
TASC Chemistry Level 4 · Understand the traps and how to avoid them.
When writing the formula of an ionic compound, the candidate copies the charge numerals directly as subscripts without reducing or checking neutrality, producing formulas such as CaCl instead of CaCl2 or AlO instead of Al2O3.
The criss-cross method is memorised as a visual trick, but the candidate does not connect the resulting formula to the requirement that total positive and negative charge must sum to zero.
What to do instead
After drafting a formula, write the charge balance underneath: for each ion, charge multiplied by subscript, then add. If the sum is not zero, adjust subscripts to the lowest whole-number ratio. Practise with polyatomic ions such as sulfate and phosphate so the bracket rule becomes automatic.
Balances a chemical equation by altering subscripts inside a formula, for example changing H2O to H2O2 or CO to CO2, rather than placing coefficients in front of the species.
The candidate treats the equation as an arithmetic puzzle and does not accept that a subscript defines the identity of the substance; changing it changes the chemical.
What to do instead
Use an atom-inventory table with one row per element and columns for reactants and products. Adjust only coefficients. After each change, recount every element, including hydrogen and oxygen, and confirm the ratio is in lowest terms.
In a limiting-reagent problem, selects the reactant present in the smallest number of moles as limiting without dividing by its coefficient in the balanced equation.
The candidate reasons from everyday 'smallest amount runs out first' intuition rather than from stoichiometric proportion.
What to do instead
For each reactant, calculate moles available divided by its coefficient in the balanced equation. The smallest quotient identifies the limiting reagent. Then use that reactant's moles to calculate product mass, and check the other reactant is in excess.
During a practical investigation, records only the final measurement and does not list controlled variables or repeat trials, so anomalous results cannot be identified or explained.
The candidate treats the practical as a sequence of steps to complete rather than an investigation designed to test a variable.
What to do instead
Before starting, draw a table with columns for independent, dependent and at least three controlled variables, and plan three repeats. Record every raw reading, then calculate a mean and flag any value that lies far from the others for re-testing.
Reads a burette from the bottom of the scale or records the initial volume as the first reading after filling, then subtracts incorrectly and reports a negative or impossible titre.
The burette scale increases downwards with zero at the top, which is the opposite of a measuring cylinder; the candidate transfers the wrong mental model.
What to do instead
Practise reading a burette diagram at eye level, using the bottom of the meniscus. Always record initial and final volumes to two decimal places, then titre equals final minus initial. Sanity-check that the titre is positive and within the expected rough range.
Writes a precipitation or neutralisation reaction as a full formula equation when asked for the ionic equation, or omits state symbols and leaves spectator ions in the final line.
The candidate focuses on getting the formulas right and does not distinguish between full, ionic and net ionic representations.
What to do instead
Use a three-line routine: full balanced equation with states; split all soluble ionic compounds into ions; cancel ions that appear unchanged on both sides. The final net ionic equation must show only the species that actually react, with states.
Draws a Lewis structure after counting only the bonding electrons, then predicts molecular shape from that incomplete structure, missing lone pairs on the central atom.
The candidate jumps to a bonding diagram without first totalling valence electrons and distributing them systematically.
What to do instead
Count total valence electrons first. Place single bonds, complete octets on terminal atoms, then place remaining electrons as lone pairs on the central atom. Check formal charges. Only after the electron-dot structure is complete should VSEPR shape be assigned.
In an extended response, uses vague phrases such as 'it reacts faster' or 'the substance changes' without naming the species, stating the evidence or linking the evidence to a chemical idea.
The candidate writes as if speaking to a classmate who can see the same experiment, rather than for an external reader.
What to do instead
Use a claim-evidence-reasoning paragraph. Claim: name the species and the effect. Evidence: quote the numerical result or observation. Reasoning: connect the evidence to a named principle such as collision theory or Le Chatelier's principle.
Assumes that a product labelled 'natural', 'green' or 'plant-based' is automatically safer or more environmentally friendly than a synthetic alternative, without comparing data.
The candidate repeats marketing language rather than evaluating evidence about toxicity, persistence, energy use or waste.
What to do instead
Compare at least two specific indicators for each option, such as biodegradability, acute toxicity, energy input or by-product mass. State the source of any claim and distinguish hazard from risk.
Evaluates an industrial process or fuel by stating a slogan such as 'hydrogen is clean' or 'recycling is always better' without quantifying trade-offs such as energy source, by-products or cost.
The candidate treats application questions as opportunities to repeat familiar environmental messages rather than to analyse a system.
What to do instead
Use life-cycle thinking: identify the raw material, energy source, main product, by-products and end-of-life pathway. Present one quantitative comparison, for example kilograms of CO2 per kilogram of product or energy per mole, and state one limitation of the comparison.