Estonian Basic-School Chemistry Exam common mistakes
Estonian Basic-School Chemistry Exam · Understand the traps and how to avoid them.
The student writes 'H₂O' as 'H2O' or omits subscripts/superscripts in chemical formulas and ionic charges, e.g., writing 'SO4-2' instead of 'SO₄²⁻'.
Students treat chemical notation like plain text, especially when typing or rushing; they may not have practised the difference between coefficient (2H₂O) and subscript (H₂O) or between charge and oxidation state.
What to do instead
Practise writing formulas by hand daily, saying the name aloud while writing (e.g., 'sulfate has two minus charges, written as superscript'). Use a checklist: subscripts for atoms, superscripts for charges, coefficients in front. In digital answers, use proper Unicode or the exam's formula editor.
The student balances equations by changing subscripts in formulas (e.g., turning H₂O into H₂O₂ to balance oxygen) instead of using coefficients.
Students focus on making atom counts equal without understanding that subscripts define the identity of the substance; they may have memorised 'balancing' as making numbers match rather than conserving atoms of the same substance.
What to do instead
Always balance by placing coefficients only in front of formulas. Use a stepwise method: list atoms, balance one element at a time, and never alter subscripts. Check by recounting atoms on both sides and verifying that formulas remain those of the correct substances.
The student calculates amount of substance by dividing mass by the sum of atomic numbers instead of by molar mass, or uses the wrong molar mass because they misread a formula.
Students confuse atomic number with relative atomic mass, or they look up the wrong element in the periodic table; they may also forget to multiply by subscripts when calculating molar mass of a compound.
What to do instead
Write the formula, list each element with its relative atomic mass from the periodic table, multiply by the subscript, and sum. Always include units (g/mol). Practise with common compounds (H₂O, CO₂, NaCl, CaCO₃) until the procedure is automatic.
The student confuses the terms 'strong' and 'concentrated' when describing acids or bases, e.g., calling a dilute hydrochloric acid 'weak' because it is dilute.
Everyday language uses 'strong' for concentrated solutions; students do not distinguish between degree of ionisation (strength) and amount of solute per volume (concentration).
What to do instead
Learn the definitions: strong acid ionises completely in water (HCl, HNO₃, H₂SO₄), weak acid ionises partially (CH₃COOH, H₂CO₃). Concentration is amount of solute per volume. Use examples: 0.1 M HCl is dilute but strong; 6 M CH₃COOH is concentrated but weak.
The student writes neutralisation reactions without including the formation of water, or forgets to balance the salt formula, e.g., HCl + NaOH → NaCl + H₂ instead of NaCl + H₂O.
Students memorise 'acid + base → salt + water' but under exam pressure they may omit water or misapply the products of a different reaction type (e.g., metal + acid → salt + hydrogen).
What to do instead
Write the general word equation first, then replace with formulas. Always check that H and O atoms from the acid and base form H₂O. Practise a set of neutralisations (HCl + KOH, H₂SO₄ + NaOH) and verify balancing by atom count.
The student identifies an unknown solution as acidic or basic using litmus paper but then gives the wrong pH range (e.g., says pH 3 is basic) or misreads universal indicator colour.
Students memorise colours without linking them to the pH scale; they may confuse red litmus turning blue (base) with blue litmus turning red (acid), or they have not used universal indicator chart recently.
What to do instead
Memorise the pH scale: 0–6 acidic, 7 neutral, 8–14 basic. Link litmus: acid turns blue litmus red; base turns red litmus blue. Practise with universal indicator colour chart and common substances (lemon juice pH ~2, soap pH ~10). Always state the pH value and the corresponding nature.
The student describes observations in a practical as 'it reacted' or 'it fizzed' without specifying what was seen (gas bubbles, colour change, precipitate, temperature change) and does not link observations to conclusions.
Students are not trained to separate observation from inference; they may think a vague statement is sufficient, or they lack vocabulary for describing changes (effervescence, precipitate, exothermic).
What to do instead
Use a structured observation frame: before, during, after. Record colour, state, gas, heat, precipitate. Then write a conclusion that names the reaction type and products. Practise with common reactions (Zn + HCl, CuSO₄ + NaOH) and compare observations with inferences.
The student names organic compounds incorrectly, e.g., calls CH₃OH 'methane' or writes 'ethanoic acid' for CH₃CH₂OH, confusing alcohols and carboxylic acids.
Students memorise a few formulas without linking functional group to name; they may not recognise the -OH group in alcohols versus the -COOH group in carboxylic acids, or they confuse the prefix for carbon count.
What to do instead
Learn functional groups: alcohols have -OH (methanol, ethanol), carboxylic acids have -COOH (methanoic acid, ethanoic acid). Use a table of prefixes (meth-, eth-, prop-, but-) and practise naming and drawing formulas. Link to everyday substances (ethanol in drinks, ethanoic acid in vinegar).
The student calculates concentration by dividing mass of solute by volume of solution without converting to moles, or uses volume in cm³ instead of dm³, giving answers off by a factor of 1000.
Students confuse the formula c = n/V with mass concentration; they forget that concentration in mol/dm³ requires amount in moles and volume in dm³, and they may not convert cm³ to dm³ (divide by 1000).
What to do instead
Always write the formula c = n/V, convert mass to moles using n = m/M, and convert volume to dm³ by dividing cm³ by 1000. Include units throughout. Practise with simple examples (0.5 mol in 250 cm³ = 2 mol/dm³) and check that the answer is reasonable.
The student writes reaction equations for metals with acids producing water or carbon dioxide, or predicts no reaction for a metal that is above hydrogen in the reactivity series.
Students mix up the products of different reaction types (metal + acid → salt + hydrogen; metal carbonate + acid → salt + water + CO₂) and may not have memorised the reactivity series or its use in predicting displacement.
What to do instead
Memorise the reactivity series (K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, H, Cu, Ag, Au). Learn the rule: metals above hydrogen displace hydrogen from acids; metals below do not. Practise writing equations for Zn + HCl, Mg + H₂SO₄, Cu + HCl (no reaction) and check products and balancing.