How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: 0.20 kg of water is heated from 25 °C to 75 °C. Take c = 4 200 J·kg⁻¹·°C⁻¹. Calculate the heat transferred to the water, and say whether this change for the water is endothermic or exothermic.
In other words — as if you have never seen this before
If you have never seen this before: Q = mcΔT. The temperature change is final minus initial. Heat entering the water is endothermic for the water. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
m=0.20 kg, T₁=25 °C, T₂=75 °C, c=4 200.
3. Which method — and why this one?
Q = mcΔT. The temperature change is final minus initial. Heat entering the water is endothermic for the water.
4. Step-by-step working
01 / 07
Subtract
ΔT = 75 − 25 = 50 °C. Use the change, not the starting temperature.
75−25=50Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 07
Write the formula
Q = mcΔT. Mass must stay in kilograms because c is per kilogram.
Q=mcΔTWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 07
Multiply
0.20 × 4 200 = 840. That is the heat for a 1 °C rise.
0.20×4 200=840Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 07
Multiply
840×50=42 000 J.
840×50=42 000Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
05 / 07
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
06 / 07
Step F — Box the answer with its unit
42 000 J, endothermic for the water.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
07 / 07
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
42 000 J, endothermic for the water.
Copy onto your answer sheet — in this order
Formula: 75−25=50
Answer: 42 000 J, endothermic for the water.
6. Check it like a marker
Using 25 or 75 as ΔT, or leaving the mass in grams without changing c, gives the wrong size. The sign of the question is heat in, so the water’s change is endothermic.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”