Chemical changes · GCSE Chemistry
Temperature change required practical
Plan the GCSE Chemistry temperature-change investigation: exothermic or endothermic dissolving or neutralisation, thermometer readings, insulation, fair test, mean temperature change and graph.
Measure the surroundings with a thermometer. Exothermic: temperature rises. Endothermic: temperature falls. Insulate with a polystyrene cup and lid. Plot mean ΔT against concentration or mass.
The important bits
What you need to know
- 1
Aim: investigate whether a process is exothermic or endothermic by measuring the temperature change of the surroundings (usually water or solution in a polystyrene cup).
- 2
Exothermic examples: neutralisation of HCl with NaOH; dissolving sodium hydroxide or anhydrous copper(II) sulfate in water — temperature of the solution rises.
- 3
Endothermic examples: dissolving ammonium nitrate or ammonium chloride in water — temperature of the solution falls. Thermal decomposition demonstrations are endothermic but are usually shown, not quantified in this practical.
- 4
Independent variable examples: mass of solid dissolved, volume or concentration of acid/alkali, or type of salt. Dependent variable: temperature change ΔT (°C) or maximum/minimum temperature reached.
- 5
Method outline: record start temperature of water or solution, add the reactant, stir gently with a thermometer, record the maximum (exo) or minimum (endo) temperature. ΔT = final − start, with sign.
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Control variables: same starting volume of water, same volume of acid/alkali if neutralising, same mass of cup, lid on, same stirring time, room draughts minimised, and same thermometer.
- 7
Insulation: polystyrene cup inside a beaker, lid with a hole for the thermometer, and gentle stirring reduce heat loss to the air (which makes exothermic ΔT look too small).
- 8
Repeat each condition at least three times, calculate mean ΔT, and plot a graph (for example mean ΔT on the y-axis against mass of solid or concentration on the x-axis). A straight line through the origin suggests ΔT is proportional to amount for that range.
Quotations worth analysing
Short evidence. Real method.
“Exothermic: energy transferred to the surroundings, temperature rises. Endothermic: energy taken in from the surroundings, temperature falls.”
The thermometer sits in the surroundings you are measuring — usually the solution. Name the direction of transfer and the sign of ΔT.
“H⁺(aq) + OH⁻(aq) → H₂O(l) neutralisation is exothermic”
Equal volumes of acid and alkali of the same concentration give a clear temperature rise. State volumes and concentrations in the method.
“ΔT = maximum (or minimum) temperature − start temperature”
Include the sign. +6 °C and −4 °C tell different stories. Mean several runs before plotting.
Go deeper
How do I write a method that would actually score?
Place 25 cm³ of water (or 25 cm³ of 1.0 mol/dm³ HCl) in a polystyrene cup. Measure start temperature T₁ to 0.1 °C. Add a stated mass of solid (for example 2 g ammonium nitrate) or 25 cm³ of 1.0 mol/dm³ NaOH, stir gently for 30 seconds without touching the cup walls with your hand. Record the lowest or highest temperature T₂. ΔT = T₂ − T₁. Repeat with fresh cup and solution three times; calculate mean ΔT. For neutralisation, use the same total volume each time (25 cm³ acid + 25 cm³ alkali). The independent variable might be the mass of ammonium nitrate (0, 2, 4, 6 g) or the concentration of acid; the dependent variable is mean ΔT. Controls: volume, insulation, stirring time, same thermometer, same room. Safety: acids and alkalis are corrosive; wear goggles; ammonium nitrate is an oxidiser — no flames nearby.
Go deeper
Why insulation and a lid matter more than “being careful”
Heat always moves from warmer to cooler surroundings. In an exothermic neutralisation, energy leaves the reaction mixture into the solution — that is what you want to measure — but then energy also leaves the cup into the bench and air. Without a lid, hot water evaporates and cools the surface, shrinking your measured ΔT. A polystyrene cup inside a beaker adds another barrier. Stir so the thermometer reads the bulk temperature, not a hot spot. Even with insulation, exothermic ΔT is usually slightly low and endothermic ΔT slightly small in magnitude — that is the standard evaluation. Improvements: lid, second cup, electronic thermometer, repeat and mean, and add a fourth mass/concentration to strengthen the graph. Validity: you are using temperature change as a proxy for energy transfer; you are not measuring enthalpy in kJ/mol unless moles are calculated on Higher tier.
Go deeper
What graph should I draw, and what does it show?
Plot mean temperature change (y-axis) against the independent variable (x-axis): mass of solid dissolved, or concentration of acid. For exothermic dissolving of NaOH, expect negative ΔT on the y-axis if you define ΔT as T_final − T_start with a rise — actually ΔT is positive for exothermic (temperature rises). For ammonium nitrate, ΔT is negative. A line of best fit through the origin suggests ΔT is proportional to amount over that range. If the line curves or levels off, the cup may be losing heat as fast as the reaction releases it (saturation). Do not join dot-to-dot if the mark scheme asks for a line of best fit. Anomalies: a low exothermic ΔT might mean a draught, no lid, or the thermometer not fully immersed. State that in the evaluation, not “human error”.
See the idea in action
25 cm³ of 1.0 mol/dm³ HCl and 25 cm³ of 1.0 mol/dm³ NaOH both start at 21.0 °C. After mixing in a lidded polystyrene cup, maximum temperature is 27.4 °C. ΔT = +6.4 °C → exothermic. Two repeats give 27.1 °C and 27.6 °C → mean ΔT = +6.3 °C. IV: neutralisation (acid + alkali). DV: temperature change. Controls: equal volumes, same concentration, lid, 30 s stir, same cup. Graph: if different concentrations of acid are tested, plot mean ΔT against concentration; steeper concentration should give larger mean ΔT until heat loss limits the reading.
Exam technique
Turn knowledge into marks
State exothermic or endothermic with the temperature change of the surroundings. Mention polystyrene cup, lid and stirring. Calculate mean ΔT from repeats. On graphs, label axes and use a line of best fit when appropriate.
Common mistakes
Do not give these marks away
- 01
Saying the reaction “gets cold” without stating that energy is taken in from the surroundings and the thermometer reading falls.
- 02
Measuring temperature once without a maximum/minimum, or forgetting to subtract the start temperature.
- 03
Changing volume and concentration at the same time, or plotting a single run instead of a mean on the graph.
The temperature of a solution falls when ammonium nitrate dissolves. What type of change is this?
AExothermic, because bonds are formed
BEndothermic, because energy is taken in from the surroundings
CExothermic, because the temperature changed
DNeither, because dissolving is not a chemical reaction
Show the answer
Endothermic, because energy is taken in from the surroundings. A fall in temperature of the surroundings means energy has been absorbed. GCSE classifies that as endothermic, whether the process is dissolving or a chemical reaction.
Quick questions
If this is the bit you searched
What is the dependent variable in the temperature-change practical?
The temperature change ΔT of the solution, or the maximum/minimum temperature reached after mixing or dissolving.
Why use a polystyrene cup and a lid?
Polystyrene insulates and reduces heat transfer to or from the air and bench. A lid cuts evaporation and draughts, so the measured ΔT is closer to the true value.
Give an exothermic and an endothermic example for this practical.
Exothermic: neutralisation of acid and alkali, or dissolving NaOH. Endothermic: dissolving ammonium nitrate or ammonium chloride.
How should you process results before plotting a graph?
Repeat each condition, calculate the mean ΔT, discard anomalies with reason, then plot mean ΔT against the independent variable.