Understanding Chemistry: Physical chemistry: Edexcel IGCSE Science Single Award Explained

Physical chemistry is where the maths meets the reactions, and it is more approachable than you might think

The physical chemistry section of the Pearson Edexcel IGCSE Science Single Award (4SS0) covers two topics that sit at the heart of how chemical reactions behave: energetics and rates of reaction. If the principles of chemistry section gave you the vocabulary and the inorganic section gave you the reactions, this is the section where you learn to measure and control those reactions. For a single award course that spans biology, chemistry and physics, these topics also build bridges to the energy transfers content in the physics paper.

These edexcel igcse science single award revision notes walk you through every specification point in the physical chemistry section. You will find worked calculations, practical guidance, and the common mistakes that lose students marks. Whether you are revising from scratch or fine-tuning your understanding, these edexcel igcse science single award notes will help you prepare for the exam with confidence.

Energetics

Exothermic and endothermic reactions

Every chemical reaction either releases energy to its surroundings or absorbs energy from them. The specification requires you to know the distinction:

Exothermic reactions give out heat energy to the surroundings. The temperature of the surroundings increases. Examples include combustion, neutralisation, and many displacement reactions.
Endothermic reactions take in heat energy from the surroundings. The temperature of the surroundings decreases. Examples include thermal decomposition and some dissolving processes (such as dissolving ammonium nitrate in water).

A useful way to remember: "exo" means out (energy exits), "endo" means in (energy enters).

Simple calorimetry

The specification requires you to describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation. Calorimetry measures the heat energy change in a reaction by tracking the temperature change of a known mass of water (or solution).

The key equation is:

Q = m x c x Delta T

where Q is the heat energy change (in joules), m is the mass of the water or solution (in grams), c is the specific heat capacity of water (4.2 J/g/degree C), and Delta T is the temperature change (in degrees C).

Here is a worked example:

Worked example: A student adds 25 cm³ of sodium hydroxide solution to 25 cm³ of hydrochloric acid in a polystyrene cup. The temperature rises from 21.0 degrees C to 27.5 degrees C. Calculate the heat energy released.

Step 1: Find the total volume of solution = 25 + 25 = 50 cm³. Assume 1 cm³ of solution has a mass of 1 g, so m = 50 g.
Step 2: Find the temperature change: Delta T = 27.5 - 21.0 = 6.5 degrees C.
Step 3: Calculate Q = 50 x 4.2 x 6.5 = 1365 J.

The reaction released 1365 J (or 1.365 kJ) of heat energy. Because the temperature increased, this is an exothermic reaction.

The specification also asks you to investigate temperature changes accompanying salts dissolving in water, neutralisation reactions, displacement reactions and combustion reactions. In each case, the method follows the same pattern: measure the starting temperature, carry out the reaction, measure the final temperature, and apply Q = mcDeltaT.

Common mistakes in energetics

Forgetting to add the volumes of both solutions when calculating mass in a neutralisation experiment.
Using the wrong sign convention: if the temperature goes up, the reaction is exothermic (energy is released). If it goes down, the reaction is endothermic (energy is absorbed).
Leaving the answer in joules when the question asks for kilojoules, or vice versa. Always check the units requested.

Rates of reaction

The rate of a chemical reaction is a measure of how quickly reactants are used up or products are formed. The specification requires you to understand how changing certain conditions affects the rate, and to describe experiments that demonstrate these effects.

Factors affecting the rate of reaction

Factor	Effect on rate	Explanation
Surface area of a solid (e.g. using smaller pieces or powder)	Increases the rate	More surface is exposed, so more collisions between reactant particles can occur per second
Concentration of a solution	Increasing concentration increases the rate	More particles per unit volume means more frequent collisions
Pressure of a gas	Increasing pressure increases the rate	Particles are closer together, leading to more frequent collisions
Temperature	Increasing temperature increases the rate	Particles move faster, so collisions are more frequent and more energetic (more particles have sufficient energy to react)
Catalyst	Increases the rate	Provides an alternative reaction pathway with a lower activation energy, so more particles have enough energy to react

A catalyst is a substance that increases the rate of a reaction but is chemically unchanged at the end of the reaction. It is not used up and does not appear in the overall equation. Catalysts work by lowering the activation energy, which is the minimum energy particles need to react when they collide.

The marble chips and hydrochloric acid experiment

The specification asks you to investigate the effect of changing the surface area of marble chips and of changing the concentration of hydrochloric acid on the rate of reaction between marble chips and dilute hydrochloric acid.

The reaction is: CaCO₃(s) + 2HCl(aq) -> CaCl₂(aq) + H₂O(l) + CO₂(g)

You can measure the rate by collecting the carbon dioxide gas produced over time (using a gas syringe or an inverted measuring cylinder over water) or by measuring the loss in mass of the flask as CO₂ escapes.

Changing surface area: Use large chips in one experiment and the same mass of smaller chips (or powder) in another. Keep the acid concentration, volume and temperature the same. The smaller chips react faster because they have a greater total surface area exposed to the acid.

Changing concentration: Use the same mass and size of marble chips each time. Change the concentration of hydrochloric acid while keeping the volume and temperature constant. Higher concentration acid produces gas more quickly because there are more acid particles available to collide with the marble surface.

Exam tip: When describing how to make this experiment a fair test, be explicit about what you keep constant (the control variables). Simply saying "keep everything else the same" is too vague. Name the specific variables: same mass of marble chips, same volume of acid, same starting temperature. Examiners award marks for precision.

Interpreting rate graphs

A graph of volume of gas collected against time typically shows a curve that rises steeply at first and then levels off. The steep part represents the fastest rate (when reactant concentrations are highest). The flat part shows the reaction has finished (one or both reactants have been completely used up). The total volume of gas produced shows which experiment produced the most product.

If you are comparing two experiments on the same axes:

A steeper initial gradient means a faster rate.
If both curves level off at the same total volume, the same amount of reactant was used (only the rate differed, not the yield).
If one curve levels off at a lower volume, less reactant was available in that experiment (for example, a lower concentration of acid).

Self-check questions

Define the terms exothermic and endothermic, giving one example of each.
In a displacement reaction, 100 cm³ of copper sulfate solution increases in temperature by 8.0 degrees C. Calculate the heat energy released. (Use c = 4.2 J/g/degree C and assume 1 cm³ = 1 g.)
List four factors that affect the rate of a chemical reaction and explain, in terms of particles, how each one works.
What is a catalyst, and how does it increase the rate of a reaction?
Describe how you would investigate the effect of concentration on the rate of reaction between marble chips and hydrochloric acid. State the independent variable, dependent variable and at least two control variables.
A rate graph shows that Experiment A has a steeper initial gradient than Experiment B, but both level off at the same total volume of gas. What does this tell you about the two experiments?
Explain why increasing temperature increases the rate of reaction, using the idea of particle energy.

Connecting physical chemistry to the wider course

The chemistry: physical chemistry edexcel igcse section connects to multiple parts of the single award specification. The Q = mcDeltaT calculation reappears in the physics energy section. The concept of enzymes as biological catalysts (from the biology section) is the biological equivalent of the catalysts studied here. Recognising these cross-disciplinary links is one of the advantages of the single award format and can help you answer questions that draw on more than one science.

For your igcse 4ss0 chemistry: physical chemistry preparation, make sure you are comfortable with the calorimetry calculation (Q = mcDeltaT) and can set up and describe the marble chips experiment. These two areas carry the most marks. Practise the calculation with different numbers until the process feels automatic, and practise describing the experiment until you can name every control variable without thinking.

Use these edexcel igcse science single award notes as your foundation, then challenge yourself with edexcel igcse science single award practice questions from past papers. The edexcel igcse science single award explained format here gives you the conceptual grounding; the past papers show you how examiners phrase their questions. The Green Bridge CBT platform offers chemistry questions sorted by topic, so you can focus specifically on physical chemistry and track your confidence as it grows.