What you'll learn

• How practical work is organised in OCR GCSE Combined Science A.
• How to plan a fair, valid investigation using variables.
• How to choose apparatus, measure carefully, repeat results and process data.
• How to answer practical-skills questions in written exams.

Why practical skills matter

Science is not just remembering facts: it is about collecting evidence and using that evidence to support a conclusion. In GCSE Combined Science A, practical skills are assessed in your written exams, and about 15% of the marks test practical understanding.

Most practical questions follow the same evidence cycle: ask a question, plan a method, collect measurements, process the results, then evaluate how good the evidence is.

The OCR PAG map

The biology PAGs include:

• Microscopy: using microscopes, magnification and labelled scientific drawings.
• Sampling techniques: using methods such as quadrats, which are square frames used to sample organisms in an area, and considering biotic factors and abiotic factors. Biotic factors are living factors; abiotic factors are non-living environmental factors.
• Rates of enzyme-controlled reactions: measuring how quickly enzyme reactions happen.
• Photosynthesis: measuring changes in plant processes, often using aquatic plants such as Cabomba.
• Microbiological techniques: working safely with microorganisms, for example growing a bacterial lawn, which is an even layer of bacteria on agar.

The chemistry PAGs include:

• Electrolysis: using electricity to break down an ionic compound.
• Distillation: separating mixtures by boiling and condensing.
• Separation techniques such as filtration, crystallisation and chromatography. Filtration separates an insoluble solid from a liquid; crystallisation forms crystals from a solution; chromatography separates dissolved substances as they move through a material.
• Production of salts: making and purifying a salt sample.
• Measuring rates of reaction: for example measuring gas produced over time.

The physics PAGs include:

• Materials: measuring density of solids and liquids.
• Forces: measuring effects such as the extension of springs.
• Motion: measuring speed and acceleration.
• Waves: measuring wavelength, frequency or speed.
• Energy: measuring energy transfers, for example specific heat capacity, the energy needed to raise 1 kg of a material by 1 °C.
• Circuits: measuring current, potential difference and resistance, and investigating I-V characteristics, meaning how current changes with potential difference.

Planning a fair investigation

An aim is what you are trying to find out. A hypothesis is a testable prediction, often linking two variables.

A fair test changes only the independent variable and keeps important control variables constant. A valid investigation is one that actually tests the intended question.

Choosing apparatus and techniques

Apparatus means the equipment you use. A technique is the way you use the equipment. Good apparatus choice depends on the quantity being measured, the range of values expected and the resolution.

Common measurements include:

• Length using a ruler, metre rule or microscope scale, in metres (m), centimetres (cm), millimetres (mm) or micrometres (µm).
• Mass using a balance, in grams (g) or kilograms (kg).
• Time using a stopwatch or light gates, in seconds (s).
• Temperature using a thermometer or temperature probe, in degrees Celsius (°C).
• Volume of liquids using a measuring cylinder, pipette or burette, usually in centimetres cubed (cm³).
• Volume of gases using a gas syringe or an inverted measuring cylinder, in cm³.
• pH using pH paper, a pH meter or a pH probe.
• Current using an ammeter, potential difference using a voltmeter, and resistance using an ohmmeter or calculated from circuit measurements.

Taking reliable measurements

Good data is not just “numbers written down”. You need readings that are careful, repeated and recorded with units.

Repeating readings helps you spot anomalous results and calculate a mean. A mean is an average:

$$mean=\frac{sum\; of\; repeat\; readings}{number\; of\; readings}$$

For many rate practicals, rate can be found using:

$$rate=\frac{amount\; of\; change}{time\; taken}$$

For gas production, this often becomes:

$$rate=\frac{volume\; of\; gas\; produced}{time\; taken}$$

Recording results and drawing graphs

A results table should be clear enough that someone else can understand exactly what you measured. Put the independent variable in the first column and the dependent variable in the next column. Headings should include units, such as “time in s” or “temperature in °C”.

Graphs help you see patterns. Usually, the independent variable goes on the x-axis and the dependent variable goes on the y-axis. Use sensible scales, plot points carefully and draw a line of best fit when the data shows a trend.

Scientific diagrams

Scientific diagrams are used to record apparatus and observations clearly.

For apparatus diagrams:

• Use simple 2D outlines.
• Label key parts, such as thermometer, gas syringe, electrodes or power supply.
• Show important connections, such as wires in a circuit or delivery tubes in a gas collection setup.

For biological drawings:

• Use clear single lines.
• Avoid shading.
• Label structures with straight label lines.
• Include magnification or scale information if required.

In microscopy, magnification tells you how many times larger the image is than the real object:

$$magnification=\frac{image\; size}{actual\; size}$$

Safety, ethics and evaluation

Safety is an overriding requirement in all practical work. A hazard is something that could cause harm, such as acid, hot equipment or microorganisms. A risk is the chance of harm happening. A control measure is an action that reduces the risk, such as wearing eye protection or using a water bath instead of direct heating.

Some practicals also involve ethical treatment of living organisms. For example, plants and microorganisms should be handled safely and responsibly, and living organisms should not be harmed unnecessarily.

Evaluation means judging how good the method and evidence are. Useful improvements include repeating readings, using a more precise instrument, controlling temperature more carefully, using a larger sample size, or taking readings at smaller intervals.