Unit 2 Practical Scientific Procedures and Techniques Assignment Answer

This Unit 2: Practical Scientific Procedures and Techniques BTEC Level 3 assignment answer guide covers the internally assessed practical unit of the Pearson BTEC Level 3 National in Applied Science (2016 specification). It explains all 17 Pass, Merit and Distinction criteria across the four learning aims, shows how to write up titration, colorimetry, calorimetry and chromatography, and includes model-answer extracts and fully worked calculations you can check your own results against.

What the Unit 2 assignments ask you to do

Unit 2 is a 90-guided-learning-hour unit, and Pearson recommends four separate assignments, one for each learning aim. Most centres set them in a workplace context, for example as a trainee technician in a quality control laboratory who must show that they can carry out standard procedures accurately and report the results.

• Assignment 1 (Learning aim A): prepare and standardise solutions, then find the concentration of unknown solutions by titration and by colorimetry.
• Assignment 2 (Learning aim B): calibrate thermometers or temperature sensors, collect temperature and time data and plot cooling curves for different substances.
• Assignment 3 (Learning aim C): use paper and thin-layer chromatography to separate mixtures such as plant pigments or amino acids, calculate Rf values and explain the technique.
• Assignment 4 (Learning aim D): review the practical skills you have developed during the unit and how they support your progression.

Your practical work is usually evidenced by your results tables, graphs and calculations, an observation record signed by your teacher, and a written report.

Learning aims for Unit 2

Pass, Merit and Distinction criteria explained

How to answer Learning aim A: titration and colorimetry

1. Write a clear method for making a standard solution: weighing by difference, dissolving, transferring with rinsings and making up to the mark in a volumetric flask.
2. Standardise your titrant and record every titre in a table, with rough and accurate runs and the mean of concordant results.
3. Calculate the concentration of the unknown, showing moles, ratio and units.
4. For colorimetry, prepare a dilution series of a coloured transition metal solution (copper(II) sulfate is common), choose a filter of the complementary colour, plot absorbance against concentration and read off the unknown.
5. For A.D1, calculate percentage uncertainties for each piece of equipment, compare titration and colorimetry, and suggest improvements.

How to answer Learning aim B: calorimetry and cooling curves

• Calibrate your thermometer or sensor using melting ice (0 °C) and boiling water (close to 100 °C, depending on air pressure) and correct your readings if needed.
• Record temperature at fixed time intervals as a substance such as stearic acid or salol cools and solidifies.
• Plot temperature against time, identify the plateau as the freezing point, and draw tangents to find rates of cooling.
• For B.M2, explain the plateau: energy is released as bonds form between particles during freezing, so temperature stays constant. Any dip below the plateau shows supercooling.

How to answer Learning aim C: chromatography

• Show photographs of each chromatogram with the origin line, spots and solvent front marked in pencil.
• Explain the stationary and mobile phases and why components move at different rates (solubility in the solvent compared with attraction to the stationary phase).
• Calculate Rf values and compare them with standards run on the same plate.
• For C.D3, compare paper and thin-layer chromatography: TLC usually gives faster runs and sharper spots, while amino acids need locating with ninhydrin.

How to answer Learning aim D: reviewing your skills

Use a skills audit table with each practical and the competency it developed, such as accurate measurement, risk assessment, teamwork and recording data. For D.M4, use teacher feedback to identify a weakness and an action. For D.D4, name a target course or job (for example a laboratory technician apprenticeship or a biomedical science degree) and explain which skills it needs and how you will close the gap.

Worked example: titration calculation and uncertainty for A.P2 and A.D1

A 25.00 cm³ sample of sodium hydroxide solution is titrated with 0.100 mol dm⁻³ hydrochloric acid. Concordant titres are 23.40, 23.45 and 23.50 cm³.

1. Mean titre = (23.40 + 23.45 + 23.50) ÷ 3 = 23.45 cm³ = 0.02345 dm³.
2. Moles of HCl = 0.100 × 0.02345 = 2.345 × 10⁻³ mol.
3. NaOH + HCl → NaCl + H₂O, a 1:1 ratio, so moles of NaOH = 2.345 × 10⁻³ mol.
4. Concentration of NaOH = 2.345 × 10⁻³ ÷ 0.02500 = 0.0938 mol dm⁻³ (3 significant figures).

Uncertainty: a class B burette is typically read to ±0.05 cm³, and a titre uses two readings, giving ±0.10 cm³. Percentage uncertainty = 0.10 ÷ 23.45 × 100 = 0.43%. A 25.00 cm³ pipette with a tolerance of ±0.06 cm³ gives 0.06 ÷ 25.00 × 100 = 0.24%. Adding these gives about 0.67%, so the result is 0.0938 ± 0.0006 mol dm⁻³. Check the tolerances printed on your own glassware, as they vary by class and manufacturer.

Rf check for Learning aim C: a spot travels 4.2 cm while the solvent front travels 7.0 cm, so Rf = 4.2 ÷ 7.0 = 0.60. Rf has no units and is always less than 1.

Rate of cooling for Learning aim B: if a tangent to the curve falls from 70 °C to 60 °C over 120 s, the rate of cooling = 10 ÷ 120 = 0.083 °C s⁻¹.

Common mistakes that cost marks

• Recording titres to one decimal place instead of two (to the nearest 0.05 cm³).
• Averaging all titres, including the rough run and non-concordant values.
• Plotting a colorimetry calibration graph without a blank or with absorbance on the wrong axis.
• Drawing the chromatography origin line in pen, which dissolves and runs with the solvent.
• Calculating the rate of cooling from two points far apart instead of from a tangent.
• Writing Learning aim D as a list of tasks completed, with no analysis of skills or progression.

How to move from Merit to Distinction

All four Distinction criteria ask you to evaluate. A.D1 needs numbers: percentage uncertainties, a comparison of the two methods and improvements that would actually reduce the largest error. B.D2 needs you to link equipment limits, such as thermometer resolution and response time, to how confidently you can draw the tangent. C.D3 needs a judgement on how reliable your identification was and a specific improvement, such as two-way chromatography or a different solvent. D.D4 needs a clear link between the skills you developed and a named next step.

FAQs

Is Unit 2 Applied Science an exam?

No. Unit 2 is internally assessed through assignments set and marked by your centre, then quality-checked by Pearson. In contrast, Unit 1 is an exam and Unit 3 is externally assessed through a set task.

What does concordant mean in a titration?

Concordant titres agree closely, usually within 0.10 cm³ of each other. Only concordant results should be used to calculate the mean.

Why use a complementary colour filter in colorimetry?

A coloured solution absorbs most strongly the colour opposite to the one you see. Using that filter gives the largest change in absorbance with concentration, which makes the calibration curve more sensitive.

Can Rf values identify a substance on their own?

Not reliably. Rf depends on the solvent, temperature and stationary phase, so you should run known standards on the same plate and compare.

Need one-to-one support from a subject specialist? Our BTEC Level 3 assignment help service covers planning, feedback on drafts and referencing.

If you would like your Unit 2 practical reports checked before submission, you can get expert help with your BTEC assignment. For the laboratory unit that builds on these skills, see our Unit 4 Laboratory Techniques and their Application answer guide, or the Unit 1 Principles and Applications of Science I guide.