Ch 1 · Experimental Design · O Level Chemistry 5070
Topic 1 of 5 in Experimental Techniques and Chemical Analysis — O Level Chemistry 5070 — 38 lessons.
Introduction
Every practical question starts with the same decisions: what to measure, which apparatus to use, and why that choice beats the alternatives. This lesson gives you the apparatus for measuring time, temperature, mass and volume, the arguments for and against each, and the six words about solutions that Cambridge defines exactly.
Key ideas — Apparatus for measurement
Use a volumetric flask to make up a solution to an exact total volume.
🧑🏫 Teacher's explanation
In chemistry, measuring things isn't just about reading a number; it's about choosing the right tool for the job. If you are measuring the thickness of a hair, a ruler is useless, but a micrometer is perfect. In the lab, if you use a measuring cylinder to measure exactly 25.0 cm^3 of liquid for a precise experiment, your results will be 'fuzzy' or inaccurate because a measuring cylinder is meant for approximate volumes.
📖 Definitions
🔍 Worked example — Easy
A student needs to measure exactly 25.0 cm^3 of distilled water to prepare a standard solution. Which apparatus is most appropriate: a measuring cylinder or a volumetric pipette? Explain why.
🔍 Worked example — Standard
A student uses a gas syringe to collect gas produced in the reaction between Mg and HCl. The gas volume increases from 0 cm^3 to 45 cm^3 in 60 s. Calculate the average rate of gas production in cm^3/\ exts.
🔍 Worked example — Exam-style
A student weighs 2.00 g of NaCl and dissolves it in water using a 100 cm^3 volumetric flask to make a solution. If the balance has an error of ± 0.01\ ext g, calculate the percentage error in the mass of the solute.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Choosing between methods and apparatus
Examiners reward a comparison, not just a name.
🧑🏫 Teacher's explanation
In chemistry, choosing your tools is like choosing a measuring spoon versus a kitchen scale. If you are baking a cake and need exactly 5.00 g of salt, using a measuring spoon is risky because salt grains vary in size; you need the scale. In the lab, if you need an exact volume for a titration, a measuring cylinder is too 'clunky' and inaccurate; you must use a pipette or burette.
📖 Definitions
🔍 Worked example — Easy
A student uses a measuring cylinder to measure 25 cm^3 of HCl. Why is a volumetric pipette a better choice for a titration?
🔍 Worked example — Standard
A student measures the volume of gas produced in a reaction by collecting it over water in an inverted measuring cylinder. The gas is NH_3. Why might the measured volume be lower than the actual volume produced?
🔍 Worked example — Exam-style
A student is investigating the rate of reaction between Mg(s) and HCl(aq). They use a mass balance to measure the mass loss as H_2(g) is evolved. Explain why this method might be unreliable for this specific reaction.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one advantage of using a gas syringe over collecting the gas over water. [1] (b) The student notices that the temperature of the reaction mixture increases significantly during the experiment. Suggest how the student could improve the experimental setup to ensure the temperature remains constant. [1] [2 marks]
Key ideas — Solutions, residues and filtrates
Cambridge expects these six definitions word for word:
🧑🏫 Teacher's explanation
Imagine you are making tea. The sugar is the solute, the hot water is the solvent, and the sweet liquid you drink is the solution. If you keep adding sugar until no more will dissolve, you have a saturated solution. If you then pour this through a tea strainer (a filter), the tea liquid that goes through is the filtrate, and the tea leaves left behind in the strainer are the residue.
📖 Definitions
🔍 Worked example — Easy
A student dissolves 10 g of sodium chloride (NaCl) in 50 cm^3 of water. What are the names of the substance being dissolved, the liquid doing the dissolving, and the resulting mixture?
🔍 Worked example — Standard
A mixture of sand and salt is added to water. The mixture is stirred and then filtered. What is the composition of the filtrate and the residue?
🔍 Worked example — Exam-style
A saturated solution of potassium nitrate (KNO_3) is prepared at 60 ^circC. The solution is then cooled to 20 ^circC. Describe what happens to the solute and why.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A student investigates the rate of reaction between calcium carbonate and dilute hydrochloric acid by measuring the volume of carbon dioxide produced every 30 seconds. (a) Name the apparatus the student should use to measure the volume of gas, and to measure the time. [2] (b) Another student suggests collecting the gas over water in an inverted measuring cylinder. Give one disadvantage of this method. [1] (c) A third student suggests placing the flask on a balance and recording the loss in mass. Give one advantage and one disadvantage of this method compared with using the apparatus in (a). [2]
🔍 Worked example — Easy
A student uses a measuring cylinder to collect gas over water. Why is this less accurate than using a gas syringe?
🔍 Worked example — Standard
A reaction produces 0.050 g of gas. A student uses a standard top-loading balance with a precision of 0.1 g. Is this suitable? Explain.
🔍 Worked example — Exam-style
Calculate the volume of gas produced if 0.10 mol of CO_2 is produced at RTP (assume molar volume = 24 dm^3 mol^-1).
⚠️ Common errors
Common mistakes
Practice
Answers (a) volumetric pipette [B1]; (b) burette [B1]; (c) thermometer [B1]. Solute: a substance that is dissolved in a solvent [B1]; solvent: a substance that dissolves a solute [B1]; solution: a mixture of solvent and solute [B1]. A solution containing the maximum concentration of solute dissolved in the solvent [B1] at a given temperature [B1]. Residue: sand [B1]; filtrate: salt solution (aqueous sodium chloride) [B1]. Human reaction time when starting or stopping the watch [B1]; repeat and take a mean, or time a longer interval, or use a light gate or data logger [B1].