Sec 3 Chemistry — Experimental Design
Prepared by Miss Clarissa Ng · www.clartutors.com
| Kind of analysis | What the experiment is doing |
|---|---|
| Qualitative | Identifying which elements and/or compounds are present in an unknown substance. It is decided by physical and chemical tests, and the answer is a name, not a number. |
| Quantitative | Measurements are taken and recorded. There are three kinds of quantitative work in this topic: volumetric (volume of reactant used), calorimetric (thermal energy released or absorbed, read as a temperature change) and rate (how quickly the reaction proceeds). |
Start from what the experiment changes, then pick the instrument. The three quantities an examiner asks about most are time, temperature and mass — and each one has an SI unit that is not the unit you actually write down in the laboratory.
| Measuring | Instrument | Accuracy | Unit you record |
|---|---|---|---|
| Time | Analogue stopwatch Digital stopwatch | ±0.1 s ±0.01 s | second (s) — the SI unit; hour (h) and minute (min) are also used. A digital stopwatch reads to 0.01 s, so quote it to two decimal places. |
| Temperature | Alcohol-in-glass thermometer Mercury-in-glass thermometer | ±0.5 °C ±0.5 °C | degree Celsius (°C) — but the SI unit is the kelvin (K). Convert with °C = K − 273. |
| Mass | Triple beam balance Electronic balance | — ±0.01 g | gram (g) — but the SI unit is the kilogram (kg). Mass is recorded when a gas escapes, because the set-up then loses mass. |
- Why the alcohol thermometer stops at about 78 °C — alcohol boils at a relatively low temperature. Above that, the mercury-in-glass thermometer is the only choice, and it brings a safety point with it: mercury is toxic and is hazardous if the thermometer breaks and mercury vapour is breathed in. The alcohol one is the less harmful option.
- Why a reaction can be followed by mass at all — if a gaseous product escapes (carbon dioxide from an acid–carbonate reaction, for instance), the set-up gets lighter, and that loss is the measurement.
Volume needs a different question: how precise does the answer have to be? The apparatus itself tells you — beakers and flasks are for holding, and only some of the glassware is calibrated for measuring.
| Apparatus | What it is for | How precise it is | Reading it gives |
|---|---|---|---|
| Beaker | Contains and mixes a larger amount of chemicals than a test tube. No calibration for measuring, so it is not the apparatus for an accurate volume. | Not for measuring | 10, 100, 250 cm³ |
| Conical flask | Holds or collects a liquid; the tapered neck makes swirling and mixing easier, which is why reagent-mixing experiments use it. | Not for measuring | 250 cm³ |
| Measuring cylinder | Measures out a volume when a rough volume is good enough. | To the nearest 0.5 cm³ | 10, 50, 100, 500 cm³ |
| Burette | Measures and dispenses a precise volume, drop by drop — the titration apparatus. | To the nearest 0.05 cm³ | 50.00 cm³ |
| Pipette | Measures and dispenses one fixed volume each time, for when the same volume must be repeated exactly. | One exact volume per pipette | 5.0, 10.0, 15.0, 20.0, 25.0 cm³ |
Volume in the laboratory is recorded in cubic centimetres (cm³), although the SI unit is the cubic metre (m³).
A gas cannot be poured, so the volume of a gas is measured by pushing it into a gas syringe — commonly 100 cm³ — and reading the scale:
Carbon dioxide from the acid–carbonate reaction travels along the delivery tube and pushes the piston out, so the reading grows. The stopwatch is started the moment the reactants are mixed, so every syringe reading can be paired with a time — which is exactly what Part D needs.
A gas collected straight from a reaction is not pure: it carries water vapour with it, and may carry unreacted chemicals or acidic and basic impurities. Purification deals with both, so that a pure and dry sample is collected.
| Drying agent | Concentrated sulfuric acid | Calcium oxide (quicklime) | Fused calcium chloride |
|---|---|---|---|
| Nature | Acidic | Basic | Neutral |
| Gases it can dry | Neutral and acidic gases | Neutral and alkaline gases | General gases |
| Examples | Chlorine, carbon dioxide, hydrogen chloride | Ammonia | Oxygen, hydrogen |
| Note | — | Must be freshly heated: it absorbs water vapour readily and also reacts with carbon dioxide in the air. | Must be freshly heated: it absorbs water vapour readily from the air. |
There are three methods, and two facts about the gas decide which one: how soluble it is in water, and its density compared with atmospheric air. Work down the tree in the order shown — the first question settles whether solubility matters at all:
Once you know which method applies, this table puts the two facts together for the gases you are expected to know. It is listed alphabetically so you can find a gas quickly, and the last column shows the method that follows from the data.
| Gas | Solubility in water | Density compared with air | Method that follows |
|---|---|---|---|
| Ammonia | Extremely soluble | Less dense | Upward delivery — being very soluble, it would dissolve in water, so density decides. |
| Carbon dioxide | Slightly soluble | Denser | Displacement of water — only slightly soluble, so little of it is lost in the water. |
| Chlorine | Soluble | Denser | Downward delivery — too soluble for water, and denser than air, so it sinks into the jar. |
| Hydrogen | Insoluble | Less dense | Displacement of water — insoluble, so it can be collected over water. |
| Hydrogen chloride | Very soluble | Denser | Downward delivery — would dissolve in water, and is denser than air. |
| Oxygen | Very slightly soluble | Denser | Displacement of water — barely dissolves, so collecting it over water is safe. |
| Sulfur dioxide | Very soluble | Denser | Downward delivery — very soluble, so it must not be collected over water. |
Take marble chips (calcium carbonate) in excess dilute hydrochloric acid:
The set-up in section 3 collects the gas in a syringe. Read the syringe at regular time intervals and plot volume of carbon dioxide against time:
- The curve is steepest at the start, so the rate is highest then — the reactants are at their most concentrated.
- It flattens when the reaction has stopped, because a reactant has been used up. That plateau is the total volume of gas produced.
- The numbers shown are a typical shape, not readings from one particular run.
The same reaction can be followed on a balance instead: the flask and its contents are weighed at regular time intervals, and the escaping gas shows up as a loss in mass.
- The mass falls fastest at the start — the steepest part marks the fastest rate.
- It levels off once the reaction has stopped, because no more gas is escaping.
- Cotton wool in the mouth of the flask stops the reactants being lost as acid spray while still letting the gas out, so the mass lost is only the gas.
Marble chips (calcium carbonate) are placed in a conical flask with excess dilute hydrochloric acid, and the flask is joined to a gas syringe. The volume of gas is read every 20 seconds.
(a) A gas syringe.
(b) The rate decreases. The acid is being used up, so its concentration falls and the reaction slows down.
(c) The graph is flat because no more gas is being produced — the reaction has finished, and the calcium carbonate has all reacted. The volume at the plateau is the total volume of gas produced.
(d) Use powdered calcium carbonate instead of marble chips (or warm the acid): the curve would rise more steeply and become flat sooner, at the same final volume.
MEASUREMENT?
HANDLE THE GAS
DRYING AGENT
FOLLOW THE RATE