Practical Chemistry and Qualitative Analysis
A chemist has to measure carefully, collect what a reaction gives off, and work out what an unknown substance contains. This chapter covers the apparatus and technique for measuring, collecting and drying gases and titrating, the tests that identify common ions and gases, and how to judge how good a measurement is.
Learning objectives
By the end of this chapter, you can:
- choose apparatus to measure time, temperature, mass and volume
- choose a method to collect and dry a gas
- describe how to carry out a titration
- identify cations with aqueous sodium hydroxide and aqueous ammonia
- identify anions and gases with their tests
- identify the A Level cations (Ba2+, Cr3+, Mg2+, Mn2+) and anions (Br−, NO2−, SO32−)
- deduce an unknown substance from a sequence of observations
- explain why the order of tests and the choice of acid matter
- tell accuracy from precision
- find the largest source of error with percentage uncertainty
7.1Measuring physical quantities
Each measurement needs a quantity, an SI unit and apparatus that is accurate enough. The accuracy of an instrument is how closely it can read, for example ±0.01 g.
| quantity | SI unit | apparatus and accuracy |
|---|---|---|
| time | second (s) | digital stopwatch ±0.01 s, analogue stopwatch ±0.1 s |
| temperature | kelvin (K) | alcohol, mercury or digital thermometer, temperature probe with data logger |
| length | metre (m) | metre rule ±0.1 cm, measuring tape ±0.1 cm to ±0.5 cm |
| mass | kilogram (kg) | electronic balance ±0.01 g |
| volume | cubic metre (m3) | pipette, volumetric flask, measuring cylinder, burette, gas syringe |
- Temperature in K = temperature in °C + 273.
- 1 dm3 = 1000 cm3 = 1 litre. Although 1 m = 10 dm, 1 m3 is 1000 dm3, not 10.
7.1.1Measuring volume
Choose the apparatus by the volume needed and the accuracy required.
| apparatus | what it measures |
|---|---|
| pipette | one accurate fixed volume, such as 10.0 cm3 or 25.0 cm3 |
| volumetric flask | one accurate fixed, larger volume, such as 100 cm3 or 250 cm3 |
| measuring cylinder | a range of volumes to the nearest 0.5 cm3 |
| burette | a range of volumes to the nearest 0.05 cm3 |
| gas syringe | the volume of a gas, up to 100 cm3 |
Read the volume with the eye level with the meniscus, the curved surface of the liquid. Reading from above or below gives a parallax error (Figure 7.1).
- Give one advantage of a burette over a measuring cylinder.
- Give one disadvantage of a pipette, even though it measures very accurately.
7.2Collecting and drying gases
Choose how to collect a gas from two properties: its solubility in water and its density compared with air. A gas syringe collects small, accurate volumes. The three methods below collect larger volumes (Figure 7.2).
| method | gas suited to it | example |
|---|---|---|
| water displacement | insoluble or slightly soluble in water, density does not matter | carbon dioxide |
| upward delivery | less dense than air, soluble or insoluble | ammonia |
| downward delivery | denser than air, soluble or insoluble | chlorine, hydrogen chloride, sulfur dioxide |
In both air-displacement methods the gas pushes air out of the jar. A gas denser than air settles at the bottom of the jar, and a gas less dense than air collects at the top.
7.2.1Drying gases
Pass the moist gas through a drying agent that does not react with it. Chapter 5 tells you which ones react: an acidic drying agent reacts with a basic gas, and a basic drying agent reacts with an acidic gas.
| drying agent | use it for | do not use it for |
|---|---|---|
| concentrated sulfuric acid (an acid) | most gases, including chlorine and hydrogen chloride | gases that react with it, such as ammonia, a base |
| quicklime, calcium oxide (a basic oxide) | ammonia | gases that react with it, such as carbon dioxide, an acidic oxide |
| calcium chloride | gases that do not react with it | ammonia |
- Gas X is less dense than air and insoluble in water. Suggest a method to collect it.
- Gas Y reacts with acids. Name one drying agent that can be used on it, and explain your choice.
7.3Titration technique
Chapter 5 used titration results. This section is about getting them. The solution of known concentration, the titrant, goes in the burette. The solution being measured, the analyte, goes in the conical flask (Figure 7.4).
- Use a pipette to put a fixed volume, usually 25.0 cm3, into a clean conical flask.
- Add a few drops of indicator.
- Fill the burette and record the starting reading, with the eye level with the meniscus.
- Stand the flask on a white tile, so the colour change is easy to see.
- Add the titrant slowly, swirling the flask all the time. Near the end, add it drop by drop.
- Stop when the colour changes permanently: the end-point. Record the final reading.
- Titre = final reading − starting reading. A burette reads to the nearest 0.05 cm3, so record readings such as 23.45 cm3.
- Repeat until two titres agree to within 0.10 cm3, and use the average of those titres.
- Why is the conical flask swirled during a titration?
- A burette reads 1.20 cm3 at the start and 24.65 cm3 at the end-point. What is the titre?
7.4Testing for cations
Qualitative analysis finds out which ions or gases are present, not how much. Cations are identified with two alkalis, aqueous sodium hydroxide and aqueous ammonia. Add each a few drops at a time, then in excess. Most metal hydroxides are insoluble, so a metal cation gives a precipitate (ppt.) of its hydroxide (Chapter 5):
Three things identify the cation: the colour of the precipitate, whether it dissolves in excess, and whether ammonia is given off. The table follows the Qualitative Analysis Notes of the A Level Data Booklet. Mg2+, Ba2+, Cr3+ and Mn2+ are the A Level additions.
| cation | with NaOH(aq) | with NH3(aq) |
|---|---|---|
| aluminium, Al3+ | white ppt., soluble in excess | white ppt., insoluble in excess |
| ammonium, NH4+ | ammonia given off on warming | – |
| barium, Ba2+ | no ppt. | no ppt. |
| calcium, Ca2+ | white ppt. only when [Ca2+] is high | no ppt. |
| chromium(III), Cr3+ | grey-green ppt., soluble in excess giving a dark green solution | grey-green ppt., insoluble in excess |
| copper(II), Cu2+ | pale blue ppt., insoluble in excess | blue ppt., soluble in excess giving a dark blue solution |
| iron(II), Fe2+ | green ppt., turning brown in air, insoluble in excess | the same as with NaOH(aq) |
| iron(III), Fe3+ | red-brown ppt., insoluble in excess | the same as with NaOH(aq) |
| magnesium, Mg2+ | white ppt., insoluble in excess | white ppt., insoluble in excess |
| manganese(II), Mn2+ | off-white ppt., quickly turning brown in air, insoluble in excess | the same as with NaOH(aq) |
| zinc, Zn2+ | white ppt., soluble in excess | white ppt., soluble in excess |
Why the precipitates behave as they do:
- The hydroxides of zinc, aluminium and chromium(III) are amphoteric, like the oxides in Chapter 5. They react with excess sodium hydroxide to form soluble salts.
- Excess aqueous ammonia forms soluble compounds with copper(II) and zinc ions, so those two precipitates dissolve. Chapter 16 explains how.
- Iron(II) and manganese(II) hydroxides turn brown as air oxidises the metal to a higher oxidation state (Chapter 6).
- Na+, K+ and NH4+ give no precipitate because their hydroxides are soluble. Barium hydroxide is soluble too.
7.4.1One test is often not enough
Several cations can give exactly the same result in one test. A white precipitate with sodium hydroxide could be any of four ions (Al3+, Zn2+, Mg2+ or Ca2+), and “no precipitate” could be NH4+ or Ba2+. A single observation narrows the list. It does not name the ion. Follow each result to the end of its branch (Figures 7.5 and 7.6), and when more than one ion is still possible, do the other test and compare:
| still possible after one test | do this | result |
|---|---|---|
| Al3+ or Zn2+ (dissolve in excess NaOH) | add excess NH3(aq) | Zn2+ dissolves, Al3+ does not |
| Mg2+ or Ca2+ (insoluble in excess NaOH) | add NH3(aq) | Mg2+ gives a white ppt., Ca2+ gives none |
| Al3+ or Mg2+ (insoluble in excess NH3) | add excess NaOH(aq) | Al3+ dissolves, Mg2+ does not |
| NH4+ or Ba2+ (no ppt.) | warm with NaOH(aq) | NH4+ gives ammonia, Ba2+ gives none |
- Compound X contains aluminium, calcium or zinc ions. Which reagent tells you which, and what would you see?
- A solution gives a light blue precipitate with aqueous ammonia that dissolves in excess. What colour is the final solution, and which cation is present?
- A solution gives a white precipitate with aqueous sodium hydroxide, insoluble in excess, and no precipitate with aqueous ammonia. Which cation is present?
7.5Testing for anions
The anion tests also follow the Data Booklet. Bromide, nitrite and sulfite are the A Level additions.
| anion | test | positive result |
|---|---|---|
| carbonate, CO32− | add dilute acid | effervescence: CO2 turns limewater milky |
| sulfite, SO32− | add dilute acid | SO2 given off: turns acidified KMnO4 from purple to colourless. With Ba2+(aq) it gives a white ppt. that dissolves in dilute strong acid |
| nitrite, NO2− | add dilute acid | colourless NO given off, which turns (pale) brown in air as it forms NO2 |
| chloride, Cl− | add dilute nitric acid, then aqueous silver nitrate | white ppt. of AgCl, soluble in NH3(aq) |
| bromide, Br− | add dilute nitric acid, then aqueous silver nitrate | pale cream ppt. of AgBr, partly soluble in NH3(aq) |
| iodide, I− | add dilute nitric acid, then aqueous silver nitrate | yellow ppt. of AgI, insoluble in NH3(aq) |
| sulfate, SO42− | add dilute nitric acid, then aqueous barium nitrate | white ppt. of BaSO4, insoluble in excess dilute strong acid. Sulfite also gives a white ppt. with Ba2+ (BaSO3), but it dissolves in the acid. |
| nitrate, NO3− | add aqueous sodium hydroxide, then aluminium foil, and warm | ammonia given off: turns damp red litmus blue |
Each positive result is a reaction from an earlier chapter: an acid with a carbonate or sulfite, a precipitation (Ag+ + Cl− → AgCl, Ba2+ + SO42− → BaSO4), or a redox reaction: aluminium reduces nitrogen from +5 in NO3− to −3 in NH3, and sulfur dioxide reduces manganate(VII). Figure 7.8 puts the tests in order.
- Acidify with dilute nitric acid first. It removes carbonate, sulfite and hydroxide ions, which would also give precipitates with silver or barium ions.
- Never acidify with hydrochloric acid before a chloride test, or with sulfuric acid before a sulfate test: you would be adding the very ion you are testing for.
- Ammonium ions also give ammonia with warm sodium hydroxide. In the nitrate test, warm with sodium hydroxide alone first. Only ammonia that appears after adding aluminium shows nitrate. Nitrite gives ammonia this way too, so rule it out with dilute acid first.
- BaSO3 and BaSO4 are both white. Only BaSO3 dissolves when dilute strong acid is added.
- Describe the test for sulfate ions in a solution.
- A solution contains sodium chloride or sodium iodide. How would you tell which?
- How would you tell a sulfite from a sulfate?
7.6Testing for gases
| gas | description | test | positive result |
|---|---|---|---|
| hydrogen, H2 | colourless, odourless | burning splint at the mouth of the tube | goes out with a ‘pop’ |
| oxygen, O2 | colourless, odourless | glowing splint into the tube | relights |
| carbon dioxide, CO2 | colourless, odourless | bubble through limewater | white precipitate, which dissolves on further bubbling |
| chlorine, Cl2 | pungent smell | damp blue litmus paper | turns red, then is bleached |
| sulfur dioxide, SO2 | colourless, pungent smell | filter paper soaked in acidified KMnO4 | purple to colourless |
| ammonia, NH3 | colourless, pungent smell | damp red litmus paper | turns blue |
Again the tests use earlier chemistry. Ammonia is the only common alkaline gas (Chapter 5). Sulfur dioxide is a reducing agent (Chapter 6). Chlorine forms an acid with the water on the paper, then bleaches it.
Worked exampleSolid K gives off a vapour on gentle heating that condenses to a liquid boiling at 100 °C. A solution of K gives a blue precipitate with aqueous sodium hydroxide, and on warming a gas turns damp red litmus blue. With aqueous ammonia the blue precipitate dissolves in excess. Acidified with dilute nitric acid, it gives a white precipitate with barium nitrate. What does K contain?
- A reaction gives off a colourless, odourless gas. Which gases could it be, and how would you tell them apart?
- Describe how to tell chlorine, ammonia and hydrogen apart.
7.7Accuracy, precision and error Core · A Level
Section 7.1 gave the accuracy of each instrument. Here we use those values to judge an experiment.
- Results can be precise but not accurate: tightly grouped, but all away from the true value (Figure 7.10).
- A better instrument or a correct reading improves accuracy. Repeating the measurement shows how good the precision is.
- Reading the meniscus from above or below adds a parallax error to every reading.
- A systematic error affects every measurement in the same way, so repeating does not remove it. A random error makes results fluctuate around their mean, so averaging repeats reduces it.
To find the largest source of error, compare the percentage uncertainty of each measurement.
Worked exampleAn experiment uses 5.0 cm3 of acid measured in a measuring cylinder (±0.5 cm3) and 2.00 g of solid weighed on an electronic balance (±0.01 g). Which measurement is the larger source of error? What change reduces it?
- Which has the larger percentage uncertainty: 20.0 cm3 in a measuring cylinder (±0.5 cm3) or 2.00 g on a balance (±0.01 g)?
- Five readings of one mass are 5.21 g, 5.20 g, 5.22 g, 5.21 g and 5.20 g. The true mass is 5.00 g. Are the readings accurate, precise, both or neither?
Answers to Check your understanding
7.1
- A burette is more accurate (nearest 0.05 cm3 against 0.5 cm3) and can measure a range of volumes.
- A pipette measures only one fixed volume.
7.2
- Water displacement, because the gas is insoluble. Upward delivery also works, because the gas is less dense than air.
- Quicklime (calcium oxide). A gas that reacts with acids is basic or reacts like a base, so concentrated sulfuric acid would react with it.
7.3
- To mix the titrant with the analyte, so the colour change appears exactly when enough has been added.
- 24.65 − 1.20 = 23.45 cm3
7.4
- Aqueous sodium hydroxide, then aqueous ammonia. A white precipitate insoluble in excess sodium hydroxide shows calcium. If it dissolves in excess sodium hydroxide, add aqueous ammonia: insoluble in excess shows aluminium, soluble in excess shows zinc.
- Dark blue. Copper(II) ions.
- Calcium, Ca2+. Mg2+ would also give a white precipitate with aqueous ammonia.
7.5
- Add dilute nitric acid, then aqueous barium nitrate. A white precipitate of barium sulfate shows sulfate ions.
- Add dilute nitric acid, then aqueous silver nitrate. A white precipitate shows chloride. A yellow precipitate shows iodide.
- Add dilute acid: a sulfite gives sulfur dioxide, which turns acidified KMnO4 colourless, and a sulfate gives no gas. (With Ba2+, only the sulfite precipitate dissolves in dilute acid.)
7.6
- Hydrogen, oxygen or carbon dioxide. A burning splint goes out with a pop in hydrogen, a glowing splint relights in oxygen, and carbon dioxide turns limewater milky.
- Damp red litmus turns blue in ammonia. Damp blue litmus turns red and is bleached by chlorine. A burning splint goes out with a pop in hydrogen.
7.7
- The cylinder: 0.5 ÷ 20.0 × 100% = 2.5%, against 0.01 ÷ 2.00 × 100% = 0.5% for the balance.
- Precise but not accurate. The readings agree closely with each other but are all about 0.2 g above the true mass.