Chapter 12 - Experimental techniques and chemical analysis

Question 1

How can time be measured?

Answer 1

Time can be measured using a stopwatch which shows time to one or two decimal places. The units used to measure time are usually seconds or minutes. 1 minute = 60 seconds.

Question 2

How can temperature be measured?

Answer 2

Temperature is measured with a thermometer. Laboratory thermometers usually have a precision of half or one degree. The units of temperature are degrees Celsius (C)

Question 3

How can mass be measured?

Answer 3

Mass is measured using a digital balance which normally gives readings to two decimal places. Balances should be set to zero before use and should be allowed time to settle on a final measurement before the reading is recorded. 1000 grams = 1 kilogram

Question 4

How can volumes of liquid be measured?

Answer 4

Volumes of liquid can be determined using burettes, volumetric pipettes or measuring cylinders. Burettes are the most accurate way of measuring a variable volume of liquid between 0cm3 and 50cm3. Read the burette scale form top to bottom as 0.00cm3 is at the top of the column. Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid. They have a scratch mark on the next which is matched to the bottom of the meniscus to make the measurement. Measuring cylinders are used when approximate volumes are required. 1 millilitre = 1cm3.

Question 5

How can volumes of gas be measured?

Answer 5

Volumes of gas can be measured using a gas syringe or the downward displacement of water. A gas syringe is more precise and accurate than the downward displacement of water. Downward displacement of water is where a measuring cylinder is inverted in water to collect the gas produced. This method doesn’t work if the gas is soluble in water.

Question 6

What are the advantages of a temperature probe?

Answer 6

More precise readings
Easy to make multiple repeat readings
Can be automated to run over long periods of time

Question 7

What are the disadvantages of a temperature probe?

Answer 7

Can be corroded by some reagents
More expensive to replace

Question 8

What are the advantages of a volumetric pipette?

Answer 8

Accurate measurement of a fixed volume

Question 9

What are the disadvantages of a volumetric pipette?

Answer 9

Harder to use than a normal pipette
Only measures one fixed volume

Question 10

What are the advantages of a gas syringe?

Answer 10

Easy to set up
Keeps the gas dry

Question 11

What are the disadvantages of a gas syringe?

Answer 11

The syringe can stick
Collects limited volumes
Expensive and delicate

Question 12

What are the advantages of microscale experiments?

Answer 12

Less wasteful
Saves energy
Safer

Question 13

What are the disadvantages of microscale experiments?

Answer 13

Hard to see what is happening
Lose a lot of material when separating or purifying the product

Question 14

What is a solvent?

Answer 14

A solvent is a substance that dissolves a solute

Question 15

What is a solute?

Answer 15

A solute is a substance that is dissolved in a solvent

Question 16

What is a solution?

Answer 16

A solution is a mixture of one or more solutes dissolved in a solvent

Question 17

What is a saturated solution?

Answer 17

A saturated solution is a solution containing the maximum concentration of a solute dissolved in the solvent at a specific temperature

Question 18

What does soluble mean?

Answer 18

Soluble is a substance that will dissolve

Question 19

What does insoluble mean?

Answer 19

Insoluble is a substance that will not dissolve

Question 20

What is residue?

Answer 20

Residue is a substance that remains after evaporation, distillation, filtration or any similar process

Question 21

What is a filtrate?

Answer 21

Filtrate is a liquid or solution that has passed through a filter

Question 22

What are titrations?

Answer 22

Titrations are a method of analysing the concentration of solutions. They can determine exactly how much alkali is needed to neutralise a quantity of acid and vice versa.

Question 23

What apparatuses are used in acid-base titrations?

Answer 23

25cm3 volumetric pipette
Pipette filler
50cm3 burette
250cm3 conical flask
Small funnel
0.1mol/dm3 sodium hydroxide solution
Sulfuric acid of unknown concentration
A suitable indicator
Clam stand, clamp and white tile

Question 24

What is the method used in acid-base titrations?

Answer 24

1. Use the pipette and pipette filler and place exactly 25cm3 sodium hydroxide solution into the conical flask.
2. Fill the burette with hydrochloric acid, place an empty beaker underneath the tap. Run a small portion of acid through the burette to remove any air bubbles.
3. Record the starting point on the burette to the nearest 0.05cm3.
4. Place the conical flask on a white tile so the tip of the burette is inside the flask.
5. Add a few drops of a suitable indicator to the solution in the conical flask.
6. Perform a rough titration by taking the burette reading and running in the solution in 1 – 3cm3 portions, while swirling the flask vigorously.
7. Quickly close the tap when the endpoint is reached. The endpoint is when one drop causes a sharp colour change.
8. Record the volume of hydrochloric acid added, in a suitable results table as shown below Make sure your eye is level with the meniscus.
9. Repeat the titration with a fresh batch of sodium hydroxide.
10. As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour.
11. Record the volume to the nearest 0.05cm3.
12. Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy.

Question 25

What are indicators used for in titrations?

Answer 25

Indicators are used show the endpoint in titration. Wide range indicators such as litmus are not suitable for titration as they do not give a sharp colour change at the end point. However, methyl organe and phenolphthalein are very suitable.

Question 26

Table of indicator colours in acids, alkali and neutral

Answer 26

Question 27

What is chromatography?

Answer 27

Chromatography is used to separate mixtures of soluble coloured substances, using a suitable solvent. It can also be used to separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent.

Question 28

What is the method used in chromatography?

Answer 28

1. A pencil line is drawn on chromatography paper and sports of the sample are placed on it. 2. The paper is then lowered into the solvent container, making sure the pencil line sits above the level of the solvent. 3. The solvent is usually water, but it can be other substances such as ethanol. 4. The solvent travels up the paper by capillary action, taking some of the coloured substance with it. 5. Different substances have different solubilities so they will travel at different rates, causing the substances to spread apart. The substances with a higher solubility will travel further than the other substances.

Question 29

What is chromatography used for?

Answer 29

You can use a chromatogram to compare the substances present in a mixture to known substances and make assumptions.

Question 30

What will pure substances show on the chromatogram?

Answer 30

Pure substances will produce only one spot on the chromatogram.

Question 31

What will impure substances show on the chromatogram?

Answer 31

Impure substances will produce more than one spot on the chromatogram.

Question 32

What will a mixture show on the chromatogram?

Answer 32

If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.

Question 33

Why is a reference spot used in chromatography?

Answer 33

It is common practice to include a known compound as a reference spot, which can help to match up to an unknown sport or set of spots in order to identify it.

Question 34

What are locating agents?

Answer 34

Locating agents can be used to see the sports. These are substances which react with the sample and produce a visible coloured spot for the products. The chromatogram is treated with the agent after the chromatography run has been carried out which makes the substances visible to the naked eye.

Question 35

What are Rf values?

Answer 35

Rf values are used to identify the components of a mixture. It is calculated by the distance moved by the substance / distance moved by solvent. The Rf value is a ratio, has no units and will always be less than 1.

Question 36

How can a using differences in solubility separate a mixture of solids?

Answer 36

Separating a mixture of solids can be completed using differences in solubility. For a difference in solubility, a suitable solvent must be chosen to make sure only the desired substance is dissolved in the solvent and the other substances or impurities in the mixture should not dissolve in the solvent.

Question 37

What is filtration?

Answer 37

Filtration is a technique used to separate an undissolved sold from a mixture of the solid and a liquid. A filter paper is placed in a filter funnel above another beaker, the mixture of insoluble solid and liquid is poured into the filter funnel, the filter paper will only allow small liquid particles to pass through in the filtrate and solid particles are too large to pass through the filter paper so they will stay behind as residue.

Question 38

What is crystallisation?

Answer 38

Crystallisation is a method used to separate a dissolved solid from a solution. The solution is heated, allowing the solvent to evaporate and leaving a saturated solution behind, you can test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution and if the solution is saturate, crystals will form on the glass rod when it is removed and allowed to cool. The saturated solution is allowed to cool slowly, solids will come out of the solution as the solubility decreases, this will be seen as the crystal growing. The crystals are then collected by filtration then they are washed with distilled water to remove any impurities and are finally allowed to dry.

Question 39

What is simple distillation?

Answer 39

Simple distillation is used to separate a liquid and a soluble solid from a solution or a pure liquid from a mixture of liquid. The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask, the vapour passes through the condenser where it cools and condenses, turning into pure water which is then collected in a beaker. After all the water is evaporated from the solution, only the solid solute will be left behind. Simple distillation can be used to separate the products of fermentation, such as alcohol and water.

Question 40

What is fractional distillation?

Answer 40

Fractional distillation is used to separate two or more liquids that form a completely uniform mixture when added together. The solution is heated to the temperature of the substance with the lowest boiling point. This substance will rise and evaporate first, passing through a condenser where it cools and condenses, this liquid is then collected in a beaker. All of the substance is evaporated and collected, leaving behind the other components of the mixture.

Question 41

How can melting and boiling points be used to separate pure substances and mixtures?

Answer 41

Pure substances melt and boil at a specific and sharp temperature. Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures.

Question 42

Anion testing table

Answer 42

Question 43

Gas testing table

Answer 43

Question 43

Aqueous cation testing table

Answer 43

Question 44

Metal ion flame testing table

Answer 44