Topic 12: Experimental Techniques

Question 1

How do you read the volume of a liquid?

Answer 1

Read the volume from the bottom of the meniscus when it is at eye level.

Question 2

Name the three methods to measure the volume of a liquid.

Answer 2

Using;

Volumetric pipettes
Burettes
Measuring cylinders

Question 3

Identify this apparatus.

Answer 3

Volumetric pipette

Question 4

How do you control the amount of liquid drawn up?

Answer 4

A pipette filler attached to the end of the pipette is used to safely control the amount of liquid you are drawing up

Question 5

State the benefit of volumetric pipettes.

Answer 5

Volumetric pipettes are often calibrated to allow for the fact that the last drop of liquid stays in the pipette when the liquid is ejected.
This reduces transfer errors.

Question 6

Identify this apparatus.

Answer 6

Burette

Question 7

How do you use a burette?

Answer 7

Take an initial reading.
Once you have released as much liquid as you want, take a final reading.
The difference between the readings tells you the amount of liquid used.

Question 8

Identify this apparatus.

Answer 8

Measuring Cylinder

Question 9

What are the disadvantages of a measuring cylinder?

Answer 9

They are less accurate than burettes and volumetric pipettes.

Question 10

When do you use a dropping pipette?

Answer 10

If you only want a couple of drops of liquid and don’t need it to be accurately measured.

Question 11

What do you use to measure solids.

Answer 11

A balance.

Question 12

Why do we collect gases.

Answer 12

To investigate the rate of the reaction

Question 13

How do you measure the volume of a gas?

Answer 13

Using a gas syringe
By displacing water from a measuring cylinder

Question 14

How do you find the volume of a gas using the displacement method?

Answer 14

1. Fill a measuring cylinder with water, and carefully place it upside down in a container of water.
2. Record the initial level of the water in the measuring cylinder.
3. Position a delivery tube coming from the reaction vessel so that it’s inside the measuring cylinder, pointing upwards. Any gas that’s produced will pass through the delivery tube and into the measuring cylinder. As the gas enters the measuring cylinder, the water is pushed out.
4. Record the level of water in the measuring cylinder and use this value, along with your initial value, to calculate the volume of gas produced.

Question 15

What apparatus do you use to measure temperature?

Answer 15

A thermometer

Question 16

How do you use the thermometer?

Answer 16

1) Make sure the bulb of the thermometer is completely submerged in any mixture you’re measuring, but not touching the inside of the container.

2) If you’re taking an initial reading, you should wait for the temperature to stabilise first.

3) Read your measurement off the scale on a thermometer at eye level to make sure it’s correct.

Question 17

What do you use to measure the time taken?

Answer 17

A stopwatch

Question 18

Describe the method you would use to do a titration.

Answer 18

1) Using a pipette, measure out a set volume of the alkali into a flask. Add a few drops of an indicator — usually thymolphthalein or methyl orange

2) Fill a burette with a standard solution (a known concentration) of acid.

3) Use the burette to add the acid to the alkali a bit at a time. Swirl the flask regularly. Go slowly (a drop at a time) when you think the alkali’s almost neutralised.

4) The indicator changes colour when all the alkali has been neutralised (the exact point when the indicator changes colour is called the end-point) — thymolphthalein is blue in alkalis but colourless in acids, and methyl orange is yellow in alkalis but red in acids.

5) Record the volume of acid used to neutralise the alkali (called the titre).

6) Repeat this process a few times, making sure you get very similar results each time (i.e. within 0.1 cm3 of each other). You can then take the mean of your results.

Question 19

State the apparatus used in a titration.

Answer 19

Volumetric pipette
Burette
Conical flask

Question 20

Define pure substance

Answer 20

A substance completely made up of a single element or compound

Question 21

Define mixture

Answer 21

Elements and compounds that are not chemically combined

Question 22

A pure substance has ______________________

Answer 22

a specific, sharp melting and boiling point

Question 23

Do mixtures have a sharp melting point?

Answer 23

No, they melt gradually over a range of temperatures.
Impurities decrease the melting point and increase the boiling point.

Question 24

Define solution

Answer 24

A mixture of a solute and a solvent that does not separate out

Question 25

Define solute

Answer 25

The substance being dissolved

Question 26

Define solvent

Answer 26

The liquid that the solute is dissolving into

Question 27

Define saturated solution

Answer 27

A solution where the maximum amount of solute has been dissolved so no more solute will dissolve in the solution at that temperature

Question 28

Define residue

Answer 28

The substance remaining at the end of a process of separation or purification, such as evaporation, distillation or filtration

Question 29

Define filtrate

Answer 29

A solution (or liquid) that has been filtered.

Question 30

How do you investigate how the solubility of ammonium chloride is affected by temperature?

Answer 30

1) Make a saturated solution by adding an excess of ammonium chloride to 10 cm3 of water in a boiling tube. You will know when ammonium chloride is in excess because it will start to sink to the bottom of the tube and remain undissolved.

2) Stir the solution and place the boiling tube in a water bath set to 25 °C.

3) After 5 minutes, check that all of the excess solid has sunk to the bottom of the tube and use a thermometer to check that the solution has reached 25 °C.

4) Filter the solution to remove any undissolved solid.

5) Weigh an empty evaporating basin. Pour the filtrate into the basin.

6) Re-weigh the basin and its contents, then gently heat it using a Bunsen burner to remove all the water.

7) Once all the water has evaporated, you’re left with just the ammonium chloride that was dissolved in the solution at 25 °C. Re-weigh the evaporating basin and its contents.

8) Repeat steps 1-6 twice more, but with the water bath at different temperatures (e.g. 35 °C and 45 °C).

Question 31

Describe the process of chromatography.

Answer 31

1) In paper chromatography, a spot of a mixture is added to chromatography paper. It is put on a baseline, which has been drawn in pencil. Then the end of the paper is placed in a solvent, with the solvent just below the baseline.

2) The solvent travels up the paper and carries the substances in the mixture with it. As they move up the paper, the substances separate out.

3) different substances will move up the paper at different rates.

4) Some will stick to the paper and others will dissolve more readily in the solvent and travel more quickly.

Question 32

How do you calculate Rf value?

Answer 32

distance travelled by substance/distance travelled by solvent

Question 33

How do you identify colourless substances?

Answer 33

to spray the chromatogram with a chemical called a locating agent to show where the spots are.