DEMO

EXPERIMENTAL TECHNIQUES

By the end of the sub topic, the learners should be able to:
  1. Name the apparatus for measuring time, temperature, mass and volume.
  2. State the S.I units
  3. Design the arrangement of apparatus given information about the substances involved.
  4. Convert units of mass, volume and temperature.

1.1 Apparatus

  • In Chemistry, appropriate apparatus for the measurement of time, temperature, mass and volume are used.

Time

  • Measurement of time is important to find out the speed of chemical reactions.
  • Time is measured using a stopwatch or stop-clock.
  • The SI unit for time is seconds (s). Other units used to measure time are minutes (min) and hours (h).
1 Fig 2.1.1_a.jpg (78 KB)
1 Fig 2.1.1_b.jpg (83 KB)

Conversion: 1 min = 60 s; 1 h = 60 min.
Example 1: Convert 100 s to min.
60 s = 1 min
100 s = 100s60s X 1 min
= 1.67 min

Example 2: Convert 2 hrs to min.
60 min = 1hr
x = 2 hr
x = 2hr1hr X 60 min
= 120 minutes

Temperature

  • A thermometer is used to measure the temperature of a substance.
  • In Chemistry, some reactions have to take place at specific temperatures so a thermometer is important in finding out if the correct temperature has been achieved. A thermometer is also vital in determining reactions which result in an increase or decrease in temperature.
  • The two types of liquid in glass thermometers that are commonly used in the Chemistry laboratory are the alcohol in glass thermometer and the mercury in glass thermometer.
  • The SI unit for temperature is Kelvin (K). The thermometers used in the laboratory have units in degrees Celsius °C.
• Conversion: K = °C + 273.
Example 1: Convert 25°C to K
K = 25°C + 273
=298
Therefore 25°C = 298 K

Example 2: Convert 300 K to °C.
300 K = °C + 273
°C = 300 - 273
= 27°C
2 Fig 2.1.2.jpg (54 KB)

Mass

  • Mass refers to a measure of the amount of matter in a substance.
  • A top pan balance is used to measure mass.
  •  A triple beam balance is less commonly used to measure mass these days.
  • The SI unit for mass is the kilogram (kg). In the Chemistry laboratory, a smaller unit of gram (g) is commonly used.
  • Conversion: 1 kg = 1000 g.
Example 1: Convert 120 g to kg.
1000 g = 1 kg
120 g = 120g1000g X 1 kg
= 0.12 kg

Example 2: Convert 5 kg to grams.
1 kg = 1 000 g
5 kg = 5kg1kg X 1000g
= 5 000 g
3 Fig 2.1.3.jpg (81 KB)

Volume

  • Volume refers to the space that a substance occupies.
  • The volume of liquids and gases are usually measured in the Chemistry laboratory.
  • The SI unit for volume is the cubic metre (m3). In the laboratory, volume is measured in cubic centimetre (cm3). Cubic decimetre (dm3) is also used.
  • Conversions: 1 m3 = 106 cm3; 1 dm3 = 1 000 cm3.
  • Example 1: Convert 0.4 m3 to cm3.
         1m3 = 106cm3
         0.4 m3 =  106cm3
        = 400 000 cm3
  • Example 2: Convert 300 cm3 to dm3.
        1 000 cm3 = 1 dm3
         300 cm3 =  1 dm3
         = 0.3 dm3
1.1 Volume of liquids
  • The apparatus used to measure volume in liquids depend on the volume to be measured and the accuracy of the measurement to be done.
  • The apparatus used to measure volume of liquids in the laboratory are beakers, burettes, conical flasks, graduated measuring cylinders, pipettes and volumetric flasks.
  • Volumetric flasks and pipettes are used to measure exact volumes and therefore they are always filled to the mark to measure specific volumes.
  • A volumetric flask is flat bottomed, pear shaped flask which has a single filling mark on its long neck.
  • The flask is used to accurately measure a specific volume when it is filled to the mark. It is also used for the preparation of solutions and temporary storage before use.

4 Fig 2.1.4.jpg (54 KB)

  • A pipette is a long glass tube with some having a bulge in the middle.
  • A pipette filler is fitted on the top of the pipette and it is used to suck liquid into the tube until the required mark is reached. The pipette filler is also used to empty the liquid in the pipette transferring it into another container.
  • A pipette is used to deliver an accurate volume of a specific amount of a liquid.
  • It is used in titration experiments to measure the volume of solution with a known concentration.

5 Fig 2.1.5.jpg (52 KB)

  • Conical flasks, beakers, graduated pipettes, burettes and graduated measuring cylinders have graduation marks along their length and for that reason they can measure varying amounts of liquids.
  • A conical flask has a wide flat bottom, a conical body and a cylindrical narrow neck.
  • It is used for measurement of approximate volumes.
  • Its long neck and wide base make it ideal for the mixing of solutions by swirling without spilling.
  • It is commonly used in titration experiments for the mixing of two solutions which are then swirled for effective mixing.

6 Fig 2.1.6.jpg (58 KB)

  • A beaker is a wide cylindrical container with a pouring sprout. It is usually used to measure large volumes of liquids. The graduated marks on a beaker are only approximate and therefore it is not ideal for accurate measurement of volume.
  • The beaker is therefore not commonly used for measuring volume but for other laboratory activities like mixing solutions and heating.
7 Fig 2.1.7.jpg (64 KB)

  • A burette is a long glass tube which is open at the top and has a tap at the bottom.
  • It is used whilst it is clamped onto a stand. Liquid is poured into the top of the burette using a funnel.
  • The initial volume is then recorded and the tap opened to release a required amount, the tap is then closed and a final reading recorded, the difference between the initial and final volumes give the volume used.
  • A burette can therefore be used to measure the volume of liquid delivered.
  • It is used in titration experiments to deliver the volume of liquid required to complete a reaction.
  • The use of a funnel is important as it has a wider opening than that of the burette which is too small and would result in the liquid spilling.
  • A burette is more accurate than a beaker or a measuring cylinder as the volume on the burette can be read to the nearest 0.1 cm3 whilst the other apparatus have scale divisions that are larger.

8 Fig 2.1.8.jpg (57 KB)

  • A graduated cylinder also known as measuring cylinder is the narrowest beaker; its length is much larger than its width and it has a wide base to make it stable and prevent falling.
  • A graduated cylinder has more measurement lines making it more accurate than a beaker for measuring volume.

9 Fig 2.1.9.jpg (60 KB)

  • It is important to read the volume of a liquid accurately by setting your eye in line with the bottom of the meniscus.
10 Fig 2.1.10_a.jpg (62 KB)
10 Fig 2.1.10_b.jpg (63 KB)
10 Fig 2.1.10_c.jpg (61 KB)
  • In fig 2.1.10, only one of the graduated cylinders has a liquid with a volume of 20 cm3. In A and B, the bottom of the meniscus is below the 20 cm3, in C the bottom of the meniscus is in line with the 20 cm3 giving a volume of 20 cm3.
  • A few liquids like mercury have a meniscus which extends outwards; the reading is therefore taken at the top of the meniscus.
  • When you are reading the mercury in glass thermometer you will take the reading at the top of the meniscus.
1.2 Volume of gases
  • The volume of gases is measured using a gas syringe.

11 Fig 2.1.11.jpg (109 KB)

  • A gas collected is connected to the gas syringe by a connecting tube; the gas pushes the plunger in the syringe as its volume increases.
  • The syringe has graduated marks along its length and the volume of gas collected can be determined.
  • It is used to measure the volume of gaseous products from a chemical reaction.
  • An inverted measuring cylinder or burette can also be used to measure the volume of a gas.
  • The measuring cylinder or burette is completely filled with water and then turned upside down in a trough of water.
  • As the gas is collected, it displaces the water and pushes it down. The amount of gas collected can then be determined.
  • Gases collected using this method should have low solubility in water. Hydrogen is insoluble in water and it can be collected using an inverted measuring cylinder, ammonia and chlorine gases on the other hand are readily soluble in water and they are collected using a gas syringe.

12 Fig 2.1.12.jpg (135 KB)

Use of apparatus

  • The apparatus presented can be combined to carry out various experiments in the laboratory.
  • The following experiments demonstrate how apparatus can be used and arranged to perform laboratory work.
  • Studying these experiments should help you to be familiar with setting up apparatus when you are given information about an experiment.
Experiment 1.1.1: Measuring the rate of a chemical reaction between magnesium and hydrochloric acid
Materials: Top pan balance; stopwatch; 250 cm3 conical flask; rubber stopper with one hole; 2 X 100 cm3 graduated cylinders; trough; delivery tube; clamp stand; magnesium ribbon; 1M hydrochloric acid in a 250 cm3 stoppered volumetric flask; water.

Procedure
Precaution: Hydrogen gas is highly flammable; ensure that there are no naked flames when performing this experiment.
The apparatus is set up as in fig 2.1.12.
  1. Measure 100 cm3 of the hydrochloric acid using one of the graduated cylinders and pour into the conical flask.
  2. Insert the delivery tube into the hole on the rubber stopper. Insert the rubber stopper on the conical flask.
  3. Half-fill the trough with water.
  4. Fill the graduated cylinder with water and invert it into the trough ensuring that it remains filled with water.
  5. Use the top pan balance to measure 1.5 g of the magnesium ribbon.
  6. Remove the rubber stopper from the conical flask and put the magnesium ribbon. Immediately replace the stopper and start the stopwatch at once.
  7. Measure the volume of gas collected at 10 second intervals until the volume of gas collected becomes constant.
  8. Measure the total volume of gas collected.
  9. Draw table of the results and draw graph of time against volume. Determine the rate of the reaction in cm3/s.

Expected observations

  • A colourless gas is collected in the inverted cylinder.
  • The gas displaces the water as it is collected.
  • With time the amount of gas in the cylinder becomes constant.
  • The gas produced in the experiment is hydrogen gas (H2) according to the following reaction of magnesium (Mg) and hydrochloric acid (HCl):
Mg(s)+2HCI(aq)MgCI2(s)+H2(g)
  • The volume of gas collected becomes constant when the reactants are finished, the chemical reaction will be complete.
  • Hydrogen gas is less dense than water and also has low solubility in water therefore it can be effectively collected using this method.
  • Other gases like carbon dioxide have a high solubility in water and it is not ideal to collect them using the above method.
  • The inverted graduated cylinder and water trough are replaced by the gas syringe when collecting these gases.

Experiment 1.1.2: Titration reaction.
Materials: 50 cm3 burette; clamp stand; funnel; 25 cm3 pipette; pipette filler; 250cm3 conical flask; dropper; small beaker; white tile; 1M hydrochloric acid prepared in a 250 cm3 volumetric flask filled to the mark; sodium hydrogen carbonate solution prepared in a 250 cm3 volumetric flask filled to the mark; bottle of methyl orange indicator.
13 Fig 2.1.13.jpg (111 KB)

14 Fig 2.1.14.png (97 KB)
Procedure
  1. Ensure that the glassware is clean before use.
  2. Clamp the burette to the stand.
  3. Put the funnel to the top of the burette.
  4. Fill the burette with the 1M hydrochloric acid. Open the burette and let the first few drops run through into the small beaker placed below the burette.
  5. Remove the funnel and read and record the volume of the acid in the burette, this is the initial volume.
  6. Fit the pipette filler on the pipette and collect 25 cm3 of sodium hydrogen carbonate up to the mark.
  7. Transfer the sodium hydrogen carbonate into the conical flask.
  8. Add a few drops of methyl orange indicator.
  9. Put the conical flask on the white tile below the burette.
  10. Open the tap on the burette and add the acid, shake the conical flask as you add the acid for effective mixing.
  11. When you notice the first signs of colour change, reduce the flow of the burette to drops and stop the flow of the burette when the colour of the solution changes.
  12. Read and record the volume of the acid in the burette, this is the final volume.
  13. To find the volume of acid used, subtract the initial volume from the final volume.
  14. The procedure can be repeated two more times or until volumes which have a difference of 0.1 cm3 are obtained. Ensure that you rinse the conical flask first with tap water and then with distilled water before reusing it.
Expected observations
  • As the acid is added to the conical flask the colour changes when the end point is reached the colour of the solution changes.
  • Volume of the acid used reacts with the 25 cm3 of the sodium hydrogen carbonate in the conical flask and at the end of the reaction the colour changes.

Indicators

  • The colour change that occurs in a titration experiment is due to the indicator added to the conical flask which changes colour at the end point of the reaction.
  • In titration reactions, indicators are substances that show the end point of a chemical reaction by change of colour.
  • The indicator selected for a particular chemical reaction should change colour as close as possible to the end point.
  • Methyl orange is one of the most common indicators used in titration experiments.
  • The indicator is used to detect the end point of a chemical reaction involving a weak base and a strong acid.
  • Methyl orange is red in the acid and yellow in the base. It gives an orange colour at the end point of the reaction.
  • The colour at the end point of the reaction is due to a mixture of the red colour produced because of the presence of the acid and the yellow colour produced due to the presence of the weak base.
  • To improve the detection of colour change for methyl orange, screened methyl orange is made.
  • Screened methyl orange is made by mixing methyl orange with a neutral dye.
  • The resulting indicator has a more distinct colour change which can be easily detected.
  • Screened methyl orange is red in acid, green in base and grey at the end point of a reaction.