Learning Objectives At the end of the lesson, you should be able to: Identify the various apparatus used in the laboratory for measurement State the accuracy of the apparatus used in the laboratory. Suggest suitable apparatus to be used for measurement for a variety of experiments In Chemistry experiments We measure 4 basic physical quantities: Mass Temperature Time Volume Measuring Mass Apparatus: Electronic balance S.I. Unit: kilogram (kg) Normally used to measure small masses of solid chemicals Smallest division: 0.1 g or 0.01 g Accuracy: 0.1 g or 0.01 g Example of how to make recording: 2.4 g or 2.45 g Measuring Temperature Apparatus: Mercury in glass ( -10 C to 110 C) thermometer or alcohol thermometer S.I. Unit: Kelvin (K) However, degree Celsius ( C) is usually used. Smallest division: 1 C Accuracy: 0.5 C Example on how to make recording: 35 C or 35.5 C and not 35.3 C Measuring Time Apparatus: Digital Stopwatch or Analogue Stop clock S.I. Unit: second (s) Smallest Division / Accuracy: Digital 0.1s / 0.01s Analogue 0.1s 1
Measuring Volume For measuring volume, the apparatus chosen to be used for an experiment will depend on: 1. The volume that you are measuring 2. How exact you need the volume to be Measuring Volume: Beaker Used to measure rough volume of liquid Unit: mili-litres (ml) or cm 3 Example: 50 cm 3 beaker Smallest division:10 cm 3 cross-view Measuring Volume: Measuring Cylinder Used to measure variable volume of liquid approximately. It is more accurate than a beaker. Smallest division: 1 cm 3 Cross-view Measuring Volume: Burette Burette Setup burette Used to measure a variable volume of liquids accurately. Often used in titration for formation of salts. Smallest division: 0.1 cm 3 It can measure volumes of liquids up to a maximum of 50.0 cm 3 and accurately to 0.1cm 3. Volume required = Initial Reading Final reading Measuring Volume: Pipette cross-view To measure a fixed, specific set of volume of liquid accurately. For example: 25.0cm 3 ; 20.0cm 3 ; 50.0cm 3 Note: Cannot measure odd volume such as 26.0cm 3 Comparison of Apparatus Apparatus Accuracy Reason Beaker Measuring cylinder Pipette Burette approximate volume and variable volume (not accurate) beaker and can measure variable volume beaker and measuring cylinder. It has the same accuracy as burette but has a fixed volume. beaker and measuring cylinder. It has the same accuracy like pipette but measures variable volume. e.g A 50cm 3 beaker has markings in divisions of 10cm 3. Readings are in 10cm 3, 20..50cm 3 e.g A 50cm 3 measuring cylinder has markings in division of 1cm 3. Reading are in 11cm 3, 12, 13,.., 48, 49, 50 cm 3 e.g A 50.0 cm 3 pipette measures fixed volume. It mean that every measurement gives a fixed volume of 50.0cm 3. It is used during titration when making salts. e.g A 50.0 cm 3 burette can measure volume such as 24.5, 24.6, 24.7.49.8, 49.9, 50.0cm 3 2
How to read the volume of a liquid? Measuring Volume of Gases When water or solution is placed in a glass container, it forms a curved surface called a meniscus. A meniscus can be concave or convex. To read the volume of a liquid, align your eyes to the liquid level. Apparatus: Gas syringe Unit: cm 3 plunger barrel concave convex How to use a gas syringe? Collecting Gases Upward delivery - To collect gas that are soluble in water and less dense than air such as ammonia downward delivery - To collect gases that are soluble in water and denser than air such as chlorine and hydrogen chloride Displacement of water - To collect gases that are insoluble in water. E.g. Nitrogen gas, hydrogen gas, oxygen gas How to determine if a gas is light or heavy? As a general guide, a gas is less dense than air if its molecular mass is less than 30. What would a chemist use to measure exactly 25.5 cm 3 of dilute hydrochloric acid? E.g. Ammonia (NH 3 ) Molecular mass of ammonia = 14 + 1 + 1 + 1 = 17 (less than 30) Hence, ammonia is a light gas. A Beaker B Burette C Pipette D Measuring cylinder ANSWER: 3
What is the reading of the volume of liquid shown in the measuring cylinder? A 28 cm 3 B 30.6 cm 3 C 36 cm 3 D 40 cm 3 ANSWER: Mingde records a temperature of 48.2 C from a mercury-in-glass thermometer. What is the mistake he made for his recording? Answer: For a mercury-in-glass thermometer, the accuracy of the apparatus is 0.5 C. Hence, it is not possible for Mingde to record a reading of 48.2 C. Name one suitable piece of laboratory apparatus for (a) measuring the volume of about 80 cm 3 of a liquid, beaker (b) adding exactly 18.5 cm 3 of liquid to a beaker, burette (c) collecting and measuring 80 cm 3 of gas, and Gas syringe (d) adding exactly 25.0 cm 3 of a solution to a flask. pipette Determining Purity A pure substance is made up of only one substance. Air is a mixture. It is made up of different gases. However, a white diamond is pure. Why is it important to determine purity of a substance? Impurities in drugs must be detected as they may cause side effects. Chemicals are often added to food and beverages. It is important to ensure that only the pure chemicals are added. How do we determine the purity of a substance? We can do so by doing one of the following: - Checking the melting point of a solid - Checking the boiling point of a liquid - Performing chromatography 4
Determining Purity by Melting and Boiling Points A pure solid has an exact and constant melting point. A pure liquid has an exact and constant boiling point. Effect of Impurities on Melting Points Impurities affect the melting point of a substance in two ways: They lower the melting point. The greater the amount of impurities, the lower the melting point of the substance. They cause melting to take place over a range of temperatures. Effect of Impurities on Boiling Points If a liquid is impure, its boiling point will increase. The liquid will also boil over a range of temperatures. The greater the amount of impurities, the higher the boiling point of the liquid. 5