Chapter 2
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The document discusses several methods for measuring important quantities in scientific experiments including time, temperature, mass, volume, and gas volume. It describes the standard International System of Units (SI units) and common lab tools used to measure each quantity, including stopwatches for time, mercury thermometers and temperature sensors for temperature, beam balances and electronic balances for mass, measuring cylinders and burettes for volume, and gas syringes for measuring gas volumes. Accuracy levels and appropriate tools depend on the specific measurement needs in each experiment.
Chapter 2
- 1. Chapter 2 Measuring time - International system of Units (S.I units) is system of different units used to measure quantities of different things. help to communicate scientific info easier , all scientists around the world use S.I units as common standard for measurements - In lab, a stopwatch or stop clock is used to measure time. - S.I unit for time is seconds (s). - Other units: minute (min) and hour (h) are used to measure longer intervals of time. - Two types of stopwatch : 1. Analogue knob – used to start, stop and reset the stopwatch secondary dial (smaller one) – measure time in minutes main dial (bigger or whole one) – measure time in seconds 2. Digital Measuring Temperature - In lab, mercury thermometer used to measure temperature. - S.I unit for temperature is Kelvin (K). - Other units : degree Celsius (°C ) Temperature K = Temperature °C + 273 Eg. 1°C = 1+273 = 274K Eg. 50°C = 50+273= 323K - Mercury thermometer measure temperature from -10°C t0 110°C Each division on scale represent one degree Celsius (1°C) Accuracy +- 0.5°C - Temperature sensor connected to data logger to measure temperature. More accurate than mercury thermometer. Useful for recording temperature changes continuously over a period of time Can be used to study and record changing temperature of substance being heated and cooled commonly used for taking measurements outdoors Eg. Humidity of different environments Eg. pH of different sources of water Measuring Mass - Mass of substance measured with a beam balance or electronic balance. - S.I n unit for mass is kilogram (kg). - Very small masses of substance needed to be measured in chemistry experiment so electronic balance used. Accuracy up to 0.01g (2 d.p.) used for precise and fast weighing
- 2. Measuring Volume - Scientific experiment involves liquids. - Lab apparatus used to measure volume of liquid. - Different apparatus different degrees of accuracy. Measuring cylinder – nearest cm³ (more accurate than beaker) Beaker – approx. 100cm³ (used for estimation , estimate volume of liquid) Pipette – 25cm³ , 20cm³(accurate measurement of fixed volumes of liquids) Burette – nearest 0.1cm³ (scale marked in 0.1cm³ divisions) - Choice of apparatus depends on volumes you are measuring and how exact the volumes need to be. - How to read volume of liquid? When water or solution placed in glass container, it forms a curved surface called meniscus. Meniscus can be concave or convex. To read volume, align eyes to liquid level. - S.I unit for volume is cubic metre (m³) - Other units : cubic centimeter (cm³) , litre (l) , millimeter (ml) Collecting gases and measuring volumes of gases - In some experiment, gas given off. - Gas can be collected and identified to find out about the reactions that occurred. - Sometimes, gases collected are used for another reaction. - Methods to collect gas : Depend on physical properties of gas solubility – how soluble is gas in water density – how dense is the gas compared to air 1. Displacement of water suitable for collecting gas that are insoluble or slightly soluble in water Eg. Carbon Dioxide, hydrogen or oxygen 2. Downward delivery used to collect gas that are soluble in water, denser than air Eg. Chlorine and hydrogen chloride 3. Upward delivery used to collect gas soluble in water, less dense than air Eg. ammonia - How to collect dry sample of gas? Pass the gas through a drying agent to attain dry gas. Eg. Commonly used drying agents – concentrated sulfuric acid, quicklime (calcium oxide), fused calcium chloride
- 3. Concentrated sulfuric acid : used to dry most gases including chlorine and hydrogen chloride but not ammonia as they react together See pg. 27 & 28 example (* tube introducing gas immersed in drying agent but exiting tube not) - How to measure volume of gas? Use gas syringe to measure volume of gas. measure maximum volume of 100cm³ made up of two parts : barrel & plunger At the start, plunger pushed in fully to expel any gas in syringe. As gas from external source enter syringe, it pushes the plunger outwards.