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.
