BPGHS 2025 Sec 3 EOY Revision (3G)

2025 Sec 3 EOY Revision | Chemistry Revision notes for the upcoming Sec 3 EOY chemistry exam 🧪 1.1 | Measurements of Physical Quantities (Experimental Chem) Measurement of Time For measurement of time, a stopwatch is used. Common apparatus include digital stopwatch. Depending on the type of digital stopwatch, it can measure time up to 2 decimal places in seconds (e.g: 20.01 s). The SI unit for time is the second (s). Other units such as minute (min) and hour (h) could also be used. Table 1.1.1. The precisions and uncertainties of digital stopwatches Digital Stopwatch Precision/s Uncertainty of measurement/s D.P. Example timing/s 0.1 ±0.1 1 28.1 0.01 ±0.01 2 28.00 28.11 ⓘ Note Precision is determined by the smallest division on the instrument whereas uncertainty of measurement corresponds to the range where the actual value could fall into. For example, if a stopwatch has an uncertainty of ±0.01s, if its reading shows 28.00 s, it means the actual value could be anywhere in the range of 27.99 s to 28.01 s. Measuring of Temperature For measurement of temperature, a thermometer is used — alcohol thermometers and mercury thermometers . Temperature measured by a thermometer is recorded to the nearest 0.5 °C. The SI unit for temperature is the kelvin (K). Other units used include degree Celsius (°C). Temperature can also be recorded using a datalogger connected to a temperature sensor . The advantages of using a datalogger connected to a temperature sensor are: ● It is more accurate than the mercury or alcohol thermometer. ● It can record data continuously over a period of time. ● It saves data (in a computer) which can be used to produce graphs and charts Page 1 . Notes prepared by BPGHS 3G 2025 :)

Table 1.1.2. The precision and uncertainty of laboratory thermometers Thermometer Precision/ ℃ Uncertainty of measurement/ ℃ D.P. Example temperature/ ℃ 1 ±0.5 1 23.0 23.5 Measurement of Length For measurement of length in laboratory, metre rule and measuring tape are used. The SI unit for length is metre (m). Other units include mm, cm, dm. Table 1.1.3. The precision and uncertainty of a metre rule Metre rule Precision/mm Uncertainty of measurement/mm D.P. Example length/mm 1 ±0.5 1 20.5 Measurement of Mass For measurement of mass, an electronic balance is used. Mass measured by an electronic mass balance is recorded to the nearest 0.001 g, depending on the type of electronic mass balance used. The SI unit for mass is the kilogram (kg). Mass can also be measured in grams (g) or milligrams (mg). Table 1.1.4. The precision and uncertainty of an electronic balance Electronic Balance Precision/g Uncertainty of measurement/g D.P. Example mass/g 0.001 ±0.001 3 2.253 Measuring of Volume (of liquids and solutions) For measurement of volume of liquids, pipette, volumetric flask, measuring cylinder and burette can be used. (Refer: table 1.1.5 ) Page 2 . Notes prepared by BPGHS 3G 2025 :)

Table 1.1.5. The precision and uncertainty of various instruments used to measure volume Apparatus Precision/ 𝑐 𝑚 3 Uncertainty of measurement/ 𝑐 𝑚 3 D.P. Example volume/ 𝑐 𝑚 3 Primary use Measuring Cylinder 1 0.5 1 15.0 Measures approximate volumes of liquids and solutions Pipette - 0.1 1 25.0 10.0 Measures accurate, fixed volumes of liquids and solutions Burette 0.1 0.05 2 25.45 Measures very precise volumes of liquids and solutions Volumetric Flask - - - 100 Measures accurate fixed volumes that are larger ⓘ Note To avoid parallax error, when reading from a burette or measuring cylinder, we should position our eyes at the meniscus, to avoid parallax error. Measuring Volume of Gases To measure the volume of a gas, a gas syringe is used. Typical gas syringes measures volume up to 100 . 𝑐 𝑚 3 Page 3 . Notes prepared by BPGHS 3G 2025 :)

1.2 | Collection and Drying of Gases (Experimental Chem) Table 1.2.1. Methods of collecting gases Water displacement Used to collect insoluble or slightly soluble gases. Density does not affect gas collection. Description Gas released is bubbled into an inverted test tube filled with water. Gas will take up the volume and push down the water level in the test tube. (Since it does not dissolve in water) Example Gases Oxygen, hydrogen, carbon dioxide Fig 1.2.1. Collection of gases using water displacement Downward delivery Used to collect gases that are denser than air. Description The gas collected will sink in the gas jar and displace the air. Gas Jar must not be sealed in order for the air to be pushed out by the gas so that the gas can enter. Example Gases Chlorine, hydrogen chloride, sulfur dioxide, nitrogen dioxide Fig 1.2.2. Collection of gas using downward delivery Page 4 . Notes prepared by BPGHS 3G 2025 :)

Upward delivery Used to collect gases that are less dense than air. Description The gas collected will rise up the gas jar and displace the air inside. Gas Jar must not be sealed in order for the air to be pushed out by the gas so that the gas can enter. Example Gases Ammonia, Hydrogen Fig 1.2.3. Collection of gases using upward delivery Gas syringe Used to measure and collect any type of gas Description Gas produced would be collected in the gas syringe and push the plunger. The gas can be measured directly. Fig 1.2.4. Collection of gases using gas syringe ⓘ Note Approximate relative mass of air is 29.0. Gases with a higher than air can be collected using 𝑀 𝑟 downward delivery while gases with a lower than air can be collected using upward delivery. 𝑀 𝑟 Page 5 . Notes prepared by BPGHS 3G 2025 :)

Table 1.2.2. Densities, solubilities and methods of collection for various gases Gases (chemical formula) Density compared to air Solubility in water Methods of collection Hydrogen ( ) 𝐻 2 Less dense Not soluble Upward delivery & Water displacement Ammonia ( ) 𝑁 𝐻 3 Less dense Highly soluble Upward delivery Oxygen ( ) 𝑂 2 Slightly denser Slightly soluble Downward delivery & Water displacement Hydrogen chloride () 𝐻𝐶𝑙 Denser Highly soluble Downward delivery Carbon dioxide ( ) 𝐶 𝑂 2 Denser Slightly soluble Downward delivery & Water displacement Nitrogen dioxide (𝑁 𝑂 2 ) Denser Highly soluble Downward delivery Sulfur dioxide ( ) 𝑆 𝑂 2 Denser Highly soluble Downward delivery Chlorine ( ) 𝐶 𝑙 2 Denser Highly soluble Downward delivery Methods for Drying Gases Neutral gases: Oxygen and Nitrogen gas Acidic gases: Hydrogen Chloride, Carbon Dioxide, Nitrogen Dioxide and Sulfur Dioxide Alkaline gases: Ammonia Table 1.2.3. Drying agents used to dry various gases Concentrated sulfuric acid Used to dry neutral/acidic gases. Fig 1.2.5. Setup to dry gases with concentrated sulfuric acid Page 6 . Notes prepared by BPGHS 3G 2025 :)

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