Apparatus for Measurements
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
Introduction to Laboratory Apparatus In chemistry, accurate measurements are essential for obtaining reliable results. Different apparatus are used depending on what needs to be measured and how precise the measurement needs to be.
Safety First Always wear safety goggles, lab coat, and closed-toe shoes in the laboratory. Handle all apparatus with care and follow proper procedures. Measuring Volume of Liquids Apparatus Typical Volumes Precision When to Use Measuring Cylinder 10 cm³, 25 cm³, 50 cm³, 100 cm³, 250 cm³ ±0.5 cm³ to ±1 cm³ Quick, approximate volume measurements Pipette 10 cm³, 25 cm³ (fixed volume) ±0.05 cm³ Accurate transfer of a fixed volume Burette 50 cm³ (variable delivery) ±0.05 cm³ Titrations - accurate variable volumes Volumetric Flask 100 cm³, 250 cm³, 1000 cm³ ±0.2 cm³ Preparing solutions of exact concentration Beaker 50 cm³, 100 cm³, 250 cm³, 500 cm³ Very approximate Rough measurements, mixing, heating Interactive Diagram Visual comparison showing all 5 pieces of apparatus to scale.
Each labeled with: name, typical volumes, graduation marks visible, and a "precision meter" showing relative accuracy (beaker=low, measuring cylinder=medium, pipette/burette/volumetric flask=high). Include annotations pointing to key features: meniscus reading line for burette/pipette, graduation marks, tap on burette, bulb for pipette.
Add color coding: red border for low precision, yellow for medium, green for high precision. Reading the Meniscus Meniscus The curved upper surface of a liquid in a tube. Water and aqueous solutions form a concave meniscus (curves downward in the middle).
How to Read a Meniscus Correctly: Always read at eye level - looking from above or below causes parallax error For water/aqueous solutions: read the bottom of the meniscus For mercury: read the top of the meniscus (forms convex curve) Position your eye so the meniscus appears as a straight line Read to the nearest graduation mark Diagram Cross-section view of a burette/measuring cylinder showing: (1) Correct eye-level reading with eye at same height as meniscus (green checkmark), showing reading at bottom of curved meniscus = 24.5 cm³.
(2) Incorrect reading from above (red X) showing apparent reading = 24.3 cm³. (3) Incorrect reading from below (red X) showing apparent reading = 24.7 cm³. Include labels: "Read here" pointing to bottom of meniscus, "Parallax error" for incorrect readings.
Show magnified view of meniscus curve with graduation marks at 24.0 and 25.0, correct reading at 24.5. Using a Pipette A pipette is used to measure and transfer an exact fixed volume of liquid (usually 10 cm³ or 25 cm³).
Procedure: Rinse the pipette with the solution to be measured (not water) Use a pipette filler/bulb - never pipette by mouth Draw liquid above the calibration mark Remove the filler and quickly place finger over the top Carefully release liquid until bottom of meniscus sits on the calibration mark Touch the tip to the side of a beaker to remove the drop outside the tip Transfer to the receiving vessel and allow to drain Touch tip to side of vessel and wait 15 seconds Remove pipette - do NOT blow out the remaining liquid in the tip Important The small amount of liquid remaining in the tip is accounted for in the calibration.
Do not blow it out as this will deliver more than the stated volume. Using a Burette A burette is used in titrations to deliver variable volumes of liquid accurately. Procedure: Rinse the burette with the solution to be used Close the tap and fill with solution using a funnel Remove funnel and open tap to fill the jet (tube below tap), removing air bubbles Ensure the liquid level is at or below the 0.00 cm³ mark Record the initial reading (to 2 decimal places, e.g., 0.05 cm³) Add liquid dropwise near the end point Record the final reading (to 2 decimal places) Calculate volume delivered = Final reading - Initial reading Reading Technique Burette readings should be recorded to 2 decimal places (e.g., 23.45 cm³, not 23.5 cm³).
The second decimal place is estimated between the smallest graduation marks. Measuring Mass Apparatus Precision Maximum Mass When to Use Electronic Balance ±0.01 g or ±0.001 g 200-500 g Accurate mass measurements for most experiments Top-pan Balance ±0.1 g 500-2000 g Larger masses, less precision required Using a Balance Correctly: Place balance on a flat, stable surface away from drafts Zero/tare the balance before use Place weighing container (boat/beaker) on balance and zero again Add substance to container Record mass to the precision of the balance (e.g., 2.45 g for ±0.01 g balance) Never place chemicals directly on the balance pan Measuring Temperature Apparatus Range Precision Advantages Disadvantages Thermometer (alcohol/mercury) -10°C to 110°C ±0.5°C Simple, no batteries needed Fragile, mercury toxic, slow response Digital Thermometer -50°C to 300°C ±0.1°C Fast response, easy to read, wide range Requires batteries, can be expensive Temperature Probe (data logger) -50°C to 5…
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