unit 1: measurements

Understanding the basic function and proper handling of common laboratory equipment is crucial for accurate and safe experimentation.

Tools: Rulers, meter sticks, calipers.
Technique: Always read measurements at eye level to avoid parallax error. Ensure the object is aligned correctly with the starting mark.
Units: Typically centimeters (cm) or millimeters (mm).

Tools: Electronic balances (analytical or top-loading).
Technique:
1. Place a weighing boat or container on the balance.
2. "Tare" or "zero" the balance to exclude the container's mass.
3. Add the substance to be weighed.
4. Read the stable mass value.
Units: Grams (g) or milligrams (mg).

Purpose: Accurately transferring specific volumes of liquid.
Types: Graduated pipettes, volumetric pipettes, micropipettes.
Technique: Always use a rubber bulb or automated pipette aid, never mouth-pipette. Ensure no air bubbles are drawn with the liquid.

Micropipettes are used for transferring very small, precise volumes (typically $\mu L$ ).
Plunger Settings:
- The volume is set by twisting the adjustment knob, which changes the digital display.
- Ensure the volume dial is set within the pipette's specified range (e.g., P20 for $2-20\,\mu L$ ).
Drawing Up Liquid:
1. Attach a disposable tip firmly.
2. Press the plunger to the first stop.
3. Immerse the tip a few millimeters into the liquid.
4. Slowly release the plunger to draw up the liquid. Avoid quick release to prevent air bubbles or splashing.
Dispensing Liquid:
1. Place the tip against the side wall of the receiving container.
2. Press the plunger to the first stop to release most of the liquid.
3. Press the plunger to the second stop (blow-out) to expel any remaining liquid.
4. Keep the plunger depressed while removing the tip from the container.
5. Release the plunger after removing the tip.
6. Eject the tip into a waste container using the tip ejector button.

Accuracy: How close a measurement is to the true or expected value.
- Expected Value: The known or theoretical correct value (e.g., a pipette set to $100\,\mu L$ should dispense $100\,\mu L$ ).
- Average Value: The mean of several repeated measurements.
- Assessment: Averages close to the expected value indicate high accuracy.
Precision: How close repeated measurements are to each other (reproducibility).
- Assessment: Low variability (e.g., small standard deviation) among multiple measurements indicates high precision.
Evaluating Pipettes: By repeatedly dispensing a known volume of liquid (e.g., water) and weighing it (since $1\,\mu L$ of water is approximately $1\,mg$ ), one can determine both accuracy and precision. Volumetric pipettes generally offer higher accuracy than graduated pipettes or micropipettes for their specified volume.

Concept: The density ( $\rho$ ) of a substance is its mass ( $m$ ) per unit volume ( $V$ ): $\rho = \frac{m}{V}$ .
For pure water at $4^{\circ}C$ , density is approximately $1\,g/mL$ (or $1\,mg/\mu L$ ).
Procedure:
1. Measure the mass of an empty container.
2. Add various known volumes of water to the container using a pipette or graduated cylinder.
3. Measure the mass of the container with water for each volume.
4. Calculate the mass of the water for each volume.
5. Plot the data to observe the relationship.

Independent Variable: The variable that is changed or controlled by the experimenter. It is plotted on the x-axis.
Dependent Variable: The variable that is measured or observed and is expected to change in response to the independent variable. It is plotted on the y-axis.
Example: In the mass vs. volume of water experiment:
- Independent Variable: Volume of water (you choose the volumes to measure).
- Dependent Variable: Mass of water (you measure the mass that results from each chosen volume).
Interpretation: A linear relationship ( $y = mx + b$ ) indicating density, where the slope ( $m$ ) represents the density of water.