Measuring techniques
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Explainer Video
Summary
Measuring techniques
In a nutshell
In a laboratory setting measuring accurately is extremely important to the validity and accuracy of an experiment. The appropriate equipment and its correct use of it provides much more accurate measurements than the wrong equipment or use of it.
Measuring mass
Mass is always measured using a balance. The proper method to use a balance to measure mass is as follows:
| 1. | Place the container you will weigh the substance in onto the balance. |
| 2. | Set the balance to zero. |
| 3. | Add the mass and read the measurement. |
When transferring a mass from one container to another, you must ensure that the measured mass is being transferred. There are two methods to do this:
- In the case of a solid substance being dissolved into a solvent, wash out any remaining solid with the solvent used to transfer it into the other container.
- Transfer the substance and reweigh the container, subtracting the second value from the first to find the mass that was transferred.
Measuring volume
There are different pieces of equipment that may be used to measure volume:
Method | Description |
| Dropping pipette (Pasteur pipette) | Less accurate, often used to transfer very small volumes of liquid. |
| Pipette | Most accurate, these come in different sizes for different volumes. A pipette filler is needed to use this equipment. |
| Measuring cylinders | These come in different sizes for different volumes. You need to choose the appropriate size for the volume you require. |
Note: When measuring volumes read the value from the bottom of the meniscus. Meniscus is the curved surface of a liquid, due to surface tension.
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Measuring equipments are shown above, such as syringes and measuring cylinders.
Measuring gas volume
As with measuring the volume of liquids, there are a number of ways to measure gas volume:
Method | Description |
| Gas syringe | This piece of equipment gives a more accurate measurement of gas volume, there are different sizes used for different volumes, so an appropriate size must be selected. |
| Upside down measuring cylinder filled with water | The gas produced gets collected at the top of the upside down measuring cylinder, so the volume can be read off. |
| Counting bubbles produced | Inaccurate method of measuring gas volume, but can provide comparable results, when the specific gas volume is not crucial to the experiment. |
Note: When measuring gas volumes, always ensure the tubing is sealed.
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Measuring the volume of solids
The volume of regularly shaped solids can be measured by measuring the dimensions of the solid and using an appropriate formula to find the volume. However, the volume of irregularly shaped solids is harder to measure, so a eureka can is used as so:
| 1. | Fill a eureka can with water above the spout. |
| 2. | Let the water run out until the water level is just below the spout. |
| 3. | Place a measuring cylinder below the spout and put the solid into the water. |
| 4. | Wait for the water to finish pouring from the spout and read off the volume measuring cylinder. |
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Measuring lengths
Straight lengths can be measured with a ruler. There are a few points to consider in order to measure a length accurately using ruler:
- Always select an appropriately sized ruler (e.g. a centimetre ruler for the distance travelled on chromatography paper, a metre ruler for larger distances).
- Sometimes it is easier to measure multiple distances and divide by the number of distances you measured (e.g. measuring the length of ten ripples of water, then divide the measurement by ten).
- When taking a measurement of the same length multiple times, always ensure the measurement is from the same points.
The above figure shows a ruler being used to measure the size of a pencil.
Measuring angles
A protractor is used to measure angles. A protractor is used in the following way:
| 1. | Line the base of the protractor with one line of the angle. |
| 2. | Put the corner of the angle into the centre of the protractor. |
| 3. | Read off where the other line comes to on the protractor. |
The above figure shows a protractor being used to measure the angle, , coloured in blue.
Measuring temperature
A thermometer is used to measure temperature. There are a couple of points to consider when measuring temperature:
- Always wait for the temperature to stabilise when measuring.
- Make sure the bulb of the thermometer is completely under the surface of the substance being measured.
The above figure shows a thermometer measuring the temperature of the solution.
Measuring time elapsed
A stopwatch is the most common piece of equipment used to accurately measure changes in time. Stopwatches can have different levels of precision, so choose an appropriate one for the purpose of your experiment. It is useful to determine when you will start/stop the stopwatch before you begin timing, to ensure your results are valid.
Measuring pH
pH can be measured using paper indicators and indicator solutions, although a more accurate way of measuring pH is by using a pH probe. There are two types of indicator paper; litmus paper and universal indicator paper.
Litmus paper tells you whether the conditions are acidic by turning red or alkaline by turning blue.
Universal indicator paper gives an estimation of the pH by changing to different colours. The colours can be compared to a chart. Follow these steps when using indicator paper:
| 1. | Touch the paper to the substance you are measuring the pH of. |
| 2. | Wait for the paper to stop changing colour. |
| 3. | If using universal indicator paper, compare the colour to the chart to find the pH. |
Indicator solutions can be used to estimate the pH of solutions, by adding a few drops of indicator to the sample and the indicator will change colour depending on whether the sample is acidic or alkaline.
Sometimes indicator paper is more suitable than indicator solution because:
- Indicator paper can be used to measure the pH of a gas, by holding some damp indicator paper in the gas.
- Indicator paper does not change the colour of the sample you are trying to measure the pH of.
- If you are measuring the pH of a solution that is coloured, this may interfere with the colour of the indicator solution, so indicator paper is more suitable.
pH probes are the most accurate method of measuring pH. This method is electronic and a specific value is displayed when the probe is put in a sample.
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The above figure shows a pH probe measuring the pH of a solution on the left hand side. On the right hand side, two litmus papers are shown.
General important points
- Always read off the measurement at eye level.
- Always choose equipment with an appropriate size and level of precision and accuracy.
- Always read off from the bottom of the meniscus, when measuring the volume of a liquid.
FAQs - Frequently Asked Questions
What colours does litmus paper change to?
Litmus paper changes to red in acidic conditions and blue in alkaline conditions.
When measuring a volume of a liquid, where should you measure from?
Always read off from the bottom of the meniscus, when measuring the volume of a liquid.
How should you read off measurements?
Always read a measurement at eye level.
What is the meniscus?
The meniscus is the curved surface of a liquid, due to surface tension.