Part 17 of Chapter 1 in Chemistry

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This section may continue into others, but we will see how far Grandma gets at Grandma's Place of Natural learning. "Two other factors to consider in measurement are 'precision' and 'accuracy'. Remember 'accuracy' and 'precision' are not the same thing. 'Precision' indicates the reliability or reproducibility of a measurement. 'Accuracy' indicates how close a measurement is to its known or accepted value. For example, suppose you are taking a reading of the boiling point of pure water at sea level. Using the thermometer in three trials, you record 96.8-, 96.9-, and 97.0 -degree C. Since these figures show a high reproducibility, you can say that they are 'precise'. However, the values are considerably off from the accepted value 100-degree C, and so we say they are not 'accurate'. In this example we probably would suspect that the inaccuracy was the fault of the thermometer. Regardless of precision and accuracy, all measurements have a degree of 'uncertainty'. This is usually dependent on two factors---the limitation of the measuring instrument and the skill of the person making the measurement." This was shown by a picture of a person looking at a cylinder of water at the Bottom of 'meniscus'. The picture shows a Graduated Cylinder Reading of 34.3mL. The book says, "The graduated cylinder in the illustration contains a quantity of water to be measured. It is obvious that the quantity is between 30 and 40mL because the 'meniscus' lies between these two marked quantities. Now, checking to see where the bottom of the 'meniscus' lies with reference to the ten intervening subdivisions, we see that it is between the fourth and fifth. This means that the volume actually lies between 34 and 35mL. The next step introduces the uncertainty. We have to guess how far the reading is between these two markings. We can make an approximate guess, or estimate, that the level is more than 0.2 but less than 0.4 of the distance. We therefore report the volume as 34.3mL. The digit in any measurement is an estimate of this kind and is uncertain." We do not want to run out of room here so Grandma will start the next section in another lesson. But Grandma will place them on the calendar so she can publish what she has for now before she finishes the Chapter for exercise questions next Friday. Each Chapter should take about 2 weeks to cover.