Observations, which are an integral part of any science, can be classified into two main types: qualitative and quantitative. In general, qualitative observations are non-numeric. As a result they are not exact and are often not reproducible (two people generally do not describe a particular color in exactly the same way). Reproducibility is an important aspect of science and so it is preferable to use quantitative observations or measurements to describe an experiment. Measurements are made with instruments that allow us to exceed the limitations of our senses. Some instruments, such as a ruler that can be used to measure length, or a balance that can be used to measure mass, are simple to use. Other more complicated instruments, such as a spectrophotometer (used for measuring the relative intensities of light in different parts of the spectrum), or a nuclear magnetic resonance (NMR) machine are more difficult to operate. Measurements can be made by one person and duplicated by another, thus making them independent of the measurer.
Measurements consist of two parts: a number and a unit. The number part of the measurement specifies the size, while the unit part of the measurement allows for standards. It would not be useful to say that the distance from A to B is five. This particular statement does not allow us to determine if the distance in question is 5 in or 5 mi. In science, the standard measuring units are metric.
Some quantities are easier to measure than others. Mass, length, and temperature, for example, can be measured directly, with one reading from one instrument.Volume, however, is not as easy to measure. For example, the volume of a solid, such as a rock, can be measured by dropping it into water and measuring the volume of the water that is displaced. The volume of a liquid can be measured directly in a graduated cylinder. The volume of a gas, however, depends on the container in which it is, the pressure surrounding it, and its temperature.
Although it is not easy to measure the volume of a gas directly, it is quite easy to calculate the volume of a gas. Calculations are used when a quantity is determined by a mathematical combination of other measurements. For example, the volume of a regular solid such as a block can be calculated by multiplying its length by its width and its height (V = l x w x h), and the density of a solid can be calculated by dividing its mass by its volume (d = m/V). When performing calculations, it is vitally important to ensure that units are consistent. For example, the volume of an object cannot be expressed in standard units if the length, width, and height are all measured using different units. In this case, it would be necessary to convert two of the units to the same as the third one. In more complex calculations, use of incompatible units can result in a completely meaningless value for the quantity being calculated. Thus, when making a measurement or when performing a calculation using measured quantities, it is extremely important to ensure that the size of the measurement as well as its unit are noted.
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