I’d like to begin this post by apologizing for the lack of posts up lately. The website at the time of posting this may only be a little less than 2 weeks old, but keep in mind that everything I have posted so far is about a day or two day’s worth of the first semester of what you might learn in your typical two-semester freshman chemistry class as an undergraduate at a four-year college or university. Keeping this in mind, I will be making an effort to post more than I have been.
In this post we’ll be discussing some of the physical quantities of the SI system. For example, what is mass? I’ve already mentioned what mass is in a previous post but I will reiterate here. In the previous post you learned that the SI unit for mass is the kilogram (kg). Often, because the kilogram is such a large unit when dealing with chemistry, grams (g) will be used instead, which are 1/1,000th of a kilogram (Petrucci et al. 2010).
Many people tend to think of weight and mass as interchangeable words. The two are not interchangeable; however, weight is certainly proportional to mass. This is because weight is defined as the force of gravity on an object, and it’s equation is W = g x m (hooray for our first equation!), where W is weight, g is the acceleration due to gravity (a constant that you will undoubtedly remember forever once you begin taking physics; hooray for our first constant!) and m is mass. Mass is commonly measured using a balance, often mistakenly called a scale (Petrucci et al. 2010).
Another important concept to have an understanding of is time, for which the SI unit is seconds (s). The exact definition of a second has always been difficult to formulate exactly, but now, very interestingly, the second is defined as the duration of 9,192,631,770 cycles of a certain radiation emitted by particular atoms of cesium, a period table element (cesium-133) (Petrucci et al. 2010).
Next is temperature. Temperature is commonly measured using increments known as degrees. The SI unit for temperature is the kelvin. Of many of the SI measurements we hear about on a daily basis, temperature is among the most familiar to us, but the SI unit is not. In America, the unit for temperature is Fahrenheit, where the melting point of ice is 32 °F, and the boiling point of water is 212 °F. In countries that use the metric system, the unit for temperature is Celsius, where the melting point of ice is 0 °F, and the boiling point of water is 100 °C. If temperatures in either Fahrenheit or Celsius are your comfort zone, you’ll have to learn to adapt to the Kelvin scale, where the lowest possible temperature is 0 K, which is equivalent to -273.15 °C. Adapting to the Kelvin scale is really not difficult if you understand the following principles, outlined by Petrucci et al.:
Below I have posted a small image of the formulas for conversion between different temperature scales. Though these formulas are common knowledge, I would like to mention that I retrieved these from the Petrucci et al. text. If you find that the diagrams thus far have not been frequent enough or detailed enough, just wait until we get into more quantitative studies of chemistry, physics, etc. Not to mention organic chemistry, which will require the drawing of structures, mechanisms, molecules, and so much more. Happy studying.
A prospective medical student, looking to help others succeed.