Volume 1

The Key Difference between Physical and Chemical Phenomena • Experiments • Exercises

1.5.1. The Key Difference between Physical and Chemical Phenomena. You boil water to cook your pasta. As the water boils at 100 oC, steam bubbles off. Both steam and water consist of the same molecules, molecules of water, H2O. If you lower the temperature of the steam on a cold surface, it condenses in the form of water. To make ice cubes, you place water in your freezer. The liquid water becomes solid ice on cooling below 0 oC. Both water and ice are made up of the same molecules, H2O. If warmed up, ice melts to give water. Ice, steam, and water are different phases of water. Their interconversions are called phase transitions. No matter how many times we repeat these phase transitions from the liquid (water) to the solid (ice) or to the gas (steam) and back, no new molecules are produced. Likewise, no new molecules and, consequently, no new substances are produced as a result of crystallizations of solids, like those described in the previous section.

Phenomena that do not result in any transformations of a substance into a new substance are physical phenomena. In contrast, chemical phenomena always involve the formation of new substances from original ones.

There are two tightly sealed containers. One is filled with water and the other with gun powder. Both containers would explode on heating due to pressure buildup. The fundamental difference between the two explosions is that one was prompted by a purely physical phenomenon, whereas the other by a chemical phenomenon. The pressure inside the container with water rose because the water started to boil to produce steam that occupies a much bigger volume. No new substances were formed. In contrast, heating the gun powder prompted a violent chemical reaction that produced, among other products, carbon dioxide (CO2) and nitrogen (N2) gases from saltpeter (KNO3), sulfur (S), and charcoal (C), the three ingredients of gun powder.

1.5.2. Experiments. Please carefully read and understand the following:

DISCLAIMER: Although most of the experiments described in this subsection and elsewhere in this website are regarded as low hazard, I expressly disclaim all liability for any occurrence, including, but not limited to, damage, injury or death which might arise as consequences of the use of any experiment(s) discussed, listed, described, or otherwise mentioned in the free online course Chemistry from Scratch. Therefore, you assume all the liability and use these experiments at your own risk (see Terms of Use).

If you decide not to do the experiments, please still read this subsection.

Experiment 4. Physical and Chemical Phenomena. To further understand the difference between physical and chemical phenomena, let us do two simple experiments using chemicals that are available in every kitchen: table salt (NaCl), baking soda (NaHCO3), and vinegar, a weak (~5%) solution of acetic acid (C2H4O2) in water. It would be convenient to have a pipette, a tool used to transfer liquids. There are many types of laboratory pipettes, but we need a very simple one, which is used to withdraw small amounts of liquids and release them drop-wise. Such pipettes are sold in some stores and pharmacies under the name of "eye dropper" or "ear dropper".

Experiment 4A. Place a pinch of table salt (NaCl) in a saucer. Using a pipette add a few drops of white vinegar to the salt. What do you see? Not much. The crystals of salt get wet and, if you add enough vinegar, will dissolve on gentle swirling, much like in water. Smell the solution. It has the characteristic smell of vinegar. If you let the solution sit for some time, the water and the vinegar will evaporate and just crystals of NaCl will be left.

Experiment 4B. Repeat experiment 4A using baking soda (NaHCO3) in place of table salt. You will see something totally different from what you observed in the experiment with table salt. Once the vinegar and baking soda come in contact, gas bubbles are produced and the smell of vinegar weakens or vanishes altogether. The gas that bubbles off is carbon dioxide, CO2, the very compound that is used to make dry ice (solid CO2) and carbonate water for soft drinks.

Where did the CO2 gas come from in experiment 4B? Neither baking soda nor vinegar contained CO2. It was formed when the two reacted in a process called a chemical reaction. As a result of the reaction between baking soda and vinegar, the two were consumed and converted to new compounds. One of them is carbon dioxide, CO2. The other products of this reaction are sodium acetate (NaC2H3O2) and water (H2O).

In our experiment, we prepared sodium acetate from baking soda. Being different compounds, baking soda and sodium acetate possess different properties and, consequently, have different applications. Baking soda is used to make baking powder, as a mild disinfectant, in personal hygiene, as a cleaning agent, and in some types of fire extinguishers. Sodium acetate is used in the textile and dye industries, as a life-prolonging agent for concrete, and as a food additive. The flavor and taste of salt and vinegar potato chips is due to the added sodium acetate (E number E262 in the food industry).
Digression. An interesting application of sodium acetate is in the smart hand warmers (watch this video) and travel baby bottle warmers. This application is based on the remarkable ability of sodium acetate to form supersaturated solutions. A solution is supersaturated if it contains more of the dissolved substance than could be dissolved under normal conditions. Supersaturated solutions are conventionally prepared by obtaining saturated solutions at a higher temperature and then slowly and carefully cooling them down. Watch sodium acetate crystallize from its supersaturated solution in this mesmerizing experiment. Not knowing the details of this experiment, one might think that introducing some substance on the spatula into the liquid prompted a chemical reaction that gave rise to the formation of a new substance that crystallized out. But, in fact, there was no chemical reaction, just crystallization from the supersaturated solution prompted by the seeding. Where does the heat come from as the sodium acetate in those hand and baby bottle warmers crystallizes? Crystallization of a substance from the molten state or solution is conventionally accompanied by the release of energy in the form of heat. The amount of heat produced upon crystallization can vary in a broad range, depending on the nature of the crystallizing substance and certain conditions such as concentration. In some rather rare cases, light is also produced (watch this). If you would like to make a large quantity of sodium acetate for experiments with its supersaturated solutions, consider using this detailed procedure.
The chemical equation for the reaction of baking soda with vinegar is as follows.

NaHCO3 + C2H4O2 = NaC2H3O2 + CO2 + H2O

If this is the first time you have seen a chemical equation, you are probably confused. That is OK, just do not be scared! Very soon you will realize that this equation is rather simple, but to get there, it would help to first consider even simpler reactions. We will do that in the next section. Meanwhile let us summarize the two experiments, 4A and 4B.

In experiment 4A, the solid NaCl was partially or fully dissolved in vinegar. No new compounds were produced, so those mixing and dissolution were physical phenomena. Then the water and acetic acid were left to evaporate, which did not produce any new compounds either, and was a physical phenomenon, too. As the vinegar evaporated, the dissolved NaCl crystallized out, also a physical phenomenon.

In experiment 4B, by contrast, the addition of acetic acid (vinegar) to baking soda triggered a transformation of these two chemical compounds into three new compounds, sodium acetate, carbon dioxide, and water. A reaction occurred, which is a chemical phenomenon.

1.5.3. Exercises. Determine if the following phenomena are physical or chemical: (a) evaporation of gasoline; (b) combustion of gasoline; (c) rain; (d) forest fire; (e) caramelization of sugar. Answer