1.5.1. The Key Difference between Physical and Chemical Phenomena.
You boil water to cook your pasta. As the water boils at 100 o
C, steam bubbles off. Both steam and water consist of the same molecules, molecules of water, H2
O. 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 o
C. Both water and ice are made up of the same molecules, H2
O. 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.
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 (C2
) 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 (NaC2
) and water (H2
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).