How do you know if a change is physical or chemical?
Chemistry is largely the study of what we cannot see with our naked eyes. We can see the changes listed above but what if you made a mixture or did something to some matter (more on the basics of that below) and it is not on the short list above? Well there are a few clues that help us decide whether or not the particle has been changed-chemical OR the particle remains unchanged-physical. Of course we cannot see the particle (molecule, ionic solid, etc) but we can look for clues like those on the right.
Mixtures
Mixtures can be classified as homogeneous or heterogeneous depending on how well mixed the mixture is at the particle level. This is mainly a visual distinction. Homogeneous mixtures look like one substance like sugar dissolved completely in water (we usually call these solutions) or brass (a metal mixture called an alloy).
Pure substances-elements
Elements are made up of one type of atom ONLY. Every atom in an element has the same number of protons. The picture above shows some of the atomic elements which are the majority by the way. This means that the particle that makes these elements up is an atom. We show this in chemistry using unique colors for different spheres indicating atoms are we use chemical symbols. See below for an image of those elements that exist as molecules (each atom is still identical).
Pure substances-compounds
We have just spent the last two weeks working on the differences between the two broad classes of compounds, ionic and covalent. Some students mistakingly look at a formula like H3PO4 and see a mixture instead of a pure substance. Any substance that has a distinct chemical formula and, therefore, a distinct particle is a pure substance as long as there are no other particles (meaning another substance is present) is mixed in.
Elements that exist as molecules.
When we start writing chemical reactions, when you see hydrogen written in, the formula is H2, not H. The same is true for all of the elements pictured here. As the picture shows, the diatomic elements are in the shape of a 7 on the periodic table (and there are 7 of them).
ALL PHASE CHANGES are Physical Changes
The one type of change that confuses students and makes them think a chemical reaction has occurred are phase changes (melting, boiling, evaporation, condensation, etc). All that is happening in a phase change is the particles move faster or slower depending on the state of matter and the distance between the particles changes as a result of the particle motion. The temperature will change once the phase change is complete. Your refreshing glass of iced tea does not warm up until all of the ice is melted. The temperature inside of a pan of boiling water does not change until all of the water is converted to gas (also called vapor).
So you have a chemical reaction (change), time to go to chemistry land!
Breathe deeply as you have learned some chemical language and now we will be putting that language into practice. If you have ever gone to another country where you know a little of the language, you know how uncomfortable it is to even try speaking the native language that you do not know well. Chemistry really is a foreign language and this is where we put it into practice-chemical reactions. The reaction part is easy, it is the language we have been working on that will be the challenge.
Types of Chemical Reactions
We will look at types of chemical reactions from two different classification systems. The one you learned in high school is shown below. Your text uses this same classification system (synthesis, combustion, displacement, and double-displacement). We will first focus on double-replacement reactions as these encompass precipitation reactions and acid-base reactions. A second classification system is introduced in what may be a new type of chemical reaction for you and that is redox reactions. Oxidation and reduction reactions are exemplified by the reactions classified as synthesis (most), combustion (all), decomposition (most) and single-replacement (all). So, really, there are two types: those in which oxidation numbers change (redox) and those in which they do not (precipitation and acid/base reactions: both of which are double-replacement).
Precipitation Reactions
This type of reaction is characterized by the presence of two compounds on both sides of the chemical reaction. The most common example is two ionic compounds switching their ions to form a precipitate (solid)-hence, precipitation reactions. However, a gas may also be formed (compound) or water can also be formed in the case of acid-base reactions. We will learn a lot about the acid-base reactions later. We will also be learning about writing net ionic equations for double-replacement reactions.
AB (aq) + CD (aq) ---> AD (aq) + BC (s)
Do all aqueous compounds "break apart" completely?
The answer is no! For most of the examples here, we are assuming that all of the aqueous species are strong electrolytes which means that every particle that is dissolved "breaks apart" into the component ions as the simulation above demonstrated. However, there are weak electrolytes as well that do not completely "break apart". This latter type will be explored further in the second half of general chemistry.Nonelectrolytes like sugar and alcohol do not break apart at all.
How do you know if a precipitate will form?
When two ionic compounds in solution (aka aqueous) are mixed, you will either see no change indicating that a chemical reaction has not occurred OR you will see the resulting mixture get cloudy (solid or precipitate is formed) or even something a bit more colorful (colored solid or precipitate is formed. But how can we predict whether this will happen or identify the product if it has happened? Solubility rules is how, see below.
Net ionic equations-more language practice
Sometimes it is helpful in many different situations to zero in "on the action". That is what we do in chemistry when we take an equation apart and end up with a net ionic equation. You will rewrite any aqueous substances in the chemical reaction (leave solids, liquids, and gases AS IS) into their ionic forms. The video link below starts with this process and then takes you through the examples in the notes.
Acids and Bases-which is which?
We measure how "acidic" an acid or base is (you can think of a base as being a REALLY WEAK acid) by the pH scale. If a substance measures 1-6.9 on the pH scale, it is an acid. If a substance measures greater than 7 on the pH scale, it is a base. pH = 7.0 is neutral. We can measure pH with pH paper or a pH meter. When we do acid + base reactions, we sometimes end up with a neutral solution but not always.
Acid + Base reactions are also double replacement reactions
When an acid and a base combine, the product is most often water and a salt. Instead of forming a precipitate, molecular water is formed which drives these reactions forward. This idea of a driving force in chemical reactions is explored further in the second half of general chemistry. The process for writing the reaction and then ionic and net ionic equations is the same. See pgs 13-14 in Vol 3 notes for examples. The beginning of lecture 13 explains these examples.
HCl (aq) + NaOH (aq) → NaCl(aq) + H2O
