Wednesday, we'll start by polishing off chemical equations, balancing equations, a little bit of cooking chemistry, and build models of organic compounds (compounds of carbon). Look over the Class-3 page again, and then read on.
What is a Gas, and What is Science?
In Class 4, we will start with another brief review the three most common phases of matter -- solid, liquid, and gas -- and then look closely at the simplest phase, the gas. This closer look at gases will include material from the One Culture essay "What is Science?", and a computer simulation of gas behavior HERE (requires Java running in your browser). This view of gases gives us ways to imagine atomic and molecular motion in liquids and vibration in solids.
Molecular motion is evident in Brownian motion, which was observed, but not understood, by botanist Robert Brown in 1827. He observed, under a microscope, the random motions of pollen grains suspended in water. He wondered if it meant that the grains were alive, but then he showed that the same thing happens with soot or other clearly non-living small particles, so he knew that the force moving the particles was not a living force. One of Einstein's first big contributions to science was, in a 1905 scientific paper, to explain precisely how collisions of moving water molecules with the pollen grains produced their motion. At the time, it was the most convincing evidence that atoms and molecules are real.
Fine particles of carmine red dye suspended
in water, magnified about 400 times. Note
especially the smaller particles being buffeted
by collisions with water molecules.
See Brownian motion of fat droplet in milk under a microscope HERE. See Brownian motion for yourself by watching dust particles in a beam of sunlight coming into a room in your home. If your home is too clean for this, next time you dust, shake a little dust from your dust cloth in light from a sunny window. Notice that the dust does not fall, but moves around at random, kept suspended by collisions with molecules of N2, O2, and other gases in the air. You are seeing direct evidence that air molecules exist, and are in rapid motion. If they were not, dust particles would fall to the floor like stones, and dust would not spread itself throughout your home.
••••••
Equilibrium Reactions: Acids, Bases, and pH
We will consider some simple equilibrium processes in class.
We will consider reactions in solution. First a few new terms. A solution is a mixture in which a solid, liquid or gaseous substance, the solute, is dissolved in a liquid, the solvent. In a aqueous solution, the solvent is water. Syrup is an aqueous solution of sugar. In other words, it is sugar (solute) dissolved in water (solvent).
(Think about the difference between dissolving and melting. Consider the melting of ice to produce liquid water, versus the dissolving of sugar in water to produce a solution.)
But even absolutely pure water (don't ask me where to find any) contains in each liter about about a hundred million billion (1 x 1017) positively charged hydrogen ions (H+) and the same number of negatively charged hydroxide ions (-OH).
Where do they come from? At any moment, any water molecule can fragment to produce these two ions. It's happening all the time, but only to a tiny fraction of the water molecules, according to this chemical equation:
H2O (l) <===> H+ (aq) + — OH (aq) (dissociation of water)
The double arrow (<===>) is the best I can do with this page editor to represent an equilibrium reaction, a reaction that does not go to completion. Instead the reaction proceeds until the rate of the forward process (water --> ions) equals the rate of the reverse process (ions --> water), and then it's stuck there, at -- erm -- equilibrium, because water molecules are forming ions at the same rate that ions in solution happen to find each other and make water molecules.
In pure water, the number of hydrogen ions exactly equals the number of hydroxide ions because every water molecule that ionizes produces one of each ion, and every time a water molecule is formed in the reverse reaction, one of each ion is consumed.
pH is a measure of the concentration of hydrogen ions in a water solution. pH of 7 means that the number of hydrogen ions equals the number of hydroxide ions, as in pure water. The pH of pure water at room temperature is 7. But I can shift the equilibrium to a new position by adding hydrogen ions in the form of a strong (HCl) or weak (acetic) acid. If I increase the concentration of hydrogen ions by ten-fold, the pH will be 6, and the concentration of hydroxide ions will be reduced by 10-fold. Such a solution is said to be acidic. A solution of pH 5 is 100 times more acidic than pure water, and a solution of pH 4 is 1000 times more acidic than pure water.
If I add hydroxide ions in the form of a strong (NaOH) or weak (ammonia) base, there will be more hydroxide ions than hydrogen ions, and the solution will be basic. A solution of pH 8 contains ten times as many hydroxide ions as does pure water, and ten times fewer hydrogen ions. A solution of pH 10 is 1000 times more basic than pure water.
Finally, two definitions: An acid is a source of hydrogen ions; hydrogen ion itself is considered to be an acid, the simplest one. A base is a consumer of hydrogen ions; hydroxide ion is the simplest base.
How is pH Measured?
The classic way is to use indicators, pigments that show different color in solutions of different pH. You can make an indicator at home, using pigments in red cabbage. Click HERE to see how.
The modern way is to use a pH meter, placing its electrode into the solution whose pH you want to know. The principles underlying the electrode are complex.
The following video shows how a pH electrode senses the level of H+ ions in a solution. It's fairly technical, but just ignore the numbers and try to get the gist of it.
All pH methods rely on calibration, which means using solutions of known pH to compare with unknowns. Calibration solutions can be made with specific pH values, by applying well-understood chemistry.
•••••
Broad Overview of Chemistry
What are fields of chemistry, and what do chemists do?
This video has a few minor errors (listed in the text material below the video), but gives a very concise and clear overview of the ideas and activities that constitute chemistry. I think it might be more effective now, after you have encountered some of the basic terms and concepts.
I hope it stimulates questions!
The central tenet of chemistry is that function comes from structure.
In chemical terms, molecular structure gives rise to function. Molecular structure is the basis of the properties of substances - their colors, textures, shapes, and tendencies to change. As we learn to make these connections, we are drawn into a world of imagination that enriches the world we see. We study chemistry because many interesting and important things we see -- the intricacy of a snow flake, the symptoms of an cancer sufferer -- are caused by things we cannot see, but must understand.
Some of the basic operations of practicing chemists are
In chemical terms, molecular structure gives rise to function. Molecular structure is the basis of the properties of substances - their colors, textures, shapes, and tendencies to change. As we learn to make these connections, we are drawn into a world of imagination that enriches the world we see. We study chemistry because many interesting and important things we see -- the intricacy of a snow flake, the symptoms of an cancer sufferer -- are caused by things we cannot see, but must understand.
Structure-function relationships are the heart and soul of chemistry.
- purification: the separation of mixtures into "pure"* substances
- analysis: detecting and identifying substances (qualitative analysis), and measuring their quantities (quantitative analysis)
- structure determination: composing and testing structural models that fit all available data on a substance, in order to learn its molecular structure
- synthesis: making complex molecules of defined structure using reactions between simpler substances
- studying reactions, particularly their rates (kinetics) and energy changes (thermodynamics), in order to establish relationships between structure and reactivity.
These operations are interdependent. For example, syntheses entails analysis to find out whether the desired product was obtained, and what impurities it contains.
* What Does Pure Mean?
No substance is completely pure.
Once more, for emphasis: no substance is completely pure.
In practice, a pure substance is one whose level of impurities is too low to have a detectable effect on what you are using the substance to do. The question is never whether there are impurities in a natural or synthetic substance. The question is how much. A liter of "pure" water, with impurities at less than one part per trillion (only 1 part in 1012 !) could still harbor trillions of impurity molecules in each liter. We are surrounded by impurities. There are likely to be atoms of every element in any sample taken on the earth, though most are at undetectable levels. There are likely to be some atoms of radioactive plutonium in our classroom.
But come to class anyway.
As scientists develop the technology to detect ever small numbers of atoms and molecules, it becomes more important to determine what level of a specific impurity actually is significant, because all imaginable impurities are probably present in some minuscule amount.
* What Does Pure Mean?
No substance is completely pure.
Once more, for emphasis: no substance is completely pure.
In practice, a pure substance is one whose level of impurities is too low to have a detectable effect on what you are using the substance to do. The question is never whether there are impurities in a natural or synthetic substance. The question is how much. A liter of "pure" water, with impurities at less than one part per trillion (only 1 part in 1012 !) could still harbor trillions of impurity molecules in each liter. We are surrounded by impurities. There are likely to be atoms of every element in any sample taken on the earth, though most are at undetectable levels. There are likely to be some atoms of radioactive plutonium in our classroom.
But come to class anyway.
As scientists develop the technology to detect ever small numbers of atoms and molecules, it becomes more important to determine what level of a specific impurity actually is significant, because all imaginable impurities are probably present in some minuscule amount.
