Sunday, September 8, 2019

Adhesion and Cohesion

What are adhesion and cohesion? To start with, they are properties of matter!

Cohesion is the tendency of molecules to stick to other molecules of the same substance. This is why drops of water bead up on a slick surface. In fact, this is why water forms drops in the first place!

The stronger the force of cohesion, the more molecules that will be able to stick to each other. Drops will be bigger!

Cohesion is also responsible for surface tension. If you've ever skipped a rock or ridden in a fast boat or jet ski, you have surface tension to thank!

Adhesion is the tendency of molecules to stick to other molecules of different substances. You probably already know what an adhesive does! An adhesive adheres to two things and makes them stick together. So, a good adhesive has a high degree of adhesion between itself and other things.

Consider this very fancy diagram (right)!

The GLUE adheres to the mug and it also adheres to the handle. Thus, the handle sticks to the glue which is stuck to the mug. The final result is that the handle sticks to the mug!

Adhesion exists beyond tapes and glues, too. The adhesive properties of water allow you to use it to stick the shower curtain to the tiles. If you have ever licked a plastic decal and stuck it to glass (or wiped it with water and stuck it to glass), it was the adhesive properties of water that allowed that to work.

Flash back to those beads of water… Car waxes brag that they cause water to bead up and run off.
What's going on there? There is at play a combination of cohesion between the water molecules and adhesion between the water and the surface.

When water falls onto a surface, there is created a balance between the forces of attraction between the molecules to themselves and the surface. Wax makes the surface "slicker." That means that the adhesive force decreases, so the cohesive forces have a bigger effect. The water sticks to itself better than it does to the wax, so the beads of water become larger.

Friday, September 6, 2019

More Physical Properties of Matter

It has been established that…

A physical property of matter is any attribute, quality, or characteristic of a material that can be observed or measured without changing the composition of the substances in the material. 

Among the many properties of matter there are:

Melting and Boiling Points

These properties were introduced HERE.

There are many more physical properties of matter, and some of them will be introduced in this article.

Some physical properties are independent of the amount of the substance preset. Some physical properties change as the amount present changes.

If the property changes based on the how much is present, it is said to be an extensive property. Some factor outside the makeup of the material—some external factor is connected to the property. For instance, mass is an extensive property; the more of something you have, the more mass you have.

If the property does not change based on how much is present, it is said to be an intensive property. The property is independent of the amount present. For instance, color is an intensive property; no matter how much you have, the color is the same.

Physical Properties of Matter

Appearance (intensive)
How does it look? What is visually identifiable about the substance?

Some aspects of a physical appearance include its color, texture, or sheen. Uniformity of these things could also be a factor, or variances might indicate that the sample has some impurities in it.

Odor (intensive)

IMPORTANT NOTE: Don't sniff chemicals! You could die!

While it is dangerous to inhale chemicals, some of them do stimulate the olfactory nerves which results in the perception of odor.

Solubility (intensive)

Solubility is the degree to which a substance (solute) dissolves in a given solvent (other substance). Not everything dissolves in everything! There is a vast degree of variance in what will dissolve in what and to what degree! You need only watch a few TV commercials to know that Brand A dish soap will dissolve grease better than all the other! Let's just skip the laundry ads!

Solubility for a given substance might be noted in relationship to various solvents. How well does it dissolve in water? How about alcohol? How about mineral spirits? How about acetone?

Nail polish is a very intuitive example. Nail polish does not dissolve very well if at all in water. (If it did, it would come off with every hand wash!) It does, however dissolve readily in acetone (nail polish removers often are acetone).

Magnetivity / Magnetism (intensive*)

The degree to which a substance is attracted to or repelled by magnets and magnetic fields. Basically, do magnets stick to it or does it stick to iron?

*Larger samples of a magnetic substance will create a larger (i.e. stronger) magnetic pull, but the property is uniform regardless of sample size)

Ductility (intensive)

The degree to which a substance can elongate when pulled.

Examples: Chewing gum is very ductile. You can pull it and stretch it really far out of your mouth (although that is gross and germy). Carrots are not, compared to gum, very ductile. They snap off.

Specific Heat (intensive)

The capacity of a substance to hold energy in the form of heat.

If you have a dishwasher and have ever tried to unload it right after it was done, you know that the glass bowls will burn your fingers more than the plastic ones, but when the door opens, they are all the same temperature. Glass can hold heat better than plastic. Thus, the specific heat of glass is higher than the specific heat of plastic.

Specific heat is measured in a unit of energy per a unit of mass such as cal/gr or J/gr.

Opacity (intensive)

The capacity of a substance to block (usually limited to visible light) electromagnetic waves. Opacity ranges from transparent (light passes without diffusion), to translucent (some light passes but is diffused) to opaque (no light passes through).

While opacity is frequently used to discuss visible light, the same term applies to other types of radiation as well.

Mass (extensive)

The total amount of matter present, the sum of all the electrons, protons, and neutrons within the sample. 

Volume (extensive)

The total amount of space occupied by the sample.

Denisty (D) is an intensive property of matter that is the ratio of mass (m) to volume (V) found by:

D = m/V