Science in Everything

It’s been quite some time since I’ve updated this!  I want to start posting new stuff soon.  Until then, here is a Note I wrote on Facebook in 2008 for my friends about how scientists figure out the age of the Earth.  I focused on uranium decay but just found a press release (last link at the bottom) from McGill University that mentions another kind of decay.  So, just keep in mind uranium is not the only element that can be useful in radiometric dating!

A process called radiometric dating is used to determine how old the Earth is. This involves measuring the amount of radioactive elements in rock. Before explaining how this works, let’s review some of the fundamentals. 

Atom Structure 

There are many radioactive elements that can be used for dating, but we’ll focus on uranium for the purpose of this post. Uranium is a well known radioactive element, but what does that really mean? You may remember that atoms are made up of protons (positive particles), neutrons (neutral particles), and electrons (negative particles). Protons and neutrons are found in the center of an atom called its nucleus. Electrons surround the nucleus. The number of protons in an atom determines what element the atom is; the number of neutrons and electrons can vary. Uranium has 92 protons. There are three “types” of uranium found in nature, each having a different number of neutrons: one type has 142 neutrons, one has 143 neutrons, and the last has 146 neutrons. Each of these types is called an isotope of uranium. The isotope with 146 neutrons is called uranium-238 (146 neutrons + 92 protons = 238). 

Radioactivity 

An element is said to be radioactive when it emits or captures particles that change the composition of the nucleus – these events even change the identity of the element. For example, an alpha particle is composed of two neutrons and two protons. If an atom of uranium emits an alpha particle, it loses two neutrons and two protons and is said to undergo radioactive decay. Remember, the number of protons determine the identity of the element. So, if uranium loses 2 protons (they left in the alpha particle), it becomes thorium, a different element. In this case, we would refer to thorium as the “daughter product.” 

The time it takes for uranium to decay into thorium is known. Rates of decay for radioactive elements are measured in “half-lives.” A half-life is the amount of time it takes for ½ of the sample to decay. So, let’s say you have a rock with uranium and thorium in it. First, you measure the amount of each element in the rock. When you know that, you can calculate how much uranium was in the rock when it formed. You can do this by figuring out much uranium it took to decay into that amount of thorium. Since you know how much uranium decayed, and you know the half-life of uranium, you can calculate how long that decay took, which is the age of the rock. 

I simplified the process a bit. Thorium itself is radioactive and decays into other elements. Ultimately, uranium decays into lead, and it is this ratio (between uranium and lead) that is used to date rocks. Still, the basic process I described above applies. 

Controversy fueled by creationist groups 

I’ve read arguments on a few creationism websites about how scientists can’t really say that the earth is 4 billion years old because carbon-14 dating is only accurate for stuff less than 10,000 years old. They are right! Because of carbon-14’s short half-life, it is not ideal to use on material older than 10,000 years old. That’s why we don’t use carbon-14 dating to date rocks that old! Uranium’s long half-life of about 4.5 billion years makes it a good element to use for dating something as old as our earth. 

Websites of interest: 

http://pubs.usgs.gov/gip/geotime/age.html 
http://www.ieer.org/fctsheet/uranium.html 
http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/cs_zircon_chronolgy.html

http://www.mcgill.ca/newsroom/news/item/?item_id=102000

When I was rummaging through the Internet finding information about copper tarnish, I noticed ketchup coming up a lot.  Weird! And gross! (Ketchup is gross!)

Apparently, you can use ketchup to clean copper.  I didn’t doubt this, but today I tried it just to see.  I started with a piece of copper pipe from my parents’ old house that I’ve had for over 10 years.  Sometimes I wear it as a pendant.  Anyway, as you can see, it was pretty dull.

Next, I collected supplies.  Ketchup is gross, but I do save the little packets that come with fast food in case I ever have visitors who want it.

So, I squirted ketchup on the copper ring and let it sit for a couple minutes.  Then I used a paper towel to rub the copper and ketchup for a couple minutes.

Wash with water, pat dry, et voila!  Shiny-like-new copper!  There are still a few smudges that didn’t come off, but like I said, this piece of pipe is old and was never cleaned before today.

The salt and vinegar in ketchup helped dissolve the copper oxides that coated the surface of the piece, leaving behind shiny copper.  Try it yourself.  Hate ketchup, too?  Use salt with vinegar or lemon juice to achieve the same effect.

You may have noticed in my last post about tarnish I used roman numerals in some of the compound names (e.g., copper (II) oxide).  Maybe you wondered why (humor me). 

Metals, like copper, and non-metals, like oxygen, form ionic bonds.  This means that if you were to pull apart the bond, the component atoms would be charged.  In the case of copper (II) oxide, the copper has a +2 charge, and the oxygen has a –2 charge. Charged atoms are called “ions”.

Many elements can only ever form ions of one charge.  For example, sodium and potassium ions are always +1.  Certain metals, however, can form ions with different charges.  Copper ions can be +1 or +2. 

So, when you pull a jar of copper oxide off the shelf, how do you know if it’s CuO (one copper +2 atom and one oxygen –2 atom) or Cu₂O (two copper +1 atoms and one oxygen –2 atom)?  You know because the charge of copper is in the name.  That’s how you notate metals that can have more than one charge.

There are two ways copper can tarnish; one way is considered attractive and the other not so much.  The attractive tarnish is colored green to blue and is called “patina”.  Of course, the most famous structure in the US with this green coating is the Statue of Liberty.  Covered entirely with sheets of copper, the Statue we know today to be light green was originally the dull orange color of copper!  During decades of exposure to moist air, the copper (Cu) reacted with carbon dioxide (CO2) and oxygen (O2) to form copper (II) hydroxide (Cu(OH)2) and copper (II) carbonate (CuCO3):

2 Cu + H₂O + CO₂ + O₂—> Cu(OH)₂ + CuCO₃

This surface coating actually protects the copper underneath.

(Photo from Wikipedia; taken by Daniel Schwen)

The other kind of copper tarnish will turn the shiny surface dull.  This happens when the copper only reacts with oxygen in air, forming copper (II) oxide (CuO).

Copper jewelry is susceptible to both kinds of tarnish.  Many people like the patina because of its pretty color.  If, however, you want to protect your copper jewelry from it and from becoming dull, it’s best to store the jewelry in a ziploc bag to protect it from air. 

It’s easy enough to go to the store and buy silver cleaner to remove tarnish from your jewelry, but here’s a way to do it using a few products you probably already have.  Line the bottom of a small pan with aluminum foil.  Put the piece of silver on top and cover with water.  Gently heat the water and add a few shakes of baking soda and stir.  Be sure the silver is in contact with the foil.  Now watch to see the tarnish disappear!  Remove the jewelry, rinse with water, and pat dry with a soft cloth.  Beautiful!  Shiny!  Magic?  Of course not – chemistry! 

In the previous post about tarnish, we learned it’s silver sulfide (Ag₂S) that turns shiny silver jewelry yellow (and eventually black).  Lucky for us, the sulfur (S) in the compound is more attracted to the aluminum than silver.  The dissolved baking soda (NaHCO₃) helps transport the sulfur to the aluminum forming aluminum sulfide (Al₂S₃) and leaving behind pure silver (Ag).  Below is the chemical reaction: 

3 Ag₂S + 2 Al —> Al₂S₃ + 6 Ag

This method of cleaning allows the jewelry to retain all the silver it started with, whereas some cleaners or polishes remove some silver each time you use it.  Give it try!