Helium Balloons and Speed of Sound in a Gas

(thanks Creative Commons)

Ever wonder why inhaling helium from a balloon increased the pitch of your voice?

The pitch of your voice depends on many factors: the shape of your throat, mouth, nasal passages, etc. But most importantly, it depends on the gas present in your lungs. Under normal circumstances, the gas in your lungs is air. The speed of sound through air at 20°C is roughly 343 m/s (the temperature in your lungs is a little bit hotter, so the speed of sound through the air in your lungs is actually about 353 m/s).

What if the gas in your lungs is helium?

In general, the speed of sound, v, in a gas with molar mass M and adiabatic constant γ (which depends on the gas), at temperature T, is given by:where R is the ideal gas constant (8.314 J·K^-1·mol^-1).

For Helium, M is about 4 g·mol^-1=0.004 kg·mol^-1 and γ=5/3, so the speed of sound at the temperature of your lungs is about 1036 m/s in Helium. This is A LOT FASTER than the speed of sound through air!

For air, which is composed of many different gases (but mostly nitrogen and oxygen), the average molar mass is about 29 g·mol^-1=0.029 kg·mol^-1 and γ=1.4, which accounts for the decreased speed of sound through air of about 353 m/s.

But why does this increase the pitch of your voice?

Just like any wave, the speed of a sound wave is related to the frequency and to the wavelength of the wave:

speed=frequency·wavelength

The pitch (frequency) of the sound wave that gives rise to your voice in helium increases because the speed of the wave increases, while the wavelength stays the same! So you might be wondering what your voice would sound like if you inhaled a really heavy gas, like argon or krypton... Your voice would get deeper! But DON'T TRY IT! You might suffocate!

Green Fluorescent Protein

The 2008 Nobel Prize in Chemistry was awarded to Osamu Shimomura, Martin Chalfie and Roger Tsien for their combined discovery and application of Green Fluorescent Protein (GFP).

GFP was isolated from a Pacific Northwest jellyfish by Shimomura, who noted its absorption of blue or UV light and resultant bright green fluorescence. It is now used widely in biochemistry and biology as a tag for proteins and as a marker for gene expression. In particular, the gene that encodes GFP can be spliced into the genome (of essentially any organism, from yeast to pigs or even bunnies...) in the vicinity of a gene that encodes a protein of interest. The expression of GFP is then linked to the expression of the protein under investigation. Whenever and wherever that protein is expressed is then easy to monitor because GFP is produced simultaneously!

(Image from Creative Commons)

You can probably imagine some of the other fascinating potential applications of GFP, ranging from tracking cancer metastasis and angiogensis in mice, to measuring calcium concentration in vivo.

Chemistry Quote of the Day

"Synthetic efficiency is in reality limited only by scientific principles and the boundary conditions of our planet, rather than by economical and biological factors. Chemists are thus encouraged to develop truly efficient processes at all costs, and the key phrase in this context should be 'Practical Elegance'."

-Ryoji Noyori, Green Chem., 2003, 5, G37-G39

LCD's, Color, and Pixels!

Did you ever wonder what a "pixel" really is?

(Pixel spout from Creative Commons)

As promised, today we're going to learn about how LCD's have been developed to produce a vast array of different colors in different positions of your computer screen (or for those more technologically advanced readers, your TV screen!).

Pixel stands for "Picture Element" and it represents one of thousands or millions of points on your graphics display, which are ordered in rows and columns. If you were to zoom in on your computer screen A LOT, you would be able to see a single pixel, but since the screen you are viewing now is so zoomed out and there are so many pixels, you cannot tell that they are just tiny dots of color. The number of colors possible for each pixel is determined by the number of bits (click the link to learn about information theory), e.g., 10 bits allows for 2^10=1024 different colors possible for that pixel, although most pixels are 8-bit, which is more than enough.

If you have a color (rather than black or white) computer screen, then each pixel is made from three dots (blue, green and red). There are two types of LCD matrices: passive and active.

In a passive-matrix LCD, a grid transfers charge to the pixels. The grid consists of two glass sheets, one lined with columns and one lined with rows of a conducting material. The rows and/or columns are hooked up to electric circuits, which control the allocation of charge. Between the two glass sheets, as mentioned in the previous entry, is a layer of LC's. And on the outsides of each sheet of glass is a polarizing film.

In order to light up a pixel, charge is transferred down a column on one of the glass sheets. When the charge reaches a point on the other glass sheet that is grounded, the pixel at that intersection will light up in response to the untwisting (see previous entry) of the LC's at that point, and voila--color!

Have you ever noticed a trail behind the arrow on your screen when you moved it around rapidly? That trail results from the inadvertent untwisting of LC's in the immediate vicinity of the pixel of interest!

Now what about active-matrix LCD's?

Active-matrix LCD's use a glass sheet, much like the ones used in passive-matrix LCD's, but instead of columns or rows, the active-matrix glass sheet consists of a matrix of transistors and capacitors. In order to charge a pixel, the appropriate row is turned on and charge is transferred down the appropriate column. In this way, the capacitor at the specified pixel gets charged. And capacitors are pretty good at holding on to charge, so the pixel stays lit for as long as you like. Depending on how much charge is transferred to the pixel, we can vary the amount of light/color that is produced, thereby displaying the elaborate set of colors and shapes that you currently see on your screen!

Liquid Crystal Displays

Did you ever wonder why your laptop computer screen display looked a bit different on a cold day?
This phenomenon has to do with the materials used to create the images you see on your display, known as liquid crystals.

"Liquid crystal" refers to a material that exists in a state somewhere between a solid and a liquid, but closer to a liquid than a solid. A small amount of energy in the form of heat is required to turn a liquid crystal into a liquid, so they are quite sensitive to temperature, which makes them great for use in computer monitor displays, or even mood rings!

Liquid crystals come in two general varieties: thermotropic and lyotropic.

Thermotropic liquid crystals are further divided into isotropic or nematic varieties, which are distinguished by the arrangement of the molecules: isotropic liquid crystals are randomly arranged, while nematic liquid crystals are ordered in response to an external factor, such as a magnetic field, or an ordered surface. The ordering of the molecules is characterized by specific layering, or spiraling of the molecules.

But I still haven't explained how they work. Let's first think about the role of light (AKA electromagnetic radiation) in this process.

Recall from the last entry the discussion of "wave-particle duality," which referred to the dual nature of matter, which displays both wave-like and particle-like behavior. Light is no exception: light "particles" travel in a line and bounce off surfaces, but light also spreads out when passed through a narrow opening and interferes with light passing through another opening, much like a wave. Light is composed of energy in the form of electric and magnetic fields (hence the name "electromagnetic radiation"), which vibrate at right angles to each other. Since light's vibrations are aligned in two different planes, it is referred to as "polarized." The polarized nature of light not only allows us to protect our eyes by wearing sunglasses, but it is also the reason why you are able to see this computer screen presently!

A Liquid Crystal Display (LCD) requires two pieces of polarized glass sandwiching a sheet of liquid crystals (which, as we noted before, can be arranged in multiple, stacked layers) and aligned at a 90 degree angle. The direction of polarization (direction of the grooves) in the lower piece of glass will determine the arrangement of the bottom-most layer of liquid crystals, while the top piece of glass determines the arrangement of the top-most layer of liquid crystals, and all the layers in between will gradually spiral towards either groove direction, depending on their proximity to the top or bottom.

As light hits the first glass sheet, it is polarized in one direction and guided through the liquid crystals. The molecules change the light's plane of vibration so that it corresponds to their own angle (with respect to each successive layer of molecules) and when the light finally reaches the other sheet of glass, it is vibrating at the same angle as the last layer of liquid crystals, which is oriented to match the grooves in the second sheet of glass so that the light passes through.

But what if we don't want the light to pass through? We certainly could not change the display on our computer screen if the same amount and color of light was always able to pass through...

Another useful property of liquid crystals is that they can change their orientation in response to an applied electric charge. So if we want them to untwist, all we need to do is add in some energy (heat or light) and the liquid crystals will straighten, thereby changing the angle of the light passing through them so that it no longer matches the angle of the grooves in the top sheet of glass. In this case, light will not pass through that area and it will appear darker than the regions in which light does pass through!

Tune in next time to learn about the different types of liquid crystals used in LCDs and the ways we can use them to get different colors in our displays!

Kinetic Isotope Effects and Tunneling

(Image taken from Creative Commons)

Did you ever wonder how Professor Johnston and his team were able to travel back to the Middle Ages in Michael Crichton's novel Timeline? Or what the "Tunnel" in Carpal Tunnel Syndrome is all about?

Well, I can't answer the latter question, but there is actually a very simple explanation for the former phenomenon: quantum mechanical tunneling!

In order to gain a basic understanding of tunneling, we should first review the so-called "wave-particle duality." In quantum mechanics, wavefunctions (solutions to the Schrodinger equation) are used to describe ANY physical system under consideration. The wavefunction's values are probability amplitudes, which can be used to determine the likelihood that the system of interest will be in a certain state at a certain time. We can make measurements on the wavefunction to determine information about the system.

For example, we can determine the frequency of a vibrating system (such as a bond between two atoms, or a guitar string). We can also determine the position of a moving particle. And if we wish to measure the frequency for this same moving particle, we will access a wave. The particle can at once be treated as a wave and as a particle. And if you are confused, ok! Most people are.

But don't let that stop us from trying to rationalize the possibility that a molecule (or "system" for that matter) without enough energy to traverse a barrier could actually just cut through the barrier to get to the other side. The "particle" wave has a finite probability of traversing the barrier.

If you recall from the entry on the Arrhenius equation, we examined the relationship between the rate constant, the temperature and the activation barrier. A modified version of the Arrhenius equation, which includes a "tunneling correction factor," Q, is used to analyze tunneling. Q depends exponentially on the mass of the tunneling system, and tunneling is more common for a light particle (e.g., a hydrogen atom or an electron). For this reason, kinetic isotope effects may be used to analyze tunneling events: substitution of a hydrogen by deuterium will lead to a dramatic decrease in the rate of a tunneling process.

An example of tunneling in an organic reaction was shown in the case of olefin generation via selenoxide elimination. In this case, a large kinetic isotope effect was observed and an unusually large difference in the A values for the D- and H- reactions was found (collisional theory would predict a 1:1 ratio of AD to AH).

At this point you might be wondering why people never tunnel through chairs when they sit down. To put it simply, our wavelengths are just too short!

To Attract or To Repel: Deciphering the Functional Dichotomy of Firefly Bioluminescence

Ever wonder why fireflies are so... fiery? Or how they decide when to glow and when not to glow?

(image taken from www.entomon.net)

The purpose of a firefly's glow is speculated to be two-fold: at the larval stage, fireflies (also called lightning bugs) emit a glowing light in order to fend off potential predators by tricking them into thinking that their prey is poisonous; adult fireflies, on the other hand, light up in order to attract mates. The adult "flash pattern" is specific to the species: some species use a continuous pulse flash, whereas others produce a single, brief flash.

So what chemical reaction is responsible for the glow?

A chemical known as "firefly luciferin," shown below, reacts with oxygen under enzyme catalysis by "flirefly luciferase," magnesium ion and Adenosine Triphosphate (ATP) to form a dioxetanone intermediate. Decomposition of the dioxetanone produces electronically excited oxyluciferin. Relaxation of oxyluciferin to its ground state is accompanied by emission of yellow light (maximum wavelength 560 nm, quantum yield 0.88, which is quite high!--compare to the bright green material in the vial shown on the side of this page, which has a quantum yield of only 0.33).

"Diamonds are Forever"

Did you ever wonder why diamond is the hardest naturally occurring mineral? Or whether diamonds really "are forever" as jewelry advertisements always claim?

Let's start by looking at how diamond is formed and how it is different from other allotropes (different forms of an element characterized by differences in bonding between the individual atoms) of carbon, a few of which are shown in Figure 1.

Diamond forms from carbon-containing compounds under harsh conditions found deep within the Earth's mantel: extremely high temperature and pressure. It consists of a covalently bonded network of tetrahedral carbon atoms (Figure 1, where n is greater than or equal to 6, technically).
Graphite, another common allotrope of carbon, consists of stacked layers of covalently bonded carbon networks. Unlike diamond, however, it is quite soft (you may have encountered it in pencils, going by the name of "lead"). You might be surprised that such a seemingly small rearrangement of the carbon atoms could produce such a dramatic change in physical properties.

As a matter of fact, the differences between diamond and graphite are vast. Let's consider their relative stabilities. On the one hand, graphite is slightly more thermodynamically stable than diamond. But the energy barrier to convert diamond into graphite is extremely high, such that the conversion would be very difficult to achieve in the lab or in nature. In order to transform diamond into graphite, or graphite into diamond, one would need to break all the carbon-carbon-carbon-carbon bonds and reform them in an entirely different way. So although diamond is not thermodynamically stable it is kinetically stable, sometimes referred to as metastable.

At least as far as carbon allotropes are concerned, diamond is "forever."