Water-Soluble Plastic

I buy boxes of dishwasher detergent containing plastic bags filled with the detergent. I place these bags straight into the dishwasher. When the dishes are clean and I remove them from the machine there is no trace of either detergent or the plastic bag that held it.

What happened to the plastic bag?

Can plastic dissolve in water?

Find out in the December 2018 edition of AUS-e-NEWS, AUS-e-TUTE's free, quarterly, newsletter for chemistry students and teachers. 

To subscribe to AUS-e-NEWS, go to https://www.ausetute.com.au/ausenews.html

Chemistry of Cement

What do the Roman Colosseum, the Roman Pantheon, a modern bridge and a modern dam all have in common?

Go to the March 2015 issue of AUS-e-NEWS to find out about the relevance of chemistry to the ancient Romans as well to modern day humans!

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World's Lightest Material?

UC Irvine, HRL Laboratories and the California Institute of Technology have announced that they have succeeded in making the world's lightest material, with a density of 0.9 mg/cm³.

This material is made up of a metallic lattice of interconnected hollow tubes with walls a thousand times thinner than a human hair. Because of this open lattice structure, the material is actually made up mostly of air, 99.99% air .

But what is the metal making up this new material?

We know the density of the new material, so we can calculate the mass of a 1cm cubed volume of this material:

1cm³ of the new material would have a mass of 0.9 mg = 0.0009g.

If 99.99% of the mass of this material is made up of air, then

the mass of air = 99.99/100 x 0.0009 = 8.991 x 10^-4g (0.8991 mg)

and the mass of metal in the new material = 0.0009 - 8.991 x 10^-4 = 9 x 10^-7g (9 x 10^-4 mg)

If we assume that 0.00001% of the volume of the new material is metal, then

the volume of metal = 0.00001/100 x 1cm³ = 1 x 10^-7cm³

So, the density of the pure metallic solid would be 9 x 10^-7g/10^-7cm³ = 9g/cm³

If we compare this calculated density of the metal to a list of common metals as shown below,

Pure Substance	State	Density (g/cm3) at 25oC and 1atm
gold	solid	19.3
mercury	liquid	13.6
lead	solid	11.4
silver	solid	10.5
copper tin	solid solid	9.0 7.3
zinc	solid	7.1
aluminium	solid	2.7

then we see it is possible that the new material is made up of copper.

Reference

T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit, W. B. Carter.Ultralight Metallic Microlattices. Science, 2011; 334 (6058): 962 DOI: 10.1126/science.1211649

Further Reading

Density Calculations

Suggested Study Questions

1. Using the table of densities above, calculate the mass in grams of a
   • cubic centimetre of gold
   • a cubic metre of copper
   • a cubic millimetre of silver
   • a cubic kilometre of zinc
2. Using the table of densities above, calculate the volume in cubic centimetres of
   • 1g of copper
   • 100mg of lead
   • 4500μg of aluminium
   • 2kg of silver
3. Brass is a mixture of copper and zinc. A sample of brass has a density of 8.5g/cm³
• What is the mass a cubic centimetre volume of this brass sample?
• If the sample were made up of equal masses of copper and zinc, what is the mass of copper in the sample?
4. A sample of brass was produced using 500cm³ of each of copper and zinc.
• What mass of copper is present in the brass?
• What mass of zinc is present in the alloy?
• Assuming additivity of volumes, what is the density of this brass sample?
5. Cymbals are commonly made of bronze which is a mixture of about 10% (by mass) tin and 90% (by mass) copper. For a 100g sample of bronze, calculate
• the mass of copper present in the sample
• the volume of copper this mass represents
• the mass of tin present in the sample
• the volume of tin this mass represents
• the density of the bronze sample assuming additivity of volumes
6. Typically, bronze contains copper and about 12% (by mass) tin. Calculate the density of a sample of this bronze.
7. Bronze coins often contain copper and about 5% tin. Calculate the density of the bronze used to make coins.
8. The brass used to make springs and screws is often 65% (by mass) copper and 35% (by mass) zinc. Calculate the density of the alloy in a brass screw.

Casting : Changes of State

The question of what happens when a material composed of more than one phase or state is heated or cooled is very important.
Many metal parts, for example, are made by casting. In the casting process liquid metal is poured into a mold and solidifies into the shape of the mold. As the liquid metal solidifies it forms tree-like structures called dendrites, and, if one of the dendrites breaks off it can lead to a change in the properties of the solidified material. The airplane industry has spent a long time developing solidification methods to avoid this problem when casting jet turbine blades.
Polymer solar cells use a complicated mixture of two polymers. When heated, the mixture evolves by a process that involves pinching which ultimately alters the properties of the mixture and the efficiency of the solar cell.
Scientists have been observing the heating process during which a rod-like phase or state embedded in another will break up into smaller domains just like droplets at the end of a stream of water, resulting in changes to the properties of the material. They have found that the shape of the interfaces during break up becomes universal, independent of the material used. This now allows them to predict the dynamics of the break-up process in a vast array of materials such as steel and polymers.

Reference:
Aagesen et al. Universality and self-similarity in pinch-off of rods by bulk diffusion. Nature Physics, 2010; DOI: 10.1038/nphys1737

Study Questions

1. Name the phase changes (changes of state) that can occur in each of the following situations:


• heating a solid


• heating a liquid


• cooling a liquid


• cooling a gas




2. Draw a sketch of the temperature-time graph expected for each of the following situations involving pure substances:
   


• heating a solid


• heating a liquid


• cooling a liquid


• cooling a gas




3. Explain why the temperature-time graph for the melting of ice differs from the temperature-time graph for freezing water.
4. Explain why the purity of a solid substance can be determined using its melting point.
5. Do you think the purity of a liquid substance could be determined using its freezing point? Explain your answer.
   
6. Explain what is meant by the term sublimation.
7. Give two examples of pure substances that undergo sublimation.

Topological Insulators

In an electrical conductor, negatively charged electrons can hop between atoms and move freely in their interior or on the surface. These free electrons are responsible for the generation of electric current. For most metals, electrons in the interior carry most of the current, while surface electrons are only weakly mobile. Some materials, such as glass, have structures that impede electron flow and are called insulators.

A topological insulator is a substance that acts as an insulator in its interior while permitting the movement of charges on its boundary. This can occur when a perpendicular magnetic field is applied, this is known as the quantum Hall effect.
Princeton scientists have discovered a new type of typological insulator, an antimony crystal, which does not require the application of a magnetic field.

Reference:
Jungpil Seo, Pedram Roushan, Haim Beidenkopf, Y. S. Hor, R. J. Cava, Ali Yazdani. Transmission of topological surface states through surface barriers. Nature, 2010; 466 (7304): 343 DOI: 10.1038/nature09189

Study Questions

1. What is required in order for a material to be considered an electrical conductor.
2. Give three examples of good electrical conductors.
3. Give three examples of electrical insulators.
4. In general, what type of substances conduct electricity?
5. In general, what type of substances do not conduct electricity?
6. If electrons are negatively charged, why are atoms considered to be neutral?
7. Why do you think electrons are more free to move within the interior of a material compared to its surface?
   

Hard Metal

Hard metal is a mixture of a hard carbide phase, tungsten carbide, and a tougher metal phase, cobalt. It is produced by sintering, a process in which fine powders of tungsten carbide and cobalt are heated up so that the cobalt melts and the material is pulled together by capillary force. This results in a solid material consisting of hard tungsten carbide grains surrounded by the tougher cobalt-rich cement phase.

The size of the tungsten carbide grains determines the hardness of the hard metal.
Scientists know that by doping the material, that is, by adding another substance in tiny amounts, they can limit the size of the grains. For example, adding a tiny amount of vanadium can limit the growth of the grains, instead of growing grains 1/1000 mm in diameter, the addition of vanadium results in grain sizes about 1/10,000 mm. Scientists at the Chalmers University of Technology in Sweden have just used high-resolution electron microscopy to observe an extremely thin layer, only 2 atom layers thick, of a cubical structure on the tungsten carbide grains which they believe is affecting the growth of the grains.

Reference:
Expertanswer (2010, June 14). Materials researchers micromanage atoms in hard metal. ScienceDaily. Retrieved June 16, 2010, from http://www.sciencedaily.com­ /releases/2010/06/100614093343.htm

Study Questions:

1. Write the symbol for each of the following elements: tungsten, carbon, cobalt, vanadium.
2. To which group of the Periodic Table do tungsten, cobalt and vanadium belong?
3. Give possible oxidation states (numbers) for tungsten, cobalt, vanadium and carbon.
   
4. Suggest a formula for tungsten carbide.
5. Would you expect larger or smaller grains of tungsten carbide to grow at higher temperatures?
   

Sticky Mortar?

1,500 years ago, Chinese construction workers developed what was probably the world's first composite mortar, a mortar made from both organic and inorganic materials. The mortar was made by combining sticky rice soup with slaked lime, limestone which has been heated to high temperatures then exposed to water. This "sticky rice" mortar was stronger and more resistant than pure lime mortar.

Scientists have recently discovered that amylopectin, a type of polysaccharide, is the ingredient in the sticky rice that is responsible for the strength of this ancient mortar. The amylopectin in the mortar acts as inhibitor, controlling the growth of the calcium carbonate crystals, resulting in a compact microstructure which leads to greater mechanical strength.

Reference:
Fuwei Yang, Bingjian Zhang, Qinglin Ma. Study of Sticky Rice-Lime Mortar Technology for the Restoration of Historical Masonry Construction. Accounts of Chemical Research, 2010; : 100510131945076 DOI: 10.1021/ar9001944

Iron and Superconductors

About 100 years ago, scientists discovered materials that could conduct electrons without losing energy to resistance, but, these "superconductors" had to be very cold. The electron-electron repulsion in these low-temperature superconductors was so weak that electrons could overcome it, pair up and move freely.

In 1986, scientists discovered new materials that became superconductors at temperatures above 100K. These high-temperature superconductors were made of layers of copper alloys sandwiched between layers of nonconducting material that were doped with trace amounts of material that could contribute a few extra electrons to the mix. If these materials were not doped with insulating material they did not conduct electricity as the electrons locked themselves at a distance from their neighbours. This locked pattern was named the "Mott localization".

In 2008 a second class of high-temperature superconductors was discovered. These pnictides are iron-based superconductors which are also layered and need to be doped. However, undoped pnictides are not Mott insulators.

Early in 2010, scientists replaced arsenic atoms in one of the intervening layers of a pnictide with slightly smaller phosphorous atoms. This brought the iron atoms a little closer together and further away from the Mott tipping point.

Rice University researchers are now using iron oxychalcogenides which are layered materials like pnictides, but with greater distance between the iron atoms, and this greater distance is enough to push the system into a Mott insulating state.

A better understanding of the behaviour of high-temperature superconductors is essential to future improvements in electric generators, MRI scanners, high-speed trains and other devices.

Reference:
Jian-Xin Zhu, Rong Yu, Hangdong Wang, Liang L. Zhao, M. D. Jones, Jianhui Dai, Elihu Abrahams, E. Morosan, Minghu Fang, and Qimiao Si. Band Narrowing and Mott Localization in Iron Oxychalcogenides La₂O₂Fe₂O(Se,S)₂. Physical Review Letters, 2010; 104 (21): 216405 DOI: 10.1103/PhysRevLett.104.216405

Scientists See in the Dark

Conventional night vision goggles use a photocathode, a cathode ray tube-like vacuum tube made of thick glass, to convert infrared light photons into electrons which are then accelerated under high voltage and driven into a phosphorous screen producing greenish images of objects invisible to the naked eye in the darkness.

University of Florida scientists have produced an imaging device that replaces the photocathode with several layers of organic semiconductor thin film materials. The photodetector is connected in series with an LED. Infrared light photons are converted into electrons in the photodetector which are then injected into the LED which generates visible light. This imaging device would be light-weight and inexpensive to produce since it could be made using the same equipment currently used to produce laptop screens and flat-screen TVs. This new night-vision technology could be used on mobile phones, car windshields, even standard glasses.

Do Young Kim, Dong Woo Song, Neetu Chopra, Pieter De Somer, Franky So. Organic Infrared Upconversion Device. Advanced Materials, 2010; DOI: 10.1002/adma.200903312