Icicle Adventures

Despite below freezing temperatures, ice can melt in sunlight. This indicates that ice molecules can easily get agitated to above freezing temperatures even if surrounding air molecules have less energy of motion. If the melt-water runs down an edge, heat can be taken away again from the dripping water. An icicle begins to form. If water keeps flowing over the bud, the icicle will grow. It not only serves as a continuous source of crystals, dripping water helps take heat away from the growing icicle.

When ice forms, it actually releases heat. What, aside from water, takes heat away so that more flowing water can freeze?   It is the up-draft of air caused by the icicle being  warmer than its surroundings. The heat is taken away more slowly at the top of the icicle, where it grows more slowly. At the base, the opposite is true; growth is faster, hence the carrot shape.

With all that mind and the expectation of seeing the same pattern again, I was of course startled when I saw this yesterday:EQlXZbGWkAIaTCz

By looking around, I soon realised that’s not what the icicle looked like when it formed. A meter to the left of that strange upside down V-formation, I noticed the following.

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In all likelihood, the odd shape originally had three components, a familiar carrot shape but joined to a pair of other icicles that had grown along the edges of the angular frame. Metal is efficient at removing heat, and it probably adds a complicating variable to the shape of the arms clinging to the metal. One of the arms detached itself from the frame upon partially melting and then rotated; the other arm broke off.

I was tweeting these two pictures when I told myself I should be out there instead, observing to verify if guess was right. The same structure to the left now looked like this:

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Seconds after I snapped the photo, the whole structure came crashing on the floor of the deck. It never got a chance to rotate as much as the original icicle.

The best, however, was yet to come. Annie van Leur saw my pictures and remarked the following:

 

 

 

Cute. However, not as interesting as our Michigan ice apples. When freezing rain coats rotting apples and the mushy rotten apple falls out, it leaves a shell of ice.

And she posted this:

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Why are there often ridges on icicles? It’s ionic impurities in the water that play an important role. Graduate student Antony Chen designed a table-top apparatus for the controlled growth of icicles. He experimented with different conditions of temperature, water supply rate, ambient air motion, and water purity. When pure water was used, ripples did not appear on icicles. They looked like the icicle in Figure 1. Even a low salt concentration (0.008%) caused ripples to grow and travel on icicles (Figure 2). In the absence of salt,  dissolved gases or non ionic surfactants had no effect on the growth of ripples. When the air was still, icicles grew more tips (Figure 3). Gently stirring the air caused the shapes to become more monotonous.

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Figure 1
figure2_008%salt
Figure 2
tips
Figure 3

 

 

SOURCES:

The Point of Icicles https://uanews.arizona.edu/story/point-icicles

Experiments on the Growth and Form of Icicles https://tspace.library.utoronto.ca/handle/1807/44105

Icicle Photos https://www.physics.utoronto.ca/Icicle_Atlas/rogues_gallery.html

 

 

 

 

 

 

Pure Water’s pH is only 7.00 at a Specific Temperature.

Pure water’s pH is only 7.00 at a specific temperature of 25.0 °C.  Students (and teachers too) hear that number so often that they forget where it comes from. And forgetting its origins makes one forget that if the temperature deviates significantly from 25.0 °C, you will get unfamiliar numbers for the pH of pure water.

At any temperature, pure water will always have the same concentration of ions resulting from a very slight splitting of the life-essential molecule into a positive hydrogen ion and a negatively charged hydroxide ion. The product of each ion’s concentration will equal its so-called Kw of 1.01 × 10-14 at 25.0 °C. To calculate the concentration of H+ ions, you first take the square root 1.01 × 10-14  and then take the negative logarithm of H+, the definition of pH. It yields 7.00.

But changing temperature usually affects any equilibrium constant, including Kw. In this case raising temperature helps water split up. You get more ions, thus a higher ion product. Kw becomes 5.48 × 10-14 at 50 ° C, Lowering the temperature has the opposite effect on equilibrium, and Kw becomes 0.29 × 10-14 at 10° C.

When you recalculate pH of pure water at 50 ° C and 10° C, we obtain pH’s of 6.63 and 7.27, respectively. The temperature does not make water either slightly acidic or alkaline. It’s just that the middle or neutrality point of the pH scale at those different temperatures changes. The 7.00 is not set in stone. The middle point of the pH scale is a setting derived from what the Kw happens to be at 25.0° C.

PHscale

The pH is also 7.00 for aqueous solutions whose solutes at that same temperature do not affect the ions that water itself produces. When the temperature changes for those solutions, the pH will change accordingly. In our bodies, if temperature was the only factor, then out physiological pH would be below 7.00. But the presence of bicarbonate ions eats up some of the hydrogen ions, setting the physiological pH at about 7.4. The  pH of the extracellular fluid of tumour cells, as determined by probing microelectrodes, is acidic. That truth has been known for at least 3 decades, and of course nothing one eats will have any impact on the pH of that fluid.

Often teachers have to reiterate to get ideas across. So indulge me. 🙂 Does temperature affect neutrality of pure water? No. The concentration of hydrogen ions will be equal to that of hydroxide ions as long as no solute interferes with one of them. Does raising temperature raise the concentration of OH? Yes. Of H+? Yes. Will that in turn affect pH? Of course, by definition.

 

The Periodic Table of the Elements’ Natural Sources

EMA1EtCWsAIopQaThere are thousands of different periodic tables in existence. Aside from the usual ones that offer atomic masses and numbers, for a long time we have had those that revealed various periodic trends. In more recent years, some have focused on their time or place of discovery, on cosmic origins of the elements, and even on endangered “species”.

Many academic institutions and a slightly-richer-than-the average-guy by the name of Bill Gates,  have placed actual samples of elements in cubicles to create a 3D-version. There are more modest tables filled with beautiful photographs of the  elements—in fact, you can even get a 1000 piece jigsaw puzzle version.

(Although it should have had 118 or 1180 pieces to match the number of elements. 🙂 ) 914eg7tsctl

I thought of creating one more, a table that focuses on some of the natural sources of the elements—even though I’m sure the idea is far from original. You probably know already that such a table will leave out synthetic ones, about 34 of them. Of course, for most of the remainder, there is more than one natural source. So anyone else who has created such a table will have put together merely one of at least millions of possibilities.

periodic

You will notice that whereas a periodic table has mostly metals, the natural source- version consists of mostly minerals, which I find more aesthetically pleasing. A rock is a heterogeneous mixture of minerals, while a mineral is similar to a chemical compound, but it is not as narrowly defined. Its composition can vary within limits; impurities can drastically change the colour of a mineral, and those impurities can sometimes be the only source of the element, as is the case with rhodium and several of the rare earth elements.

The list of elements that can be found in their native, non-bonded state is longer than most of us imagine. It includes four of the five elements in group 15: nitrogen, arsenic, antimony and bismuth; all three mintage elements: copper, silver and gold; iron and nickel in meteorites—in fact native iron can also be found in basalt; five other heavy metals: osmium, rhodium, iridium, palladium and platinum; oxygen, sulfur and, of course, all six noble gases.

For four of the elements, helium, gallium, rubidium and cesium,  I included spectra, which is how those elements were discovered. In helium’s case, the scientists were looking at the sun’s outer layers during the eclipse of 1868. Only decades later was helium gas found on earth when it was found to be released from a uranium ore. Soon after, they realized that lots could be extracted from natural gas sources.

If you refer back to the first table I listed, that of the endangered elements, you will notice that helium is one of them. The number of suppliers worldwide is limited. If one  of them experiences issues, shortages quickly develop. This leads to a spike in prices for the simplest but most essential of the noble gases. Helium is used as a coolant in MRIs, smartphone-manufacturing and other applications.

It’s just not recycled enough, if at all, as is the case with many of the endangered elements.

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