Re: crystal growth and reasons why temperature and different
Postby tdaly » Sat Oct 25, 2014 12:15 pm
Thank you for this additional information. It is very helpful! It sounds like you controlled all of the right variables so that the difference must be due to using different kinds of water.
Take a look at the answer Vince Calder (of Argonne National Laboratory) gave when someone asked a similar question: http://www.newton.dep.anl.gov/askasci/g ... n99889.htm. I agree with him--other ions dissolved in tap water are probably interfering with crystal growth in your daughter's experiments. Tap water may have additional nucleation sites (which other scientists at Argonne point out in that post), but the impurities in tap water can actually stop crystals from growing larger. Those impurities aren't present in distilled water, so the crystals in distilled water can grow to larger sizes. That's the explanation for the result of your daughter's experiment.
As far as presenting her hypothesis and findings goes, your daughter's hypothesis shouldn't change after her experiment. A hypothesis is an educated guess about what will happen--you make it before doing the experiment. Her conclusion should (1) state whether her hypothesis was correct or incorrect, (2) say what the results were (crystals grew larger in tap water), and (3) briefly explain why the experiment turned out the way it did (other ions--impurities--in the tap water interfered with the growth of the crystals, keeping them from growing as large as the one in distilled water).
Last edited by tdaly on Fri Oct 31, 2014 7:29 am, edited 1 time in total.
All the best,
A chemical garden is an experiment in chemistry normally performed by adding metal salts such as copper sulfate or cobalt(II) chloride to an aqueous solution of sodium silicate (otherwise known as waterglass). This results in growth of plant-like forms in minutes to hours.
The chemical garden was first observed and described by Johann Rudolf Glauber in 1646. In its original form, the chemical garden involved the introduction of ferrous chloride (FeCl2) crystals into a solution of potassium silicate (K2SiO3).
The chemical garden relies on most transition metal silicates being insoluble in water and coloured.
A metal salt such as cobalt chloride will start to dissolve in the water. It will then form insoluble cobalt silicate by a double decomposition reaction (anion metathesis). This cobalt silicate is a semipermeablemembrane. Because the ionic strength of the cobalt solution inside the membrane is higher than the sodium silicate solution which forms the bulk of the tank contents, osmotic effects will increase the pressure within the membrane. This will cause the membrane to tear, forming a hole. The cobalt cations will react with the silicate anions at this tear so forming new solid. In this way growths will form in the tanks; these will be coloured (according to the metal) and may look like plants. The crystals formed from this experiment will grow upwards, since the pressure at the bottom of the tank is higher than the pressure closer to the top of the tank, therefore forcing the crystals to grow upwards.
The upward direction of growth depends on the density of the fluid inside the semi-permeable membrane being lower than that of the surrounding waterglass solution. If one uses a very dense fluid inside the membrane, the growth is downward. For example, a fresh, green solution of trivalent chromium sulfate or chloride refuses to crystallise without slowly changing into the violet form, even if boiled till it concentrates into a tarry mass. That tar, if suspended in the waterglass solution, forms downward twig-like growths because all the fluid inside the membrane is too dense to float and thereby to exert upward pressure. The concentration of sodium silicate becomes important in growth rate.
After the growth has ceased sodium silicate solution can be removed by continuous addition of water at a very slow rate. This will prolong the life of garden. In one specific experimental variation a single tube can be obtained 
Common salts used
Common salts used in a chemical garden include:
While at first the chemical garden may appear to be primarily a toy, some serious work has been done on the subject. For instance this chemistry is related to the setting of Portland cement, hydrothermal vents and during the corrosion of steel surfaces tubes can be formed. A chemical garden helps one to understand the nature of that chemical substance.
The nature of the growth also is useful in understanding classes of related behavior seen in fluids separated by membranes. In various ways it resembles the growth of spikes or blobs of ice extruded above the freezing surface of still water, the patterns of growth of gum drying as it drips from wounds in trees such as Eucalyptus sap, and the way molten wax forms twig-like growths, either dripping from a candle, or floating up through cool water.
- ^Balköse, D.; Özkan, F.; Köktürk, U.; Ulutan, S.; Ülkü, S.; Nişli, G. (2002). "Characterization of Hollow Chemical Garden Fibers from Metal Salts and Water Glass". Journal of Sol-Gel Science and Technology. 23 (3): 253. doi:10.1023/A:1013931116107.
- ^Cartwright, J; García-Ruiz, Juan Manuel; Novella, María Luisa; Otálora, Fermín (2002). "Formation of Chemical Gardens". Journal of Colloid and Interface Science. 256 (2): 351. Bibcode:2002JCIS..256..351C. CiteSeerX 10.1.1.7.7604. doi:10.1006/jcis.2002.8620.
- ^Thouvenel-Romans, S; Steinbock, O (Apr 2003). "Oscillatory growth of silica tubes in chemical gardens"(PDF). Journal of the American Chemical Society. 125 (14): 4338–41. doi:10.1021/ja0298343. ISSN 0002-7863. PMID 12670257.
- ^Glauber, Johann Rudolf (1646). "LXXXV. Wie man in diesem Liquore von allen Metallen in wenig Stunden Bäume mit Farben soll wachsen machen. (How one shall make grow—in this solution, from all metals, in a few hours—trees with color)". Furni Novi Philosophici (German-language 1661 ed.). Amsterdam: Johan Jansson. pp. 186–189.
- ^Glaab, F.; Kellermeier, M.; Kunz, W.; Morallon, E.; García-Ruiz, J. M. (2012). "Formation and Evolution of Chemical Gradients and Potential Differences Across Self-Assembling Inorganic Membranes". Angew. Chem. Int. Ed. 51: 4317–4321. doi:10.1002/anie.201107754.
- ^Julyan H. E. Cartwright, Juan Manuel García-Ruiz, María Luisa Novella, and Fermín Otálora, J. Colloid Interface Sci. 2002, 256, 351–359. "Archived copy". Archived from the original on 2007-03-11. Retrieved 2006-02-05.
- ^"Ice Formations Diurnal Freeze-Thaw Cycles".