Formation of crystals

Formation of crystals: Hi. Is copper sulfate recommended for crystal making in high schools—I thought that it was no longer permissible to heat copper sulfate in school laboratories. I have suggested that potassium aluminum sulfate is a safer and cheaper alternative.

Thanks.

 

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Publication Date: 03 August 2015
Asked By: mazza
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Formation of crystals

Expert Answer

Both chemicals are considered safe to use with students from Years 7–12(1) if proper controls are put into place to deal with the hazards. Students should wear safety glasses and avoid contacting the materials with the skin. Hands should be washed immediately after completing the activity and students should not be allowed to take the crystals home. It is recommended that any crystals formed are recycled for future use.

There are many substances that can be used to grow crystals in school science labs. Common examples include copper (II) sulfate and aluminium potassium sulfate (potash or common alum). However, sugar and salt could also be used. A comparison of copper (II) sulfate and aluminium potassium sulfate shows the following:

 

copper (II) sulfate

aluminium potassium sulfate

Signal word

Warning

Warning

Pictogram Hazard

Health hazards

Environmental

Health hazards

Hazard phrases

Harmful if swallowed

Causes skin irritation

Causes serious eye irritation

Very toxic to aquatic life

Very toxic to aquatic life with long lasting effects

Harmful if swallowed

Causes skin irritation

Causes serious eye irritation

May cause respiratory irritation

Repeated exposure may cause skin dryness and cracking

ADG

9

NA

Poisons schedule

S6

NA

Both have identified hazards, however aluminium potassium sulfate is less hazardous to the environment than copper (II) sulfate and is not a scheduled poison.

Copper (II) sulfate is commonly used as a fungicide, herbicide, and pesticide. It will produce striking, blue, diamond-shaped crystals in the laboratory.

The heating of copper (II) sulfate, is relatively safe to do if it is in a solution form. More hazards arise when the solid chemical is heated. If copper (II) sulfate is heated to decomposition (above ~250 °C), then there are irritating copper oxide fumes and the corrosive gaseous sulfur trioxide is produced. The temperatures involved in crystal making are less than 100 °C, nowhere near hot enough to cause decomposition.

Nevertheless, the heat from a Bunsen burner is in excess of that required for crystal making and rapid evaporation from copper (II) sulfate solutions can result in hazardous fumes. Therefore, we recommend using hotplates or water baths where possible for these activities to prevent the heating of the salt solutions to boiling point. We also recommend that the work area is well-ventilated.

Aluminium potassium sulfate (potash or common alum) is found in industrial, medicinal, cosmetic and food products and is widely used as a preservative in food processing. It can produce large clear crystals in the laboratory.

The solubility of aluminium potassium sulfate increases significantly with temperature. At 25 °C, the solubility is about 14g/100mL, whereas at 100 °C, this increases to about 160g/100mL. Therefore, 100 mL of hot, saturated solution may cool to give an excess of over 140g of alum, which would be unlikely to precipitate as large, well-formed crystals.

Note: The purchase of some technical grade chemicals e.g. copper (II) sulfate may be an issue as some contain impurities, which will need to be filtered when recycling for reuse.

Background information

Using chemicals to grow crystals is a common activity in school science laboratories. Crystals are solids that form when molecules join together in a regular repeating pattern. You can easily grow crystals by adding a crystal-forming chemical to hot water to form a saturated solution and wait for the water to cool or evaporate.

The following are some different crystal-growing activities.

  1. Based on the solubility characteristics of the salt (available from a chemical data book), specify the amount of a solid for students to dissolve, the volume of water to use and the temperature to which the water should be heated (i.e., less than 100 °C)
  2. Provide the class with a saturated solution to which students add a specified amount of the solid and heat until it is dissolved. For example, the dissolution of 2–3 teaspoons of alum in 40–50 mL of a saturated alum solution will, with good technique, give a single crystal of alum.
  3. Provide the class with a saturated solution in which a seed crystal will grow with the slow evaporation of the water (see http://practicalphysics.org/growing-crystal-alum-or-copper-sulfate.html)
  4. Instruct students to dissolve as much solid as possible in a known volume of hot water. This method is commonly suggested, however, it does not produce good results for all salts. Depending on the salt, it can lead to a much-too-concentrated solution that will not produce very nice crystals, such as with alum. However, it may work well for some salts where there is not such a large variation in their solubility versus temperature, such as sodium chloride or copper (II) sulfate. The following links also provide good information on crystal growing.

http://www.raci.org.au/document/item/1862

http://www.iucr.org/__data/assets/pdf_file/0005/15863/20_US.pdf

Science ASSIST recommends a risk assessment be conducted prior to the use of any chemicals, and all appropriate control measures be identified and implemented. Science ASSIST has developed a one-page risk assessment template, see Risk Assessment Template.

References:

(1) 'Recommended list of chemicals for science in Australian schools' Science ASSIST website. /resource/3052/list-recommended-chemicals-science-australian-schools

‘Safety Data Sheet: Copper (II) sulfate’, January 2013 Chemwatch.

‘Safety Data Sheet: Aluminium potassium sulfate’ March 2014, Chemwatch.

Formation of crystals

It is toxic to heat the solid copper sulfate, however for crystallisation we gently heat the saturated solution then add another spatula of solid (teacher can do this part), then pour an amount into a small glass petri dish (preferably an old one with plenty of scratches - site of crystallisation) This method gives some pretty impressive large crystals.

if you don't want to heat at all pour a small amount of saturated copper sulfate into a glass petri dish and seed with a tiny amount of solid or previously made crystals.

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