Lab: Crystals

Here is my latest lab. I’ve pasted the text of the background into this post. The link takes you to the actual hand-out. The at-home portion of the lab involves the use of chemistry to create amazing crystalline ornaments. Happy holidays!

Source: Lab: Crystals

Solubility

Water is a substance that is easy to take for granted. We require it to live but it usually plays the role of a theatrical prop, easily ignored in the excitement of events going on around it. The chemical truth of the matter, though, is that water is a miraculous material. It is safe and non-toxic but still it is one of the most powerful solvents known to science. So many substances easily dissolve in water that it is fairly called the universal solvent. For example, salt (NaCl) and sugar (C12H222O11) both dissolve in water. These are very different kinds of chemical substances, for all that they look alike. Salt is an ionic compound of a metal and a non-metal and the bonds between its atoms are among the strongest known. Salt has a very high melting point as proof of this strength: 801°C (1,474°F). When salt melts the bonds between the atoms must be broken—the high melting point shows that a large amount of energy is necessary to break these bonds. Sugar is a compound of non-metal atoms bound by covalent bonds. The bonds holding the atoms together are reasonably strong but they are not broken when sugar melts. The bonds that do break are based on the weak forces of attraction holding one molecule to the other in a solid or liquid form. As proof of these weaker bonding forces, sugar melts (if you haven’t burned it first) at 186°C (367°F). Despite their differences, salt and sugar are both very soluble in water.

Although they both dissolve in water, there is a difference between sugar and salt in how well they dissolve. Salt will dissolve in water only until it makes up about 26% of the solution by mass. That is, when 26 g out of 100 g of the solution are salt. The other 74 g are water. Sugar, on the other hand, will dissolve until it makes up 40% by mass of the solution. Sugar is much more soluble than salt. Different substances all have their own unique amounts that represent the maximum mass that will dissolve in a given amount of water. When the maximum amount of any substance is dissolved in water we call the solution saturated . There is an activity available in which students can explore solubility as a function of temperature in a pencil-and-paper exercise in which they put data on a graph and answer questions about it. Find it on my site here.Saturated solutions will not allow any more of the dissolved material to dissolve. Additional solid that is added appears to remain at the bottom of the container, unchanged. Chemists measure the solubility of a substance based on its concentration when the solution is saturated. Substances with higher solubility make saturated solutions with larger dissolved masses.

Solubility is also subject to changes in temperature. Gases, such as oxygen (O2), dissolve better in cold water than in hot water. Solids, on the other hand, usually have a higher solubility in hot water than in cold water. For example, at a temperature of 90°C a saturated solution of sugar will be 45% sugar by mass instead of 40%. By making a solution at a high temperature it is possible to dissolve more solid than at low temperature. Compared to the amount that dissolves at room temperature, a solution made at high temperature is super-saturated . A super-saturated solution is one in which more material is dissolved than the usual maximum amount at a given temperature. When the temperature falls as the solution cools off the extra dissolved material will form crystals.

Crystal Formation

NaCl.Solid (204K)

Image source: Aaron Keller
Salt at the Atomic Level

Square.Salt.Crystals (168K)

Image source: Aaron Keller
Salt Crystals Showing Typical Square Shape

A crystal is a special solid form of a pure substance that has a regular three-dimensional geometrical shape. Each substance in the

Borax.Water.Molecule (155K)

Image source: Aaron Keller
A borax water molecule

Borax.Snow.Flake (166K)

Image source: Aaron Keller
A borax snowflake

world has its own unique crystalline shape. The shapes of crystals do fall into groups that can be readily classified. Some are cubic others are octahedral, rectangular solids, or other shapes. The shape of a crystal provides information about how the atoms or molecules in the crystal are arranged. At the molecular scale atoms in a crystalline solid are arranged in a regular repeating pattern which gives rise to the overall shape we see.

Crystals will form from a saturated solution left out on a table simply because the water evaporates away. When there is less water, less of the solid can remain dissolved. As a result, the solid begins to form crystals. These crystals may be so small that they appear to be like a fine powder. Visible crystals form when crystallization is slow. If evaporation takes a long time then bigger crystals have time to form. In the picture of salt crystals at right it is possible to see that they all have the typical square shape of salt crystals. Their different sizes are a result of different amounts of time spent growing as the water evaporated. This type of crystal formation is one that the in-class portion of this activity addresses. The ‘magic trees’ that you grow are made of crystals of salt that form as a result of the rapid evaporation from the edges of a piece of paper soaked in the solution.

Another way in which crystals can form is when a solution which is saturated at a high temperature is allowed to cool down. As it cools the solution becomes super-saturated and the excess solid crystallizes out. If the solution cools quickly then the crystals that form will be small. On the other hand, if the solution is allowed to cool more slowly then larger crystals may form. In the at-home portion of this activity borax will be dissolved in water at high temperature. Next it will be allowed to cool so that crystals may form. It is usually possible to create very attractive decorative ornaments by allowing the crystals to form on pipe cleaners which have been bent into pleasing shapes. For example, see the pictures at left.

Crystallization in Mixtures

When salt is dissolved in water there are two substances mixed together. Each maintains its own identity and could be physically separated from the other. For example, by arranging to make the water evaporate, leaving behind the salt crystals. But what if two or more substances are dissolved in the same water? When the water evaporates away the two different substances will crystallize only with other particles of the same kind, as long as they are distinctive enough. But if the atoms or molecules are similar enough to one another they may form crystals that are really a mixture of the two dissolved materials. For the in-class portion of this lab activity you will mix up some saturated salt water and then add ammonia and laundry bluing. The ammonia will make a mixed crystal with the chloride ions from the salt because the ionic form of ammonia (the ammonium ion, NH4+) is similar to the sodium ion (Na+). In addition, the ammonia, because it has a basic pH, causes the iron ions in the laundry bluing to precipitate as iron(III) hydroxide (Fe(OH)3). The laundry bluing is a colloidal suspension of ferric hexacyanoferrate: Fe4[Fe(CN)6]3 (also called iron(III) hexacyanoferrate(II)). It is a blue dye that can be used in washing white clothing and linens which have become yellow. This makes them appear white again. It is not commonly used in recent times, though in the past it was a household essential. Most people now use bleach instead, which works in a completely different way. The iron that is part of the cyano complex remains dissolved and also participates in forming the crystals you will observe in the lab.

Because of the mixed nature of the crystals that form by mixing salt, ammonia, and laundry bluing they have a completely different form. Instead of forming perfect cubes as sodium chloride would normally do, the crystals are tree-like or dendritic crystals. These crystals are a complex mixture of all the ingredients. By cutting out a small tree from an absorbent piece of paper the crystals can be made to form along the edges of the paper where the water evaporates most quickly. Left overnight, the crystals will form all over the entire tree.

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