The Fascinating World of Minerals

Growing crystals in the kitchen: simple and safe recipes

Below is a little exercise that you can easily do at home under adult supervision.  The purpose of this exercise is to grow and observe different crystal forms and to both describe and recognize some of their diagnostic properties.

MATERIALS:

• Pans
• Water (from tap)
• 3 spoons
• Measuring cup
• 3 transparent glass containers
• Stove
• 2 25 cm lengths of string
• 2 Wooden splints
• Small rocks (3-5)
• 3 paper towels
• 3 Rubber bands
• Sucrose (C₁₂H₂₂O₁₁) - sugar
• Halite (NaCl) – kitchen salt
• Alum (KAL(SO₄)₂*12H₂O)

PROCEDURE:

You will be creating super-saturated solutions by adding solid compounds to water in specific ratios.  Because the crystals will grow out of these solutions, it is important to be careful when measuring the amount of each compound as well as the amount of water.

• Sucrose

You will grow your own ROCK CANDY!! (although I wouldn’t advise you to eat it…)

• Measure 200 ml of tap water into a pan.
• In another container, add 230 grams of sucrose (sugar).  When measuring out the sucrose, use only the utensil found with the sucrose to avoid contamination with the other compounds.
• Place the beaker on the stove and turn the dial medium-high.  Gradually add the measured sucrose to the water while stirring the solution with a spoon.
• Continue heating the solution until all of the solid sucrose has dissolved and the solution begins to boil.
• Once the sucrose has dissolved (this may take a long time) and you first start to see evidence of boiling (bubbles forming), remove pan from heat.  Let the solution cool a little and then transfer it in a transparent glass container.  Be carefull, boiling water and glass are a dangerous combination.

• Tie one of the 25 cm strings around the middle of a wooden splint so that about 15 cm are left loose.  Tie one of the small pebbles to the bottom of the string.  Lay the splint across the mouth of the beaker and hang the string into the solution.

• Cover the beaker with a paper towel and secure it with a rubber band.  The point of this is not to obtain an airtight seal, but rather to prevent foreign material from falling into the solution.  Label the beaker with its contents so you’ll know what it is when you look at it later.  Set it aside.

• Halite

Now you will grow halite (salt) crystals.

• Follow steps 1-7 outlined above, this time using a ratio of 100 grams of halite to 200 ml of tap water. 

• ALUM

In this last exercise you will grow your own crop of alum crystals.

• Follow steps 1-5 outlined above in the sucrose experiment, using a ratio of 40 grams of alum to 200 ml of tap water.

• This time, rather than dangling a rock on a string, gently slide a few small rocks into the solution and let them settle to the bottom.  They will provide a rough surface on which the crystals can grow.

• Cover the beaker with a paper towel and secure it with a rubber band. Be sure to label the beaker with its contents.  Set it aside.

The crystals you have just created will need some time to grow.  Generally within a week or two they will germinate and grow to a fairly decent size, so keep an eye on them and have fun!