Valentine’s Day Experiment: Science behind making rock candy

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SAVANNAH, Ga. (WSAV) – The sweetest holiday of the year is this weekend – Valentine’s Day! I know I am not the only one looking forward to the candy. You know what is more fun? Rocking out and making your own! In this episode of Exploring the Atmosphere with Alysa, let’s fall in love with a science treat that is sweet enough to eat!  

MAKING YOUR OWN ROCK CANDY

SUPPLIES

  • 1 Cup of Water
  • 2-3 Cups of Sugar
  • Large Pan or Pot
  • Glass Jar or Cup
  • Thick String/Pipe Cleaner/Stick
  • Popsicle Stick/Pencil
  • LOTS OF TIME

*If you are making more than 1 cup of rock candy, times the supplies by that number. The ratio should be around 3:1 of sugar and water.

EXPERIMENT

MAKING SUGAR WATER

  1. Pour 1 cup of water into a pot or pan and bring it to a boil.
  2. Pour 1/4 cup of sugar into boiling water until it dissolves.
    – You’ll need to keep adding 1/4 cup of sugar into the water. Each time waiting until it dissolves. You’ll stop adding sugar when no more can be dissolved.
    -When sugar stays on top of the mixture and falls to the bottom in clumps, you are getting close to this point.
    -By the end, the the mixture will have a cloudy tint by the end.
  3. Once no more sugar can be dissolved, remove it from heat and allow mixture to completely cool.
    -You’ll know you’ve done this correctly, if a sugary film forms on top as it cools.

SETTING UP SKEWER

  1. While it cools, get your rock growing skewer set up.
  2. IF YOU ARE USING A STRING: measure the string to make sure it does not touch the bottom of the jar. Tie the string to a popsicle stick or pencil.
  3. IF YOU ARE USING A PIPE CLEANER: Shape the pipe cleaner in the shape you want the sugar to grow on. Make sure the shape is small enough and won’t touch the bottom or sides of glass jar. Tie pipe cleaner to popsicle stick or pencil with string or another pipe cleaner.

CREATING SUGAR SEED

Here is a fun tip: if you dip the string in the sugar/water mixture and roll it in sugar ahead of time, you will be giving the tiny sugar particles a great place to start crystalizing once in the solution! This can act like seed crystals. Seed crystals will be able to connect with other sugar particles and a crystal begins to grow.  

If you do this, make sure the string is completely dry before putting into the mixture in the jar otherwise the sugar seeds will fall off.

GROWING THE SUGAR CRYSTALS

  1. After it cools, pour mixture into the glass jar or cup.
    -You can add food coloring to mixture and stir with a spoon.
  2. Put your cut string or shaped pipe cleaner into the mix.
  3. Put the pencil/popsicle stick on top of the jar to hold the string steady.

HOW LONG WILL IT TAKE

This is the worst part of the experiment…waiting! The sugar will slowly grow, but it will take time! You’ll need to wait 5-7 days for the sugar crystals to completely form. If you have enough patience, you are in for a sweet treat!  

SCIENCE BEHIND ROCK CANDY

As you are mixing the sugar and water together, you are slowly creating a super saturated solution. This means you will end up with more dissolved particles of solute (SUGAR) than of the solvent (water) can normally dissolve. By adding more and more sugar into the hot water, the sugar is dissolving faster than the fast moving (heated) water molecules are able to dissolve.

This creates an unbalance in the solution – there is now more sugar particles than water in the mixture. This is where you stop adding sugar. As the water cools, a large amount of sugar particles remain in the solution and this creates the supersaturated solution.

HOW CRYSTALS GROW:
SUPERSATURATED SOLUTION & EVAPORATION

There are two different methods that contribute to the growth of the sugar crystals. The first is to create a supersaturated solution. This is done by heating a supersaturated sugar solution and then allowing it to cool. This is a solution in which no more sugar can dissolve at a particular temperature. This is creates an unstable balance – it contains more solute than can stay in a liquid form. The sugar that forms out of this solution is called precipitate and this method is called precipitation.

The other is evaporation where the water slowly evaporates from the solution. As water evaporates, the solution becomes more saturated and sugar molecules continue to grow. The rock candy crystals grow molecule by molecule.

If you soak the string/pipe cleaner before hand: it needs to be soaked and then dried – the string will provide a surface for crystals to grow. As water evaporates from the string, small crystals of sugar will encrust the string. These tiny seed crystals provide starting points for larger crystals. Further growth will be concentrated around these points. 

As you pour the mixture into the jar, the sugar particles will begin to settle. They join together and form crystals with other sugar molecules on the string or pipe cleaner.

The key is giving them lots of time to grow. As water evaporates, the solution becomes more saturated and sugar particles continue to grow until you pull it out of the solution and enjoy the candy! You will also be able to see the crystallization both on the bottom and side of the jar as well.

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