Hot ice

Sodium acetate solution "freezes" instantly

Difficulty:
Danger:
Duration:
1 hour
Experiment's video preview

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
  • Observe safety precautions when working with boiling water.
  • Keep a bowl of water nearby during the experiment.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

In the step 3, sodium acetate doesn’t dissolve completely. What should I do?

Pour out hot water from the beaker and pour in freshly boiled water. Place the bottle with sodium acetate in the beaker and wait for 10 more minutes.

The bottle is very hot — how do I safely pour out sodium acetate from it?

Wait several minutes for the bottle to cool down a bit and continue the experiment.

Why do we wipe one end of a wooden stick with a cloth?

Moisture prevents crystallization. We remove this water, but the stick remains covered in tiny crystals of sodium acetate. Upon contact with the solution, they initiate the crystallization process.

Solution in the test tube crystallized before I’ve touched it with a stick.

Crystallization may start earlier than expected if sodium acetate in the bottle hasn’t dissolved completely. Also, crystallization might have been initiated by a dust particle that accidentally fell into the test tube. To learn in detail how crystallization works, refer to the “What happened” section.

In order to repeat the experiment, immerse the opened test tube in freshly boiled water. Wait until sodium acetate dissolves completely (20–25 min.) and continue the experiment starting with the step 7.

The sodium acetate doesn't crystallize. What should I do?

Add 1ml of water into the test tube and then put it in a beaker with hot water. Place the beaker on the stove and heat for 10-15 minutes. Then place the test tube into a plastic cup and leave it to cool for 15 minutes. Touch the solution with the wooden splint to start the crystallization.

Step-by-step instructions

  1. Place three candles on the solid fuel stove and light them. Set the flame diffuser on the stove.
  2. Place the beaker onto the flame diffuser and pour in 125 mL of freshly boiled water.
  3. Immerse a bottle with sodium acetate CH3COONa in hot water. Wait 20 min.
  4. Using forceps, pull the bottle out of hot water.
  5. Take a test tube from the set and stick a thermochromic sticker on it.
  6. Pour sodium acetate from the bottle into the test tube.
  7. Place the test tube into a plastic cup. Dip one end of a wooden stick into the solution in the test tube.
  8. Wipe wet the end of the stick with a cloth.
  9. Wet the test tube rubber stopper with hot water.
  10. Close the test tube with the rubber stopper. Wait 15 min.
  11. Touch the solution with the wooden stick.
  12. Solution will start to crystallize! As it does so, it will release heat causing the thermosticker to change its color.
Graphical step-by-step instruction

Expected result

A touch of a splint triggers immediate crystallization of the supersaturated sodium acetate solution

Disposal

Dispose of solid waste together with household garbage.

Scientific description

How does a sodium acetate solution freeze so quickly?

Sodium acetate CH3COONa is a salt that crystallizes both on its own and in combination with water molecules CH3COONa · 3H2O . Such compounds are called crystalline hydrates, and the water in their composition is called water of crystallization. One of the most common properties of hydrates is their relatively low melting temperature or melting point. When a crystalline hydrates melts, water breaks away and starts acting as a solvent in the resulting solution.

In this experiment, we are using such a hydrate. The formula for the hydrate of sodium acetate is: CH3COONa · 3H2O. Sodium acetate, with a melting point of 58C, melts and forms a solution even at slight heating. The resulting solution is “supersaturated”. This means the amount of sodium acetate in a given volume is higher than that if we tried to dissolve sodium acetate in water at room temperature.

It is important to remember that under normal conditions sodium acetate CH3COONa is a solid at room temperature, whereas the crystalline hydrate of sodium acetate CH3COONa · 3H2O forms a supersaturated solution. This solution is just a step away from a transition into solid sodium acetate CH3COONa and water. All that is required is a “trigger” to start the process.

Learn more

Looking more closely at the structure of the hydrated sodium acetate, CH3COONa · 3H2O we see there are three water molecules for every sodium acetate. In the figure below, you see many red balls — these are the oxygen atoms. The red balls (oxygen atoms) attached to the white balls (hydrogen atoms) are water molecules. While the red balls (oxygen atoms) attached to the black balls (carbon atoms) are part of the negatively charged ions CH3COO.

Acetate

How is crystallization triggered?

During our experiment, we treated the splint with a supersaturated solution of CH3COONa · 3H2O and then wiped it dry. As a result, a small amount of crystalline sodium acetate remained on the surface of the splint.

Sometimes a simple physical impact may not be enough to trigger the solidification process. A tiny amount of CH3COONa · 3H2O, such as on the end of a splint, acts as a nuclei crystal and helps to start the avalanche-like process of solidification.

That's interesting!

What is a supersaturated solution?

Imagine yourself standing in the middle of a very busy and crowded shopping area at noon. There are many people rushing around trying to get their shopping done during their lunch break, others are trying to grab a bite to eat, still, others hope to get a haircut or have their nails done, while others need to visit a doctor or dentist. Under these circumstances, it’s likely you’ll be jostled, bumped, little space. You might very well feel that it would be best to leave

Using this analogy, imagine that this is how the particles in a supersaturated solution “feel”. All the particles are constantly being jostled and are vying for space. They “understand” that there isn’t enough room for them in the solution, and that at least some of them should leave. Although they “understand” that it would be best for some of them to leave, they might need a little nudge of encouragement to leave the solution (with a splint, for example, as in this experiment). The little nudge is the “trigger” which starts the crystallization process.

Let us take a closer look at the difference between a saturated and supersaturated solution. A saturated solution is one in which no more solute can be dissolved in a solvent at a given temperature. A supersaturated solution contains more solute than that of a saturated solution at the same given temperature. How is this possible? Typically, when the temperature of a solution is increased, more particles can be dissolved, thus increasing the amount of solute. This is followed by a period of cooling down to the initial temperature. The solution now has more solute than if had not been heated. As you can imagine it is very crowded in this supersaturated solution and particles are quite happy to leave if they have the opportunity. Thus, if presented with a trigger (such as the splint) some particles will leave the solution and form a precipitate.

supersat