How to make hot ice with vinegar and baking soda?

There are a large number of home experiments that serve to illustrate many of the most important aspects of science, and specifically chemistry. One of these experiments is that of hot ice, which consists of the rapid and exothermic crystallization of an aqueous solution of sodium acetate. Next, we show everything necessary to carry out the synthesis of sodium acetate from vinegar and sodium bicarbonate, and we will also prepare a supersaturated solution of this organic salt that, when it crystallizes, will illustrate the power of exothermic reactions.

What is hot ice?

Despite what the name suggests, hot ice isn’t actually ice. Let’s remember that ice is nothing more than the common name with which water is known in a solid state. Under ambient conditions of temperature and pressure, water is in a liquid state and its melting point is 0 °C. At the standard ambient pressure of 1 atmosphere, it is not possible to heat ice beyond its melting point, since any additional heat we supply will be spent melting the ice rather than heating it. For this reason, the idea of ​​making true ice (solid water) at ambient pressure and at room temperature, or worse, at a higher temperature, violates the laws of thermodynamics. In other words, it is impossible.

The hot ice is actually a chemical compound called sodium acetate . This compound is an organic salt formed by acetate ions, which correspond to the conjugate base of acetic acid, contained in the vinegar we use in the kitchen, and sodium ions. Acetic acid is an organic acid belonging to the family of carboxylic acids and is the second simplest member of that family. Hence, its salts are called organic salts.

The formula for sodium acetate can be written in different ways. Its chemical formula is NaC ₂ H ₃ O ₂ . In semiexpanded form it is written CH ₃ COONa, and the acetate ion is sometimes abbreviated Ac ^– , so sodium acetate is represented as NaAc. The chemical structure of this compound is:

Despite this being its structure, sodium acetate crystallizes from aqueous solution in the form of the trihydrated salt with the formula NaC ₂ H ₃ O ₂  · 3 H ₂ O.

Sodium acetate synthesis reaction

Salts can be synthesized in different ways and from different chemical reagents. One of the easiest ways to do this is through the neutralization reaction between an acid with the appropriate anion (acetic acid in this case) and a base containing the appropriate cation. Acetic acid is a weak acid that is readily available and safe to handle. As for the base, there are several options that would work for sodium acetate, but the two most suitable are sodium hydroxide (caustic soda) and sodium bicarbonate .

The first base, sodium hydroxide, is a good choice for rapid synthesis because it is a strong base that reacts rapidly with acetic acid to form sodium acetate, and the by-product of the reaction is water. , which is already present anyway.

In this case, the chemical reaction would be:

However, this reaction has two major problems. First, although sodium hydroxide is readily available, as it is the main component of most caustic oven cleaners and grease removers, it is a fairly dangerous substance that must be handled with great care. .

Secondly, it is necessary to carefully measure the amounts of sodium hydroxide added to the solution and the pH should be checked using litmus paper or something similar to indicate when all the acetic acid has been neutralized, since if an excess is added On this basis, the salt solution can be strongly alkaline and dangerous to handle.

A more suitable and much safer alternative that provides the most benefits of sodium hydroxide, but none of the drawbacks, is baking soda. This salt corresponds to the sodium salt of the conjugate base of carbonic acid. It is an amphoteric compound that can react with both acids and bases and is capable of neutralizing acetic acid, forming sodium acetate and carbonic acid. The latter could interfere by contaminating the sodium acetate formed, but it rapidly decomposes producing water and gaseous carbon dioxide which quickly escapes in the form of an effervescence visible to the naked eye.

The synthesis reaction for sodium acetate using sodium bicarbonate is:

This is the reaction that we will use in the present synthesis of sodium acetate.

White vinegar is a purified aqueous solution of acetic acid that generally contains between 3% and 5% by mass of the acid. With a density very close to that of pure water, this means that 1 L of vinegar contains approximately 30 to 50 g of acetic acid. This means that for them to react in stoichiometric proportions, a liter of vinegar requires approximately between 42 g and 70 g of sodium bicarbonate. If found in the maximum proportion, around 113 g of trihydrated sodium acetate would be formed.

Procedure to prepare hot ice at home

reagents

• 1 L White vinegar (acetic acid approx. 5%)
• 100 g of baking soda

materials and equipment

• Stove to heat.
• Beaker of at least 1.5 L capacity. If you don’t have one, you can replace it with a pot or pan made of glass and, ultimately, metal.
• Glass container of 200 to 250 mL capacity. The container must be very clean and must not be scratched.
• Kitchen scale, preferably digital.

Preparation of supersaturated sodium acetate solution

To synthesize sodium acetate follow these steps:

1. Weigh the baking soda on a scale and set aside.
2. Add the vinegar to the beaker and slowly add the baking soda to the solution in small portions, stirring with a spoon until fizzing stops before adding the next portion. This is to prevent too much carbon dioxide from being generated and overflowing over the sides of the beaker. At this point the sodium acetate has already been formed.
3. Bring the solution to a boil over medium heat and continue heating to evaporate the water and reduce the volume until sodium acetate crystals appear at the edges of the container. At this point the volume should be approximately 100-150 mL.
4. At this point, add a small amount of water (about 5 mL should be sufficient) and continue heating until all solid formed is completely dissolved.
5. Carefully transfer the hot solution to the second glass container. If any crystals form during transfer, heat container in a boiling water bath until dissolved. If crystals persist, add a small amount of water again and heat a little longer. It is very important that there is not a single crystal in this solution before the next step.
6. Cover the container containing the solution with plastic wrap and place it in the refrigerator, avoiding hitting or rough handling of the container.

hot ice experiments

The cold solution thus obtained is a supersaturated solution of sodium acetate which will crystallize rapidly after any disturbance, forming a hot block of crystals resembling ice.

• You can experiment with this solution in different ways. Try adding small crystals on the surface and you will see how they grow in all directions forming beautiful needles.
• You can also transfer a small amount of solution to a Petri dish and another shallow glass container and seed crystals at two ends to see which grow faster.
• Finally, many people enjoy slowly pouring the solution over a small handful of crystals on some surface. As the solution falls, it crystallizes forming a vertical stalagmite of sodium acetate.

Explanation of the hot ice experiment

The rapid crystallization of sodium acetate is due to the fact that the cold solution prepared is a supersaturated solution of the salt. This means that it contains more dissolved sodium acetate than water is capable of dissolving at that temperature. This means that the solution is in a metastable state that can be easily precipitated by any disturbance that generates the formation of the first crystals, or by the addition of some crystals that serve as nuclei for the formation of new crystals.

This disturbance may consist of seeding a small crystal on the surface, touching the solution with a finger, or scratching the surface of the container with a metal implement. Sometimes simply hitting the container is enough to trigger crystallization

The crystallization process will spread rapidly through the entire solution. The heat comes from the fact that the chemical reaction of crystallization is exothermic, that is, it releases heat.

Security measures

Fortunately, none of the chemical reagents used in this experiment are toxic or dangerous in any way. Sodium acetate is also not a dangerous compound. In fact, it is used as an additive in the food industry.

However, the experiment requires heating a solution to boiling and handling the hot solution, which poses a significant burn hazard. This experiment should not be carried out without adult supervision, and without using thermal gloves to protect the hands from the heat.

References

Amochemicals. (nd). Caustic soda: what it is, characteristics and precautions . amochemicals.com. https://www.amoquimicos.com/caracteristicas-de-la-soda-caustica

Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ^ed .). McGraw-Hill Education.

Experciencias. (2017, February 7). Instant hot ice . Youtube. https://www.youtube.com/watch?v=ujuwKEZevfY

Laboratorium Discounter. (2021). Sodium acetate . laboratoriumdiscounter.nl. https://www.laboratoriumdiscounter.nl/en/chemicals/az/n/sodic-acetate/

Editorial CuídatePlus. (2020, August 27). bicarbonate . Take carePlus. https://cuidateplus.marca.com/alimentacion/diccionario/bicarbonato.html

Seara, B. (2017, February 7). Instant Hot Ice – amazing experiments . experScience. https://www.experciencia.com/hielo-caliente/