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Name: Lauren
Status: student
Grade: 6-8
Location: NC
Country: USA
Date: Summer 2012


Question:
Why does water turn into a solid when you freeze it, while juice and soda turns into more of a slushy form?



Replies:
Lauren

If you drop the temperature on juice and soda far enough they will freeze. Water molecules form into rigid crystals when you remove the heat energy from the liquid.

Sincere regards, Mike Stewart


Hi Lauren,

The short answer is that with soda pop and juice, the water portion freezes first, and the sugar-syrup portion, which needs a colder temperature to freeze, does not. You might ask _how_ and _why_ they separate... read on.

Different materials turn solid ("freeze") and different temperatures. The temperature at which a material freezes is called its freezing point. If a material is at a temperature above its freezing point, it melts -- which means it turns into a liquid state. If you heat up ice, it melts and turns into water. Glass can also melt, but at a much higher temperature than water. Plastic (there are many different kinds) also will melt. Sugar also can melt -- if you put it into a sauce pan and heat it up (although much hotter than it takes for water), it will melt.

Things get even more complicated when you make mixtures. It turns out, when you dissolve salt or sugar into water, it actually lowers its freezing point. In other words, water with salt or sugar dissolved in it will stay unfrozen at a lower temperature than pure water. This is how salt put on the road prevents ice (depending on where you live in NC, you may or may not have encountered the practice of putting salt on roads during snowy weather). And, this partly explains why juice or soda pop only partially freeze in your freezer.

Soda pop and juice are a mixture of water, sugar, and other things. In both cases, sugar is dissolved in water. With the soda pop, you also have carbon dioxide dissolved in the water too. The freezing point of each is lower than that of pure water. Your freezer is designed to operate at around 15F, which is low enough to freeze pure water, but is only cold enough to partially freeze the juice or soda.

But that does not explain why the juice or soda only partially freeze.

To explain the partial-freezing, let us examine how "freezing" actually happens. When a liquid freezes, what actually occurs is a few molecules of the liquid collect and form a tiny ice crystal called a "nucleus" (not the same kind of nucleus inside a cell, and also not the same kind of nucleus of an atom either). This nucleus then grows as liquid molecules "join the party" and attach to the nucleus. This is how ice crystals form. Depending on how cold the liquid's surroundings are, you may have many nucleii forming at once, (meaning lots of little ice crystals), or you might have just a few slow-growing ice crystals (meaning fewer larger ice crystals). But, the physics of the ice crystals means that water is favored over sugar to add to the nucleus. As a result, freezing juice actually forms a mixture of water-enriched ice crystals (meaning the ice crystals have more water in them than the juice), and a liquid phase of sugar-enriched (meaning the liquid phase is concentrated in sugar compared with juice). As the mixture freezes, you get a slush of water crystals and concentrated juice-syrup.

Hope this helps, Burr Zimmerman


Lauren,

A slushy feel to a solidifying liquid means that small crystals are present with some liquid. A blocky feel to a solidifying liquid means that all the crystals merged into one crystal and any liquid is outside the crystal.

There are several factors controlling the way a liquid solidifies. The main factors are: temperature, whether the liquid is homogeneous or not, whether the liquid is pure or a solution, and whether the liquid was agitated during freezing or not. We will tackle each one in turn.

Temperature: this controls how fast the liquid is cooled. If the liquid is cooled very fast, a "shock treatment" with cold temperature, it can happen that smaller crystals of the solid will form and it will form every where in the solution all at once. If the temperature is high enough that not all the liquid solidifies during this "shock treatment", this will lead to a slushy feel. Homogeneity: If the liquid has little bits of solid (like fruit pulp, grains of undissolved sugar, even dust), these bits of solid become places where the liquid can solidify. If the liquid starts forming crystals in many different places within the liquid and those crystals might not merge together. If they don't merge together, this will leas to a slushy feel.

Purity: When comparing a pure liquid to the same liquid in which something else has been dissolved, we find that the pure liquid freezes at a higher temperature than the solution - so pure water will freeze at 0degC, but sugar water will freeze at a lower temperature than that. This goes back to the temperature control mentioned above.

Agitation: If the liquid is being stirred, mixed, or in any way physically agitated during the freezing process, the growing crystals tend to be small and numerous, and these crystals might not merge - again this will lead to a slushy feel.

So, let's say that we remove the agitation factor - we place a container of juice and another of water in the freezer. This means that both liquids experience the same temperature. But, the juice is probably not homogeneous (there are bits of pulp in it; the soda also has carbonation which also makes it not homogeneous). This means that the juice or soda will form smaller crystals. The fact that the juice or soda are solutions means that they actually freeze at a lower temperature than pure water. So, depending on the temperature of the freezer, it can happen that when you check the containers that some liquid in the juice or soda will still not be frozen - again giving it a slushy feel.

You can get soda or juice to freeze in one solid block by controlling the temperature so that the it is close to where the freezing begins. This would mean that the crystal formation would be very slow and crystals can merge into one block. Then when most of the liquid has frozen, if you then bring it to a lower temperature so that all the liquid freezes, this also helps connect the crystals together and form one solid block.

Greg (Roberto Gregorius) Canisius College


When you cool water to 0 C., ice (solid water) begins to freeze. The water will continue to freeze at 0 C. until all the liquid water is frozen to form ice (solid water). However, in the case of juice and soda, what is in the liquid is not pure water. It is a solution of water, plus sugar and various salts. When the water begins to freeze, water does begin to form ice (solid water). However, the liquid solution remaining is not pure water. It is a solution of water and the various components present in juice and/or soda. These remain dissolved, so a slush of ice (solid water) and the dissolved other components (sugar, various salts, etc.) remain dissolved in the water keeping part of the solution as a liquid solution. So what is present are ice crystals and the liquid solution of dissolved sugar, various salts, etc. This forms the “slush”.

If you continue to cool this mixture, eventually you will reach a temperature where not only does ice form but the solution containing the other dissolved components also begins to freeze, but the solid is not pure solid water. It is a solid containing ice and/or solid sugar, salts, etc. Actually measuring the components present is not easy to do, because the solid is a complicated mixture of the various other components. And the composition of both the solid part and the dissolved solution will depend upon the detailed composition of the particular juice and/or soda.

Vince Calder


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