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Like all matter, sulfuric acid can exist as a solid, liquid, or gas. The temperature of the acid determines which state of matter it is in, with lower temperatures slowing the atoms down until they become solid and higher temperatures creating enough movement in the atoms that it becomes first a liquid and then a gas. Though sulfuric acid is a liquid at room temperature, it is possible to create solid sulfuric acid by cooling it to a temperature below 50 degrees Fahrenheit (10 degrees Celsius). When sulfuric acid is diluted with water, the temperature needed to create solid sulfuric acid will vary depending on the dilution.
In order to create solid sulfuric acid, the acid needs to be kept below its melting point. For pure sulfuric acid, this means that the acid needs to be kept below 50 degrees Fahrenheit. If the acid reaches this temperature, it will quickly melt into a liquid. Sulfuric acid is usually thought of as a liquid because it takes this form at room temperature, which is how it is usually stored.
Though pure sulfuric acid solidifies at temperatures below 50 degrees Fahrenheit, most sulfuric acid is diluted with water. This type of acid reacts with water readily, making it difficult to keep it pure. Additionally, the acid is more stable at certain dilutions so it is usually kept partially diluted. At 99% sulfuric acid, 1% water, the acid will freeze when it reaches a temperature of 41 degrees Fahrenheit (5 degrees Celsius). At 98%, creating solid sulfuric acid requires temperatures below 30 degrees Fahrenheit (-1.1 degrees Celsius).
It is important to check the specific melting point for each concentration of sulfuric acid that is going to be kept frozen as the temperature needed to transform liquid sulfuric acid into a solid does not follow a regular pattern. A dilution containing 93% sulfuric acid, which can be used to create hydrogen flouride, becomes a solid at -21 degrees Fahrenheit (-29.4 degrees Celsius), a temperature lower than that is needed to freeze 98% acid. A dilution of 78% acid becomes solid at 11.5 degrees Fahrenheit (-11.4 degrees Celsius).
Industries are most likely to use sulfuric acid in a liquid form. Though solid sulfuric acid is not particularly useful, it is important for anyone who stores this material to be aware of the freezing point for the dilution of sulfuric acid they are storing, and the acid should be kept at temperatures that keep it either in solid or liquid form. The physical properties of the acid, including the amount of volume it takes up, can change if the acid changes from one phase to another, and an unexpected change in volume could cause stress to a container that is being used to store the acid.
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