Sulfuric acid can exist as a solid, liquid, or gas depending on temperature. Solid sulfuric acid can be created by cooling it below 50°F, but dilution with water affects the freezing point. It is important to store sulfuric acid at the correct temperature to avoid unexpected changes in volume.
Like all matter, sulfuric acid can exist as a solid, liquid, or gas. The temperature of the acid determines the state of matter it is in, with lower temperatures slowing the atoms down until they become solid and higher temperatures creating enough motion in the atoms that they first become a liquid and then a gas. Although sulfuric acid is a liquid at room temperature, solid sulfuric acid can be created by cooling it below 50 degrees Fahrenheit (10 degrees Celsius). When sulfuric acid is diluted with water, the temperature required to create solid sulfuric acid will vary depending on the dilution.
To create solid sulfuric acid, the acid must be kept below its melting point. For pure sulfuric acid, this means the acid must be kept below 50 degrees Fahrenheit. If the acid reaches this temperature, it will quickly dissolve 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.
Although pure sulfuric acid solidifies at temperatures below 50 degrees Fahrenheit, most sulfuric acid is diluted with water. This type of acid reacts readily with water, making it difficult to keep it pure. Also, the acid is more stable at certain dilutions, so it is usually kept partially diluted. At 99% sulfuric acid, 1% water, the acid freezes 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 will be kept frozen because the temperature required to turn 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 fluoride, becomes a solid at -21 degrees Fahrenheit (-29.4 degrees Celsius), a temperature lower than that required to freeze 98% acid. A 78% dilution of acid becomes solid at 11.5 degrees Fahrenheit (-11.4 degrees Celsius).
Industries are more likely to use sulfuric acid in liquid form. While solid sulfuric acid is not particularly useful, it is important that anyone storing this material be aware of the freezing point for the dilution of the sulfuric acid they are storing, and the acid should be maintained at temperatures that keep it solid or liquid form. The physical properties of the acid, including the amount of volume it occupies, 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 used to store the acid.
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