Which of the following is a physical change? A) Formation of HBr from H₂ and Br₂. B) Melting sulfur. C) Rusting of iron. D) Formation of CO₂ gas from mixing baking soda and vinegar. Or E) combustion, burning, of wood.
A physical change is distinguished from a chemical change as a change that does not alter the chemical nature of the substances involved. There is some debate about the boundaries, but good examples include state changes like boiling and melting, size changes like increasing the volume of a gas or grinding up a solid into a powder, and mixing nonreactive chemicals together.
There are a few things that we could look out for that would indicate what we’re dealing with was a chemical rather than a physical change. On the most basic level, a chemical change involves the formation or breaking of bonds. So, if we see a new type of chemical substance being produced, then we’re dealing with a chemical change. So, let’s have a look at the options.
The formation of HBr from H₂ and Br₂ looks distinctly like a chemical change. We’re producing the new substance hydrogen bromide. And we can see in the equation that we’re consuming H₂ and Br₂. So, the formation of HBr from H₂ and Br₂ is a chemical change, not a physical one.
Melting sulfur is an example of state change. Melting is the process whereby a solid turns into a liquid. And we can see, as sulfur goes through the process, all that changes is the state symbol, not the chemical nature of the substance. So, we found our answer. Melting sulfur is an example of a physical change. But just in case, let’s have a look at the others.
The rusting of iron is one of the most well-known chemical reactions. Historically, rusting of iron has caused the destruction of countless pieces of machinery and weaponry. And today, an astronomical amount of money is spent preventing it from occurring. There is no single product of the rusting of iron. It very much depends on the conditions. But they’re all part of the family of hydrated iron oxides. So, absolutely, we’re looking at a chemical change.
The formation of CO₂ gas from mixing baking soda and vinegar is also clearly an example of a chemical change. Baking soda, otherwise known as sodium hydrogen carbonate, and vinegar, acetic acid, react to form the novel CO₂ gas. The word formation kind of gave this away. It’s a classic example of the reaction of a hydrogen carbonate plus an acid producing a salt, carbon dioxide, and water, yet another example of a chemical change.
The next example, combustion of wood, is a little bit more subtle. On first glance, it might look like combustion is a physical change because we’re seeing a solid turn into gas. But what we’re seeing is most definitely a chemical change. Wood is a very complex mixture of organic compounds which reacts with oxygen to produce the very simple carbon dioxide and water among other products. So, this too is an example of a chemical rather than a physical change. Meaning that the only physical change we’ve identified out of the five is melting sulfur.