 Magnesium metal reacts with hydrochloric acid to produce hydrogen gas and dissolves magnesium. We can test the presence of hydrogen gas by placing a lid splint at the mouth of a test tube. If we hear this distinctive squeaky pop, it means that hydrogen has indeed been produced. The unbalanced equation for this reaction looks like this. In another lesson on this channel, we learned about balancing chemical equations. Can you try balancing this equation yourself? Grab a pencil and a piece of paper, pause the lesson, and resume when you are ready. Because atoms are never created or destroyed during chemical change, only rearranged, the number of atoms for each element on the reacting side must be equal to the number of atoms for each element on the product side. So the balanced equation will look like this. As there are two chlorine atoms on the product side, there must also be two chlorine atoms on the reactant side. You can also see that the number of hydrogen atoms is also balanced because there are two hydrogens in the hydrochloric acid on the reactant side, and there are two hydrogens on the product side. Now, we are not quite done with this chemical equation. For this equation to be complete, we must also include the state symbol for each reactant and product. There are four possible states, each with their own unique state symbol. Solid, represented by lowercase s. Liquid, represented by lowercase l. Gas, represented by lowercase g. And aqueous, or dissolved in water, represented by lowercase aq. This is written after each reactant or product in brackets and in subscript, meaning a bit lower than the actual text. As magnesium metal is solid, we represent this state with lowercase s in brackets, in subscript right after the symbol. Hydrogen gas has a state symbol g, because it is a gas. Note that hydrochloric acid is in fact hydrogen chloride dissolved in water. So it has a state symbol aq, much like dissolved magnesium chloride. Remember that squeaky pup described at the start of the lesson. This is in fact the reaction of hydrogen gas with oxygen in air to produce liquid water that you can observe at the mouth of a reaction vessel. Since we have learned about state symbols, why not try the following. Write the chemical equation for the above reaction, balance it, and add state symbols for each reactant and product. Pause the lesson and resume when you are ready. Hydrogen gas reacts with oxygen to produce water. As hydrogen and oxygen are a diatomic gas, we must also represent that in the equation. Balancing the equation gives us, adding the state symbols completes the equation. In summary, state symbols represent each of the reactant and each product in reaction, and must be included in every chemical equation.