Mixing a base with an acid results in a chemical reaction called neutralization. The result is a perfectly balanced solution of salt and water with a pH of 7 if the acid and base are balanced properly. Depending on the bases and acids used, it can be a dangerous experiment.
Mixing an acid and a base results in neutralization, but the results are potentially dangerous. No matter which acid or base is used, the resulting solution is water and varying types of salt. The process of neutralization often involves the substances heating up when they come together. If the solution heats up too much or too fast, a violent explosion or the creation of harmful or flammable gases is a possibility. This occurs when the chemicals are mixed too quickly, the acid and base are too strong or if there is no available salt to be made in the solution.
A mild example of this is when baking soda (a base) is mixed with vinegar (an acid). The solution bubbles out of control when the substances meet. Since acids and bases are often caustic and harmful to body tissues, the experiment is dangerous for people who are using unfamiliar chemicals.