Hydrochloric acid is a strong acid, meaning it dissociates almost completely in water to yield hydronium ions (H3O+) and chloride ions (Cl-). Since it's a strong acid, it has a significant effect on the pH of a solution. Consequently, hydrochloric acid is corrosive and can cause severe burns if it comes in contact with eyes or skin. Nonetheless, hydrochloric acid is also a very useful industrial chemical found in a number of household items.
Some tile cleaners, especially the heavy-duty brands available at some hardware or home improvement stores, contain hydrochloric acid. This ingredient acts to help dissolve scale and other debris on the surface of the tile, making it easier to remove stubborn stains. Unfortunately, it can also make these tile cleaners corrosive and create irritating or unpleasant fumes. Sodium bicarbonate will neutralise hydrochloric acid, so it can help in cleaning up spills. Often, however, there are other ingredients that cannot be so simply neutralised, so follow the manufacturer's instructions if dealing with a spill.
Toilet Bowl Cleaners
Some toilet bowl cleaners also contain hydrochloric acid, again for much the same reason as the tile cleaner -- it helps to dissolve some really tough types of stains like mineral deposits, rust and scale. The concentration of hydrochloric acid -- and hence the pH of the solution -- varies depending on the product, so some of these are stronger and more corrosive than others. Just as with the tile cleaners, however, it's important to follow the manufacturer's instructions when you're working with these products to ensure your own safety.
Hydrochloric acid is a common pool chemical, often sold under the name muriatic acid. If the pH of your pool is too high, adding small amounts of hydrochloric acid to your pool increases the hydrogen ion concentration, thereby reducing the pH. It's important to observe good safety precautions when working with muriatic acid, because if you're careless or sloppy while diluting it, you can burn yourself severely.
Your pets, your children and your significant other also have hydrochloric acid in their bodies. Parietal cells in the gastric glands of the mammalian stomach secrete hydrogen ions and chloride ions separately. This highly acidic environment aids in digestion and helps to activate an enzyme called pepsin, which cleaves bonds inside proteins in the food you eat. No need to worry, however. The mucus lining of your stomach protects you from being burnt by your own digestive juice. If you have a stomach ulcer, however, the mucus lining of your stomach has been eroded, so the hydrochloric acid damages your stomach lining -- much in the same way that muriatic acid is harmful to your skin.