Reader Question: How Do Crocodiles Digest Entire Animals?

Reader Ken McMurdy of Mahwah, NJ was interested in the fact that our stomachs contain hydrochloric acid. But, he asks, what about the stomachs of animals, like alligators and crocodiles, that can digest the entire bodies of their prey?

Good question! Gators and crocs, as it turns out, have pretty amazing digestive systems. And that's a good thing, because they eat huge meals, weighing about one-fourth of their body mass! If you weigh 80 pounds, that's like eating 20 pounds of food, all at one sitting!

First of all, their stomachs are divided into two parts. These critters actually swallow small stones, which end in in the first part of the stomach. These little rocks are called gastroliths (Greek for "stomach stones"), and they help to break the food up into smaller pieces. The second part of the stomach contains hydrochloric acid, but it's about ten times more concentrated than the acid in human tummies. In fact, it's the most acidic gastric (stomach) juice of any animal! This acid can dissolve teeth, bones, and feathers (but not hair!). Also, when alligators and crocodiles are digesting food, extra blood flows to their stomach to help move the meal along.

But even with the stomach stones, supercharged gastric acid, and extra blood, it takes 10 to 20 days for one of these animals to completely digest a meal! In humans, this process takes 12 to 24 hours. So the next time you eat too much, and don't feel too good because of it, be happy you're not a gator or croc!

Strong vs. Weak

By now you've experimented with a number of different acids, and while they have some pretty interesting reactions, they haven't been at all scary to handle. So why do acids have such a bad reputation as dangerous chemicals?

The simple answer is that there are two kinds of acids - strong acids and weak acids. You probably won't be surprised to learn that everyday acids, like vinegar, lemon juice, or cola, are weak acids. The really dangerous acids are strong, and you won't be doing any experiments with them! OK, strong vs. weak acids - isn't that just like strong vs. weak coffee? What if you just add more water to a strong acid - doesn't that just make it weak? No, the difference isn't that simple. Some acids are weak, and some are strong, and it doesn't matter how concentrated the acid solution is. You can't turn a weak acid into a strong one, or the other way around.

The important difference between strong and weak acids has to do with those hydrogen atoms that you noticed are present in all acids. The picture below shows this difference. When a molecule that makes a strong acid is dissolved in water, all of the hydrogens (gray balls) separate from the other part of the molecule (green balls), and it's those hydrogens that make the solution acidic. But when a molecule that makes a weak acid is dissolved, very few of the hydrogens (gray balls) separate from the rest of the molecule (purple balls). In a weak acid, most of the molecules stay together.
So the "dangerous" part of an acid is the unattached hydrogen atoms - the more of them, the stronger the acid. Even though strong acids have to be handled very carefully, they are still very important. In fact, you carry a strong acid around with you all the time! Your stomach contains hydrochloric acid, which helps to digest your food.

Acid Reactions

One thing that acids do really well is to react with other chemicals. If you did the dirty-penny-in-cola experiment, you've already seen one such reaction, when the phosphoric acid in the cola removed the copper oxide coating from the penny. But the reaction itsefl wasn't very interesting while it was happening. Here are some acid reactions that are more exciting. They're kind of messy, but safe, as long as you follow the instructions carefully.

Acids really like to react with chemicals known as bases, which you can think of as "anti-acids" - they neutralize each other. You can collect some acids and some bases that you might have around your home, and observe their reactions.

What you'll need:
Small plastic cups
Eye dropper
Teaspoon
Mortar and pestle
Water
Marker
Newspaper or plastic sheeting
Household acids, such as vinegar, lemon juice, cola, aspirin, Vitamin C tablets
Baking soda (sodium hydrogen carbonate, which is a base)

First spread the newspaper or plastic sheeting around the area where you will be working because things may get messy! If you are using any acids in solid form, such as aspirin, grind them with the mortar and pestle and dissolve in water. Place a small amount of each acid into a plastic cup and label it. Add about half a teaspoon of baking soda to each acid, and stand back! What happens?

You should see a very fast and exciting reaction, with lots of foamy bubbles forming. These bubbles are filled with carbon dioxide, which is released by the reaction of the acid with baking soda. You might want to test other household acids to see if they have the same reaction.

The Big H

Which element did you find to be common in all acids? Hydrogen is the answer!

Yes, all acids contain hydrogen. There are many compounds that have hydrogen that aren't acids (like water), but all acids must have hydrogen in them.

Acids Around You

You use a lot of chemicals that are actually acids. Here is a list of some of them, along with their chemical names:

• Vinegar is acetic acid, usually about 4-8% in water. Its formula is C₂H₄O₂, with two carbons (C), four hydrogens (H), and two oxygens (O).
• Citrus fruits like lemons, limes, and oranges, contain a lot of Vitamin C (ascorbic acid, C₆H₈O₆) and citric acid (C₆H₈O₇).
• There is formic acid (CH₂O₂) in bee venom, but this isn't what makes bee stings painful.
• Salicylic acid (C₇H₆O₃) is found in many skin care products. A closely related chemical, acetylsalicylic acid (C₉H₈O₄) is more commonly known as aspirin!
• Lactic acid (C₇H₆O₃), or milk acid, is particularly concentrated in sour dairy products like yogurt and sour cream.
• Grapes and wine contain tartaric acid (C₄H₆O₆).
• Tannic acid (C₇₆H₅₂O₄₆) is found in wood, especially oak and walnut. From the big numbers of carbons, hydrogens, and oxygens, you can tell that this is a huge molecule!
• Uric acid (C₅H₄N₄O₃) also contains 4 nitrogens (N). It is found in urine.
• Your stomach holds some hydrochloric acid (HCl), which helps you digest the food you eat.
• Boric acid (H₃BO₃) has hydrogen, boron (B), and oxygen. It is used to kill bugs, particularly cockroaches.

What do all of these acids have in common? Look over the list of formulas, and see if you can tell which type of atom (C, H, O, N, Cl, or B) can be found in every acid listed. In fact, this chemical element is found in every kind of acid!