Shedding Light on Acids and Bases Episode 2: Acids and Carbonates

The Shedding Light on Acids and Bases series makes it easy for students to learn all the basics (pardon the pun) of acids and bases! Students will come away with a deep understanding of what acids and bases are and they will learn about how much acids and bases affect their lives, given that acids and bases can be found everywhere from our farms to our kitchens and from our power stations to our industrial plants.
In Episode 2, Acids and Carbonates, we describe what carbonates are and explain how they react with acids. If it wasn’t for carbonates, our own stomach acid would kill us, acid rain would have destroyed our forests, and, worst of all, our pancakes wouldn’t be as soft and fluffy as they are.

A 3½-minute excerpt followed by a 1-minute trailer.

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The Transcript (which can be used as a textbook)

Contents
Part A: Introduction
Part B: Acids and Bi-Carb Soda in Cooking
Part C: Acids and Bi-Carb Soda in our Bodies
Part D: Teeth
Part E: Acid Rain

Part A: Introduction

Acids are chemicals that play a hugely important part of our lives. We’ve already looked at some common acids in the first episode of our series. These included hydrochloric acid, which is used in, for example, pickling metals, and acetic acid (or vinegar), used in pickling foods.

 

 

 

 

In this program we’re going to look at how acids chemically react with carbonates. Carbonates? Yes, carbonates. You don’t hear that word every day, but carbonates are everywhere. These are small pieces of calcium carbonate, CaCO3.

Many types of rocks like limestone and marble are made in large part of crystals of calcium carbonate. The crystals come in different forms, which give the rocks different properties. Limestone is a key ingredient in concrete and the pyramids in Egypt are made of limestone blocks, while marble is used to make statues among other things. Calcium carbonate is also a key component of egg shells, and sea shells.

Our pancreas, tucked in behind our stomach, makes a chemical called sodium hydrogen carbonate, NaHCO3, without which our own stomach acid would kill us. More about that later. It’s also called bi-carb soda which is used in cooking.

Carbonates are any substances that have this CO3 group of atoms as part of chemical formula. Other carbonates include

  • sodium carbonate, Na2CO3, which is used in the production of glass, paper, and soap,
  • potassium carbonate, K2CO3, which is also used mostly in the production of glass and soap, and
  • hydrogen carbonate, H2CO3, which is used to make carbonated soft drinks because of the tang it gives to their flavour.

Acids readily chemically react with carbonates.

 

 

 

 

If I pour some hydrochloric acid onto some crushed calcium carbonate that I’ve got in this gas jar, a chemical reaction takes place. The gas that is forming is carbon dioxide. The equation for the reaction is: hydrochloric acid + calcium carbonate  —> carbon dioxide + water + calcium chloride (which is a type of salt).

In fact whenever any acid comes into contact with any carbonate, carbon dioxide, water, and a type of salt are produced.

We can tell that the gas being produced is carbon dioxide because if I place a burning stick into the gas jar, the flame immediately goes out. Wood needs air (or more accurately the oxygen in the air) to burn, but the carbon dioxide rising up into the gas jar pushes all the air (along with all the oxygen) out of the gas jar. So, the flame goes out.

Reactions between acids and carbonates are very common. For example, they can actually cause marble and limestone sculptures to prematurely wear away and they stop our own stomach acid from burning up our intestines. It’s literally life and death. But let’s start with something light and fluffy.

Part B: Acids and Bi-Carb Soda in Cooking

This is vinegar which is acetic acid mixed in water and this is bi-carb soda AKA baking soda AKA sodium bi-carb AKA sodium bicarbonate. Its chemical formula is NaHCO3. On top of all its common names, its scientific name is sodium hydrogen carbonate. In other words, bi-carb soda is a carbonate.

If I pour a little vinegar onto some bi-carb soda the two will chemically react to produce carbon dioxide, water, and sodium acetate. The frothing up that you see is caused by the production of carbon dioxide gas.

Bi-carb soda is used in cooking as what’s called a leavening agent. Leavening agents, like bi-carb soda, produce carbon dioxide which makes, for example, pancakes more fluffy and airy (and therefore better). The carbon dioxide that produced these bubbles was produced when the bi-carb soda reacted with an acid, in this case, vinegar.

To make pancakes, add 1 cup of self-raising flour, a tablespoon or so of caster sugar, a pinch of salt, a ¼ teaspoon of bicarb soda, and 1 egg to a bowl. In a separate container, mix 1 cup of milk and 1 teaspoon of vinegar (there’s the acid). You then combine all the ingredients and whisk them all together until the mixture thickens. This will take 2 or 3 minutes or so.

This Acids and Bases video has suddenly turned into a cooking show, but acids and bases, and science in general, are everywhere!

Once done, leave it to rest for 5 minutes or so. The batter now contains millions of tiny bubbles of carbon dioxide… produced in the reaction between the vinegar and the bi-carb soda.

On top of that, the self-raising flour itself contains some bi-carb soda and an acid in powder form. One common acid used is called disodium pyrophosphate. The two don’t chemically react in the flour because they’re dry, but as soon as they get wet, they do react and produce even more bubbles of carbon dioxide. The carbon dioxide produced is the reason that the flour is called self-raising flour. A lot of science goes into the things we eat!

Na2H2P2O7 + NaHCO3 —> Na3HP2O7 + CO2 + H2O

disodium pyrophosphate + bi-carb soda   —>   sodium hydrogen diphosphate + water + carbon dioxide

When the batter is cooked on a medium-hot non-stick frying pan (a lot of science goes into non-stick frying pans as well), little bubbles of carbon dioxide soon appear. These bubbles are made of the carbon dioxide that is produced in the reactions that we just looked at. (To complicate things even further, any unreacted bicarb soda decomposes in the frying pan and turns into sodium carbonate, water and carbon dioxide again.) When the whole pancake is covered in bubbles, you then flip the pancake over and cook the other side. Aim for about 1 minute per side. As I said earlier, the bubbles of carbon dioxide in the batter leave the finished pancake soft and light.

Serve with your favourite topping. Mine’s cream and maple syrup. Mmmmmm.

So as I eat my snack and resort to using a voiceover, let me say that there’s another chemical reaction involving an acid and bi-carb soda that occurs in our bodies, and if it didn’t we would die. Let’s take a look at it.

Part C: Acids and Bi-Carb Soda in our Bodies

You probably already know that when we eat food and swallow it, it goes into our stomachs.

The stomach plays a really important role in the digestion of our food. It contains hydrochloric acid and other chemicals that begin the process of breaking down the proteins that we eat (in meat for example) into smaller units called amino acids which then enter the blood stream later on through the walls of the small intestine.

If I place some small pieces of meat and potato into two beakers that contain hydrochloric acid and then come back a day later, we can see that the acid has broken down the meat and it’s turned into a sludgy mess. Digestion of meat in the stomach takes only about 4 hours because the stomach doesn’t just contain acid, it also contains chemicals called enzymes. Enzymes speed up the process of digestion. The potato pieces in contrast have not really been affected as much by the acid. Hydrochloric acid doesn’t break down carbohydrates, the main nutrient found in potatoes, other chemicals do.

The hydrochloric acid in our stomachs is reasonably concentrated, but if it can digest meat, why doesn’t it digest the stomach itself? Well it doesn’t because the inside of the stomach continuously produces a layer of mucus, which is a little like the snot in our noses.

The slimy stuff you’re looking at here is a mixture of corn syrup, gelatine and green food dye and it’s similar to the mucus in our stomach although stomach mucus is more kind of off white, not green. The acid doesn’t react with the mucus and so the stomach is protected.

This is an actual stomach, a sheep stomach in fact, that’s very similar to a human stomach. I got it from a butcher. The food enters through a small valve called the lower oesophageal sphincter which then closes up. The stomach muscles churn up the food and mix it with the hydrochloric acid and the other chemicals. This helps break the food down. The “gastric folds” in the stomach allow chemicals and water to be secreted into and absorbed from the stomach more easily and they allow the stomach to expand.

By the way, the massive hole we’re looking through is not a feature of the stomach. It was cut by the butcher. I’m not sure why.

Once the stomach has done its job, the stomach muscles squeeze the partially digested food into the small intestine through another valve, called the pyloric sphincter, where the food that has been partially digested is digested further.

However, the small intestine is not protected by any mucus, so why doesn’t the hydrochloric acid burn away our intestines? Well, it’s thanks to the pancreas.

The pancreas is a small organ that sits just behind the stomach. The pancreas produces sodium hydrogen carbonate (that is, bi-carb soda) and releases it into the small intestine through the pancreatic duct as the food is squeezed into the small intestine by the stomach. The acid and the bi-carb soda chemically react and the acid is neutralized by the bi-carb soda.

It’s basically the same as the reaction between bi-carb soda and vinegar that we saw earlier. The water, the sodium chloride, and the carbon dioxide produced end up passing into our blood. We breathe the carbon dioxide out and the water and sodium chloride are used by the body for other things. The food that has already been partially digested in the stomach continues on its way down the small intestine and is digested further.

If the pancreas loses its ability to produce bi-carb soda, due to pancreatic cancer for example, our stomach acid would painfully eat away our intestines and we would die.

Now stomach acid can sometimes come out of the stomach the wrong way.

Some people suffer from a condition called heartburn, which despite the name has nothing to with the heart. They get a burning sensation about here somewhere that’s caused by the hydrochloric acid in the stomach rising up into the oesophagus.

The oesophagus is the tube that carries food from the mouth down to the stomach. The lower oesophageal sphincter opens to let the food in and then is pulled tight by the stomach muscles around it which stops the acid coming out the wrong way as the stomach churns away, but if there’s even a small leak the stomach acid can burn the oesophagus and cause damage… and pain.

I’ve never had it, luckily enough, but apparently it’s not a good feeling. To relieve heartburn, sufferers take an antacid tablet or an antacid drink.

These antacids have among other things, our good friend sodium bicarbonate in them. Antacid is short for anti-acid, as in, they neutralize acids. The ingredients react with the rising stomach acid and neutralize it, providing much-needed relief. The carbon dioxide is often burped out.

(The stomach acid, the hydrochloric acid, also kills the germs that we swallow, as long as we don’t swallow too many germs, in which case we might get food poisoning.)

So hydrochloric acid plays a hugely important role in our stomachs and the sodium hydrogen carbonate produced by our pancreas plays a huge role in neutralizing the acid as it is squeezed into our small intestine. However, reactions between acids and carbonates are occurring constantly in our mouths, and these are not so good.

Part D: Teeth

The outer layer of a tooth is made of what’s called enamel. It’s the hardest substance in our body and it’s held in place by the rest of the tooth, including the dentin underneath, which is also hard but not as hard, and the root that attaches the tooth to the jaw bone.

The enamel is made of, get this, carbonate-substituted hydroxyapatite crystals!! But you probably already knew that :). These crystals are really hard. The chemical formula of these hydroxyapatite crystals varies a little, but here are some examples:

Ca10(PO4)5(OH)5

Ca10(PO4)5(OH)1(CO3)2

Ca10(PO4)4(OH)2(CO3)3

This is calcium, and we also have groups of atoms called phosphates, hydroxides, and good ol’ carbonates. I told you carbonates are everywhere. It’s a very slow process but acids over time can react with the enamel and weaken it. It’s a little like the reaction between vinegar and bi-carb, but obviously much much slower.

So where do the acids come from? Well, all the food that we eat is slightly acidic. Fruits, vegetables, grains, meat, all food contains weak acids. These acids react with the enamel. Since the acids are weak though and they don’t stay in your mouth for very long, they don’t really do much damage.

Fizzy soft drinks, though, are an exception. These drinks are so acidic that they eat away at the enamel significantly even in the short amount of time that they’re in our mouths. It’s best to avoid them.

The biggest problem though is the food that gets stuck on or between our teeth. There are various types of bacteria, called lactic acid bacteria, living in our mouths that survive by consuming the food that gets stuck on and between our teeth.

These bacteria absorb the food stuck on and between our teeth and use it to grow and reproduce. They produce an acid called lactic acid as a waste product. This lactic acid that gets produced day and night eats away at the enamel and puts holes in it. In fact, the acid produced by these bacteria is responsible for most of the enamel loss that people typically experience… and the tooth decay that follows. Once the enamel has been eaten away (by the acid produced by the bacteria), the tooth is weaker and is more likely to break. The hole, or “cavity”, allows more bacteria to get into your tooth, where they can then rot the tooth away from the inside. This weakens the tooth further and there’s always a lot of pain involved.

 

 

 

 

To maintain dental health and to avoid tooth decay, we have to clean our teeth to remove as much of the food that the bacteria live on as we can. If they get no food, they can’t produce their acidic waste product!

Brushing regularly, morning and night, removes any leftover food stuck on the exposed surfaces of our teeth, but brushing doesn’t get between teeth. So we should also use dental floss. Dental floss should be scraped along the tooth surfaces up and down and side to side. These things, called interdental brushes, are brilliant. You put a bit of toothpaste on them and you can actually brush the surfaces that a normal tooth brush can’t get to. They come in different sizes for the different sized gaps that we have between our teeth. Of course, a regular visit to the dentist is also a good idea, as is avoiding sugary sticky foods that stick to our teeth and don’t come off easily.

Tooth decay is one of those problems in society that in most cases just doesn’t need to occur. The problem is that the decay process is really slow and we don’t realize that a hole is forming in our enamel until there’s a hole in our enamel!!

If we spend a few minutes every day in the morning and in the evening looking after our teeth with brushing, flossing, and interdental brushing, they should last a lifetime.

If you’re young, don’t make the mistake of thinking that your teeth are invincible. They aren’t. They need looking after.

Now another really slow process is the decay of these sculptures and the destruction of these forests that you can see in this old footage from the 1980s. What caused these problems and how were they fixed? Let’s take a look.

Part E: Acid Rain

Rain water and the water in streams and lakes is ever so slightly acidic. This is because certain gases dissolve into the water in clouds and rain which makes the water slightly acidic.

Firstly, a very small amount of carbon dioxide in the atmosphere dissolves in water and then chemically reacts to produce carbonic acid. This makes rain water a very very weak acid. The equation is
carbon dioxide + water à carbonic acid. CO2 + H2O —> H2CO3.

Secondly, volcanos produce gases like sulfur dioxide, nitrogen dioxide, and nitric oxide that also make rain water acidic. Some of the SO2 for example reacts with the oxygen in the air to produce sulfur trioxide, SO3. (SO2 + O2 —> SO3) and then some of the sulfur trioxide chemically reacts with water in the air to produce sulfuric acid, H2SO4. (SO3 + H2O —> H2SO4). Nitric acid is also formed in a similar way from nitrogen dioxide (NO2) and nitric oxide (NO).

The very slight acidity of rainwater is natural and doesn’t cause any problems (except for plants right next to volcanos I suppose!).

However, in the about the middle of the 20th century, the acidity of rainwater increased thanks to the increased burning of fuels like coal in coal-fired power stations and fuel in cars and trucks.

This led to severe damage of many forest ecosystems. You can see the damage done in these two photos taken 20 years apart. Populations of aquatic animals living in rivers and lakes also plummeted. So how did this happen and what’s been done about it? Let look at coal first.

Basically, in a coal-fired power station, coal is burned in a boiler which creates steam under high pressure which then turns the turbines (which are like giant fans) which then turn the generator. The exhaust gases produced by the burning coal itself exit the boiler and go up the chimney.

Burning coal in power stations produces mostly carbon dioxide and water. However, though coal is mostly made of carbon, it also contains small amounts of sulfur, and when the coal burns, the sulfur burns too and produces sulfur dioxide. This sulfur dioxide was leading to an increase in sulfuric acid in rainwater because of the chemical reactions it was undergoing with the air and with the water in the air.

Burning liquid fuel, like petrol in a car engine, also produces mostly carbon dioxide and water, but small amounts of nitrogen dioxide and nitric oxide are also produced because a tiny tiny amount of the nitrogen in the air that gets sucked into the engine reacts with oxygen to produce these gases. These two gases lead to the production of nitric acid in the atmosphere.

The increased amounts of sulfuric and nitric acids in the rain was making the rain more acidic than it was naturally and it was called acid rain. The increased acidity was affecting the growth of leaves and the way that nutrients were being absorbed from the soil, so in parts of the world which had large populations and which relied heavily on fuels, like Europe and North America, the health of forest trees and aquatic life in rivers started suffering.

It didn’t really affect Australia because there aren’t all that many people in Australia.

Marble statues also experienced degradation as you can see from these photos. We’ve seen how acids chemically react with carbonates like calcium carbonate to produce water, CO2 and a salt. Since marble statues are made mostly of calcium carbonate, they were very slowly being eaten away by the acid rain.

Buildings made of limestone, which is also a rock made up mostly of calcium carbonate, also began to degrade relatively quickly. I actually remember being taught about acid rain back in the 1980s when I was at school.

The acid rain wasn’t very concentrated. You could drink it, you could swim in rivers and creeks and you wouldn’t notice it, but that slight increase in concentration of sulfuric and nitric acid in the rain over what was natural was slowly causing enormous damage.

Now in the 1980s and 1990s, scientists and governments came together to tackle the problem of acid rain caused by power stations and vehicles.

To do this, sulfur dioxide emissions from power stations, and nitrogen dioxide and nitric oxide emissions from vehicles had to be cut.

The exhaust gases of coal-fired power stations now have to pass through giant so-called “scrubbers”, which remove the sulfur dioxide before it gets out into the atmosphere.

Basically, the exhaust gas from the boiler where the coal burns, which contains small amounts of sulfur dioxide, is directed into the scrubber. At the same time wet and crushed limestone, that is calcium carbonate, and oxygen are also pumped in. A number of chemical reactions takes place which results in the production of carbon dioxide, which just goes up the chimney, and solid calcium sulfate which is collected at the bottom. The calcium sulfate is used to make plaster sheets that the walls of a typical house are made of. Thanks to these scrubbers, that contain the hero of this video: a carbonate, only tiny amounts of pollutants now come out of the chimneys of coal-fired power stations and forests and waterways in areas formerly affected by acid rain have now largely recovered.

CaCO3(s) + SO2(g) + O2 —> CO2 + CaSO4

The reactions:

  1. CaCO3(s)+ SO2(g) —> CaSO3(s) + CO2(g)
  2. CaSO3(aq)+ 2H2O(l)+ ½O2(g) —> CaSO4 · 2H2O(s)

Overall: CaCO3(s) + SO2(g) + O2 —> CO2 + CaSO4

It’s a good news story.

Also all cars and trucks now have what are called catalytic converters fitted to their exhaust systems. Catalytic converters reduce acid rain by removing nitrogen dioxide and nitric oxide from a car’s exhaust. Let’s take a very quick look at how a car engine and its exhaust system works.

Fuel and air are drawn into the cylinder of the engine, where the fuel is set alight by the spark plug. The expanding gases push on the piston and make the engine turn. The waste gases then leave the cylinder and the engine and eventually come out of the exhaust pipe. However, before they do they pass through the muffler, that reduces the noise levels of the engine, a series of resonators, which also help reduce noise levels, and, if we keep following the pipe back towards the engine, this thing here: the catalytic converter.

Here’s a larger brand-new one. Now as I said, the main waste products of burning fuel are carbon dioxide and water, but tiny tiny amounts of nitrogen dioxide and nitric oxide (NO2 and NO) are also produced. These two gases are collectively called NOx for short. Each car produces very small amounts of NOx, but when you’ve got millions of cars, it all adds up. Catalytic converters convert the small amount of NOx produced in the engine as the fuel burns into nitrogen gas and oxygen gas, the two gases that make up most of the atmosphere anyway. Since catalytic converters became standard, the NO2 and NO emissions from vehicles have been slashed and much less nitric acid is produced in the atmosphere as a result, which means less acid rain.

Once again, a good news story, and it goes to show how technology can often cause problems, but once problems are discovered, we can often work on new technology to overcome those problems.

So there you have it. Acids and the chemical reactions that they take part in, play a huge role in our lives. In this program we’ve looked mainly at the reaction between acids and carbonates. But, so far I haven’t mentioned bases much and this series is called Shedding Light on Acids and Bases after all. Well it turns out that carbonates, bicarb soda for example, are bases. So what makes a base a base? That’s what we’ll be looking at in our next episode. See you then.

CREDITS:

https://commons.wikimedia.org/wiki/File:Lactobacillus_paracasei.jpg by Dr. Horst Neve, Max Rubner-Institut. CC license.

https://commons.wikimedia.org/wiki/File:Cross_sections_of_teeth.svg by Goran tek-en. CC license.

https://en.wikipedia.org/wiki/File:Dental_Caries_Cavity_2.JPG by Suyash.dwivedi. CC license.

https://en.wikipedia.org/wiki/File:Smooth_Surface_Caries_GIF.gif by ADuran. CC License.

https://commons.wikimedia.org/wiki/File:Decayed_wisdom_tooth.jpg by ProjectManhattan.

https://commons.wikimedia.org/wiki/File:Pit-and-Fissure-Caries-GIF.gif by ADuran.

Root canal treatment for dental decay | Sukhibhava | 17th June 2019 | ETV Andhra Pradesh by ETV Andhra Pradesh: https://www.youtube.com/watch?v=MpJb1lTmvp0. CC License.

https://en.wikipedia.org/wiki/File:Flue_gas_desulfurization_unit_EN.svg by Sponk. CC license. (Our scrubber animation was based on this diagram)

https://commons.wikimedia.org/wiki/File:Tractus_intestinalis_base.svg by Orem. CC License.

Make your own Mucus experiment” by Kids Fun Science: https://www.youtube.com/watch?v=gmJv2Lhd0jM. CC License.