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 1, Acids in Industry, we look at what acids are, how they’re made, and how they’re used in steel making and agriculture. We also take a quick trip back to the 1770s to look at how acids were used by Captain James Cook to save his crew from the disease that used to kill more sailors than shipwrecks and sea battles!
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The Transcript (which can be used as a textbook)
Part A: Introduction
Part B: What are Acids?
Part C: Pickling (preserving foods in acids and treating metals with acids)
Part D: Sulfuric Acid: the Acid Most in Demand
Part A: Introduction
In this series, we’re going to look at acids and bases. There are literally millions of different chemicals and substances on Earth so scientists try to classify them into groups based on what atoms they’re made of or on how they chemically react with other substances. Two really important classes of chemicals are acids and bases. Acids have certain things in common with each other as do bases.
Acids include hydrochloric acid, which is in our stomachs, sulphuric acid, which is used to make certain fertilizers, acetic acid, which is the acid that gives vinegar its sour taste, and carbonic acid, the acid in fizzy drinks that gives them their tang. Most foods in fact contain weak acids such as citric acid in citrus fruit and lactic acid in yoghurt.
Bases include sodium hydroxide, which is used in drain cleaners and calcium hydroxide, which is used to reduce the acidity of acidic soils.
There are huge industrial plants all around the world that every year make hundreds of millions of tonnes of various acids and bases because of the fact that acids and bases are so widely used, not just on their own, but also to make other important chemicals. The examples shown here are examples only. There are many more acids and bases and they all have lots of uses.
Now not only do all acids have certain things in common with each other, as do bases, but acids chemically react with bases in a specific way that relates them to each other.
In this series we’re going to examine how acids and bases chemically react with each other and how they chemically react with a range of other substances. By the end of the series you’ll know how widespread the use of acids and bases are in the production of everything from steel to the foods we eat. So let’s begin.
Part B: What are Acids?
This bottle contains concentrated sulfuric acid and the glass contains sugar. Sulfuric acid is the most widely used industrial acid on the planet. Hundreds of million tonnes are produced every year. About 60% of the output is used to make fertilizers and it’s also used in the production of metals, explosives, synthetic fibres, plastics, and much more. The acid in the so-called lead-acid batteries in our cars is sulfuric acid. Other acids, like hydrochloric acid, are also widely used in industry.
Concentrated sulfuric acid reacts with sugar and slowly turns it black. After about a minute the reaction starts producing hot jets of steam and a column of black carbon rises out of the glass. The carbon rises because the chemical reaction produces water in the form of steam. The steam creates bubbles within the carbon that is left behind so the whole mixture rises up. An acrid choking gas is also given off that burns your throat if you stand too close. Clearly you don’t want to be spilling concentrated sulfuric acid onto your skin and it should be treated with extreme caution. Acids chemically react with a lot of different substances but most don’t do this much damage as they’re usually watered down significantly when they’re used in manufacturing.
This is also sulfuric acid, the same sulfuric acid that we’ve just seen, but it’s been watered down, so much so, that even if I touch it, which I shouldn’t really do, I can’t even tell that it’s an acid. However, if I drank it or any got into my eyes, it would still do a lot of damage. If you ever spill acid on yourself, you should try to wash it off as soon as you can.
If we look at the chemical formulas of some common acids we can see something that they all have in common: they all have hydrogen atoms. When dissolved in water, some or all of the hydrogen atoms of an acid can separate from the rest of the molecule and chemically react with things. Now not all substances with hydrogen atoms are acids. Methane and ethanol, just to name two examples, are not acids, because the hydrogen atoms don’t separate from the rest of the molecule when these two molecules are in water. At a simple level then, an acid is a chemical that has hydrogen atoms that separate fairly easily when the chemical is dissolved in water and that makes acids fairly reactive.
For example, acids chemically react quite readily with a lot of metals.
If I pour some hydrochloric acid (which as I said is HCl dissolved in water) into a test tube that has magnesium metal in it, the hydrochloric acid reacts with the magnesium and produces hydrogen gas and magnesium chloride. The chemical equation for the chemical reaction is
hydrochloric acid + magnesium à hydrogen + magnesium chloride.
HCl + Mg —> H2 + MgCl2
You can’t see the magnesium chloride because it’s dissolved in the water but the bubbles of hydrogen gas are pretty obvious.
We can tell that it’s hydrogen gas because the gas is flammable. If I collect the gas in an upside down test tube and hold a match next to it, it catches fire and makes a popping noise. The actual flame lasts for only a fraction of a second, but we can see it here in this one frame of the video.
Many metals in fact chemically react with hydrochloric acid (and other acids): magnesium, zinc, iron, and aluminium, for example, though some metals are more reactive than others. However, looking at the chemical equations, there’s an obvious similarity in the chemical reactions going on.
metal + acid —> hydrogen + a salt
It turns out that whenever any metal reacts with any acid, hydrogen gas and a type of salt are produced. We normally think of the word salt as meaning table salt, the salt we add to food, sodium chloride, NaCl, but there are actually many different types of salts. In these reactions the salts produced are all different types of what are called chloride salts. We can’t see any of them because they dissolve into the watered down acid.
When iron reacts with sulfuric acid, hydrogen gas and iron sulfate are produced. Iron sulphate is a type of salt as well. You can’t see it being produced in the reaction, because, once again it remains dissolved in the watered down sulfuric acid, but the hydrogen gas being produced is obvious. Now not all metals react with acids, but many do.
Just as some metals are more reactive than other metals, some acids are more reactive than other acids.
The acid on the left is hydrochloric acid, while the acid on the right is acetic acid which is the acid in vinegar. Hydrochloric acid is a strong acid while the acetic acid in vinegar is a fairly weak acid. You can see that its reaction with magnesium is relatively slow. So, though all acids have things in common, some are strong and some are weak.
We’ll get into a little more detail in a later episode.
Apples contain a weak type of acid called malic acid, while strawberries, oranges and lemons contain citric acid which is a little stronger. However, the citric acid is far more concentrated in lemons than it is strawberries and oranges, so lemon juice has a more sour taste and it’s more reactive.
You can actually use lemon juice to clean a rusted BBQ plate because the citric acid reacts with the rust and dissolves it. But this isn’t just a BBQ hack, acid on metal reactions are really important in industry.
Part C: Pickling
The reaction between iron and acid is used to clean steel before it’s used to manufacture steel beams, steel sheets, and a million other steel products. https://www.sms-group.com/press-media/media/video-gallery
Steel is about 99% iron and, as it’s being produced, some of it can rust immediately while it’s still hot and form what’s called mill scale on its surface. Mill scale is composed in large part of iron(III) oxide, Fe2O3, so, to get rid of it, the steel sheet is passed through a pickling line where it is bathed in hydrochloric acid. This process is called pickling.
The acid does two things. Firstly, it reacts with the Fe2O3 to form what’s called iron(III) chloride, which is soluble and just washes away easily. HCl + Fe2O3 —> FeCl3 + H2O
Secondly, a very thin layer of the surface of the steel (that is, the iron) that the mill scale is stuck on reacts with the acid and turns into iron(II) chloride. HCl + Fe —> FeCl2 + H2. The iron(II) chloride is then washed away, along with the mill scale that had been stuck onto the steel, leaving clean steel behind. Obviously the manufacturers try to use the right amount of acid so that they lose very little actual iron in the process. The steel comes out perfectly clean and it is then ready to be made into products.
I can demonstrate the process of pickling with this piece of corroded magnesium.
The magnesium metal has chemically reacted with the oxygen in the air to produce the grey magnesium oxide layer that we can see. Mg + O2 —> MgO
If I now dip it in acid, the acid reacts with the magnesium oxide to form magnesium chloride, which is water soluble, and it just falls off the magnesium strip leaving a clean piece of magnesium behind. Now the surface of the magnesium, the actual metal, also reacts with the acid to produce magnesium chloride and sure this causes any magnesium oxide stuck to it to fall off, but, as I said, in industrial steel pickling, they try to limit the exposure of the steel to the acid so that they lose as little of the valuable steel as possible.
MgO + HCl —> MgCl2 + H2O
I can do the same with this rusted iron barbecue plate. If I pour some vinegar onto it, vinegar is made of acetic acid, the surface of the iron and the rust itself reacts with the acid to make iron chloride which can easily be wiped off. As we saw earlier, rubbing the iron plate with a lemon does a similar thing thanks to the citric acid that lemons have.
The word pickling is also used to describe the process of preserving certain food products in acids, mostly in vinegar (acetic acid) or in lactic acid. The bacteria that cause foods to rot away are all killed by the acids so the food doesn’t spoil. These are pickled onions and these are pickled cucumbers.
This is sauerkraut. Sauerkraut is pickled cabbage.
Food pickling is not a huge industry these days because we have refrigerated trucks that can quickly carry produce from the farms where it’s grown to our grocers and supermarkets, where most of us buy our food.
The process of rotting or spoiling doesn’t just occur by itself. It’s caused by bacteria and other microscopic organisms. Refrigeration doesn’t kill them but it does slow down their reproduction so the food lasts longer when it’s cold, and most of the fruit and veg that we buy spends very little time in transit between the farms and our kitchens anyway.
However, back in the days of long ship voyages by English and European sailors, in the period between the 1500s and the 1800s, way before refrigeration was available, fresh fruit and vegetables were not eaten much because they spoil fairly quickly. Lacking fresh produce, many sailors would get a disease called scurvy.
These are some images drawn by doctors back in those days of people suffering scurvy. Scurvy causes, among other issues, blood vessels and bones to weaken and skin and other tissues to become less elastic resulting in internal bleeding, susceptibility to bruising, severe skin infections, bone breakages, bleeding gums, tooth loss, and ultimately to death if left untreated.
Scurvy results from a lack of Vitamin C, which is found in fresh fruit and vegetables, for example, citrus fruits and cabbage, which is what sauerkraut is made of. On long sea voyages, the sailors ate mainly dry biscuits and heavily salted and dried out meat, which kept for months and months, but unfortunately, dry biscuits and meat don’t contain much Vitamin C.
Ships often carried far more sailors than were necessary because the government knew that many of the sailors would die before the ship reached its destination. Imagine signing up for that job. More sailors died of scurvy back in those days than any other cause, including shipwrecks and battles.
It had long been suspected that the lack of fresh fruit and vegetables caused scurvy on these long sea journeys but no-one was sure. In the 1760s and 1770s, the famous Englishman Captain James Cook made three long scientific voyages of discovery around the Pacific Ocean.
The ship you’re seeing is a replica of the Endeavour, the ship that he captained on the first of these voyages. Cook took tonnes of sauerkraut (pickled cabbage) with him. Sauerkraut contains lots of Vitamin C, and it lasts for years because the acid that it’s in kills all the bacteria that would otherwise cause it to rot.
Cook insisted that the sailors all eat it and not a single sailor got scurvy on any of his voyages. What a legend!
*Eats sauerkraut* It’s not for everyone. I think I’ll stick with oranges.
It was only in the 20th century that it was discovered that Vitamin C is essential in the production of a type of protein called collagen, which keeps skin, blood vessels, bones, and other parts of our body healthy. It turns out that Vitamin C is also an acid, a weak acid called ascorbic acid.
Various weak acids are actually used extensively in the food industry. Citric acid, for example, that is found in oranges and lemons, so called citrus fruits, is used to add flavour and aroma to certain foods and helps to preserve them.
Part D: Sulfuric Acid: the Acid Most in Demand
Sulfuric acid is the most widely used acid in the world. Hundreds of millions of tonnes of it are produced every year, mostly so that it can be used in the production of fertilizers. So, how are fertilizers made and why is it necessary to use them? Humans (and animals) need to eat food to obtain energy and to obtain the nutrients that we need to grow and maintain our bodies. Plants are different though. They don’t eat, but they still need certain raw materials which they use as building blocks.
Plants absorb carbon dioxide from the air through their leaves and they absorb water from the soil through their roots. In a process called photosynthesis, they use the light energy that comes from the sun to produce glucose and oxygen.
Glucose is a type of sugar that, when dry, looks exactly the same as table sugar which is technically called sucrose. The glucose is used as fuel to power all the chemical reactions that go on in the plant that allow the plant to live and to grow. We need to eat our fuel that is our food for the energy that it provides, but plants make their own fuel: glucose. The glucose is also used to make… cellulose (which makes up the cell walls of the cells), sucrose and other sugars for storage, starch (which is again an efficient way of storing the glucose for later use), and various fats and oils. Now all these substances are made up of carbon, hydrogen, and oxygen atoms, which together make up about 98% of the mass of plants.
However, to make other chemicals, plants also need small amounts of other atoms, mostly nitrogen, phosphorus, and potassium atoms (but also a few others as well).
DNA, for example, is made up partly of nitrogen and phosphorus atoms which on the diagram are drawn purple and yellow. They get these atoms from various chemicals in the soil with technical names like ammonium nitrate, NH4NO3, ammonium sulphate (NH4)2SO4, and monocalcium phosphate Ca(H2PO4)2.
These and other related plant nutrients exist naturally in soils, but, unfortunately, most soils don’t contain enough of them to allow for continuous production year after year. They get used up. So, farmers have to add fertilizers to replenish these important nutrients.
This is a spring-onion farm and the plants are growing healthily thanks to the use of fertilizers. Scientists have actually been studying the use of fertilizers in a wide range of agricultural settings for hundreds of years. This small wheat field has been set up specifically for research into wheat growth.
Here we can see that this wheat is not growing very well because no fertilizer has been added to the soil, but here the wheat has grown taller, and there is much more grain to be harvested. Fertilizers help plants to grow more quickly and they allow farmers to grow more plants in a given amount of space.
Sulphate of Ammonia, also called ammonium sulfate, for example, (NH4)2SO4, from which plants get can get nitrogen and sulfur atoms is a very common fertilizer. About 10 million tonnes of this stuff is produced every year.
It’s produced in industrial plants by passing ammonia gas into sulfuric acid. The chemical reaction produces ammonium sulfate. 2 NH3 + H2SO4 —> (NH4)2SO4.
Now the ammonia is produced by reacting hydrogen gas with nitrogen gas in pressurized steel vessels at high temperatures.
Sulfuric acid is made in a series of steps using sulfur, oxygen, and water.
Sulfur, S, is a yellow solid that occurs naturally in the earth’s crust. It’s typically extracted from the crude oil that is brought up in huge quantities by oil rigs from underground or undersea oil deposits. (Crude oil is a dark liquid and it’s the raw ingredient from which we get most of our fuels and from which we produce most of our plastics and the fabrics that we use to make clothes.)
Sulfuric acid is made by burning the sulfur in huge industrial plants to produce sulfur dioxide, which is then purified. More oxygen is then added at high pressure and the two chemically react to produce sulfur trioxide.
S + O2 —> SO2
2 SO2 + O2 —> 2 SO3
H2SO4 + SO3 —> H2S2O7
H2S2O7 + H2O —> 2 H2SO4
The next step involves passing the SO3 through sulfuric acid that has already been produced to produce what’s called oleum. Now this may seem weird because sulfuric acid is what they’re trying to make (!) but bear with me. When water is then added to the oleum, the two react to produce twice as much sulfuric as the amount that was used up in Step 3. Now the sulfur trioxide (SO3) can be made to react with water to produce H2SO4 directly and this is how they used to make it, but this process is really slow. Scientists discovered about a hundred years ago that they can produce the acid at a faster rate if they use some sulfuric acid that has already been made. For every one molecule of H2SO4 you use in the second last step, you end up producing 2 molecules of H2SO4 in the last step. It’s amazing how people are always inventing better and more efficient ways of manufacturing things.
Sulfuric acid plays a key role in the production of lots of fertilizers, ammonium sulfate, being just one example, while other acids are used to produce other fertilizers. As I said, fertilizers provide plants with the essential nutrients that they need for healthy growth.
Of course, fertilizers are not used just in the production of the plants that we eat. Huge quantities are also used on farms that raise livestock. The grass grows more quickly if the soil is fertilized which means more animals can be raised in a given amount of land.
It’s been estimated that about 50% of all the nitrogen atoms in our bodies originated in the artificial fertilizers that are produced with the help of acids.
So, in summary, acids are hugely important. Most of the food that we eat has been grown on farms with the help of fertilizers that are produced with the help of acids and every piece of steel you’ve ever seen, ever, has been treated with acids. And we’ve still only barely scratched the surface of how widespread the use of acids is.
There’s a huge world out there of people who produce things that we don’t really think about much, especially when we’re young. If you’re a student watching this, then soon, you’ll become part of that world.
In our next episode, we’re going to look at more examples of how acids affect our lives and we’re going to introduce what are called bases. See you then.
Sulphuric acid + sugar by Lejf Diecks. Creative Commons Licence: https://www.youtube.com/watch?v=njEXWKgF7lw
Wheat farming and fertilizers footage © Agrium Inc. Used with permission.
HM Bark Endeavour refit Whitby 1997 by Jeff Freestone. CC Licence
https://commons.wikimedia.org/wiki/File:Endeavour_entering_Fremantle.jpg by John M Wheatley (on worksheet).