Shedding Light on Acids and Bases Episode 3: Neutralization

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 kitchens to our power stations and from our industrial plants to our farms.
In Episode 3, Neutralization, we explain what bases are and then take a look at what happens when acids and bases chemically react. We also take a look at acid-base indicators, which are chemicals that change colour depending on whether they are in an acid or a base.

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The Transcript (which can be used as a textbook)

Contents
Part A: Introduction
Part B: What’s a Base?
Part C: Indicators

Part A: Introduction

Acids and Bases. That’s what we’re looking at in this series. We’ve already looked extensively at acids so far in this series, the hydrochloric acid in our stomachs for example. In this episode, we’re going to continue to explore the world of acids but also look at bases. By the end of the program you’ll know what a base is and how bases and acids are related.

We’ve actually already looked at bases in our last episode. Carbonates, like bicarb soda, are in fact examples of bases, although I didn’t say that until the very end.

Now in Episode 1, I told you that an acid is a chemical that has hydrogen atoms that separate from the rest of the molecule when they’re dissolved in water. This gives them the ability to react with metals, for example. So what’s a base? Let’s take a look.

Part B: What’s a Base?

This is solid sodium hydroxide and this is sodium hydroxide dissolved in water. Sodium hydroxide is a base. It’s one of the most important bases used in industry.

Millions of tonnes are produced every year and it’s used in, for example, the production of soap, paper, and textiles.

It doesn’t chemically react with metals, but it does chemically react with proteins and fats so it should be treated with caution.

It can cause severe chemical burns on your skin, so you have to be very careful.

The sodium hydroxide in this beaker is heavily watered down, so even if I touch it, which I shouldn’t really do, it doesn’t really do much damage, but if you do get any on yourself, you should try to wash it off as soon as you can. It kind of feels soapy because it chemically reacts with the oils on my skin and turns them into chemicals that are a little like the chemicals in soap. As watered down as this is, if it gets in your eye it can do major damage, so again be careful. If you’ve ever felt the sting of soap in your eyes, getting this in your eyes is much much worse.

Bases are hugely important in lots of industries. For example,

Sodium hydroxide, NaOH, is used, as I said, in the production of soap, paper, and textiles;

Lithium hydroxide, LiOH, is used to produce lithium ion batteries in phones, computers and electric vehicles;

Potassium hydroxide, KOH, is used to produce alkaline batteries and fertilizers;

Calcium hydroxide, Ca(OH)2, is used to change the acidity of soils on farms, and in the production of many processed foods; and

sodium hydrogen carbonate (bi-carb soda), NaHCO3, which we looked at extensively in our last episode, is used in the food industry a lot and our bodies produce it to neutralize the stomach acid that enters the small intestine.

At a very basic level, pardon the pun, maybe I should say, at a very simple level, a base is a chemical that reacts with an acid to produce water and a salt. This reaction is called a neutralization reaction, because the acid and the base neutralize each other.

Let me demonstrate.

As we’ve seen, hydrochloric acid reacts with magnesium and that’s what’s happening in these two test tubes. Now if I pour water into the test tube on the left, I water down the acid even more than it already is (that is I’m diluting the acid even more than it already is) and so the reaction slows down a little, but it continues because there’s still acid present.

Now if I add some sodium hydroxide (a base) into the test tube on the right, the hydrochloric acid immediately reacts with it and water and sodium chloride are produced. The Mg is not producing bubbles anymore because there is literally no acid in the test tube anymore. The acid has chemically reacted with the base so now I just have a piece of magnesium lying in a test tube that has water and salt in it. So in general, an acid + a base à water + (a type of) salt, and this is called a neutralization reaction.

The mixture of salt and water is neither acidic nor basic. It’s neutral.

Now when a neutralization reaction occurs, it’s not always obvious… but heat is always generated. The sulfuric acid and the sodium hydroxide here are both about 15°C but when I mix them, a chemical reaction takes place which produces water and sodium sulfate and the temperature quickly rises to about 20°C. This temperature change tells us that a reaction has occurred, but as I said it’s not obvious because the sodium sulfate remains dissolved in the water. The equation for this reaction is

sulfuric acid + sodium hydroxide —> water + sodium sulfate.

Once again the general equation holds, but the salt produced this time is sodium sulfate.

As I’ve mentioned, bi-carb soda (that is, sodium hydrogen carbonate) is also a base. If I add some hydrochloric acid to the bicarb soda they both neutralize each other.

Hydrochloric acid + sodium hydrogen carbonate —> water + sodium chloride (a salt) + (this time we also get) carbon dioxide, most of which just floats away.

Now in these neutralization reactions, what’s left in the glassware may still be acidic or basic because the amount of acid and base I added and/or how concentrated they were may not have been the same. Neutralization still occurred, but some acid or base may still be left over.

In our last episode, we saw that the stomach acid (carrying the partially digested food) that gets squeezed into the small intestine chemically reacts with the bi-carb soda that is produced by our pancreas (that sits just behind our stomach), and water, sodium chloride, and carbon dioxide are produced. This is an example of a neutralization reaction. There are cells in our small intestine that can measure the level of acidity and they send signals so that the process is carefully controlled. You don’t want too much bi-carb and you don’t want too little. Our bodies are amazing.

Now on a more technical level, bases either contain these hydroxides OR they chemically react with water to produce these hydroxides (in the water). A hydroxide is made of an oxygen atom and a hydrogen atom but it has an extra electron which is negatively charged and which gives it an overall charge of 1-, so it’s called a hydroxide ion or an OH ion. An ion is an atom or a group of atoms that have lost or gained electrons. Ions are very common in nature. The Na and the Li atoms here are actually Na+ and Li+ ions because they’ve lost an electron each to the hydroxides, so overall the positives balance out the negatives. Negative ions never exist on their own, they only exist when positive ions are around, and vice versa. We’ll look at ions in a little more detail in our next episode.

So if we take sodium hydroxide as an example of a base, when dissolved in water, the OH’s separate from the Na+’s. The hydroxide ions floating around in the water are quite reactive, and can react readily with acids or other chemicals. Sodium hydrogen carbonate (bi-carb soda) is also a base but it doesn’t contain hydroxides. When it’s mixed in water there are no hydroxide ions initially but a very small amount of the bi-carb reacts with the water to produce a very small amount sodium hydroxide… and hydrogen carbonate. The presence of the dissolved NaOH makes the water basic, but it’s a weak base because only a tiny amount of NaOH is produced. Strong bases produce heaps of hydroxides, weak bases produce relatively few hydroxides.

NaHCO3 + H2O —> NaOH + H2CO3

Other common examples of bases include metal oxides like sodium oxide and lithium oxide. When sodium oxide is mixed with water for example, it reacts with the water to produce dissolved sodium hydroxide, which once again, makes the solution basic.

Na2O + H2O —> NaOH

So, let me repeat, bases are either made of hydroxide ions OR they chemically react with water to produce hydroxides ions.

So, hydrochloric acid and sulfuric acid are two examples of acids (the word acid is in their names of course), sodium hydroxide and bi-carb soda are two examples of bases, but pure water is a neutral substance, being neither acidic nor basic, as is any salt solution (that is water with any type of salt dissolved in it). When acids react with bases, water and salt are always produced but in the case of carbonates, carbon dioxide is also produced.

So, how do you tell the difference between an acid and a base? Well, the easiest and the safest way is to use an indicator.

Part C: Indicators

The word acid comes from the Latin word acidum (ahCHIdum) which means sour. So acids are sour. Vinegar, which is sour, is made of about 5% acetic acid and 95% water. The word acetic also derives from the Latin word acidum—sour.

The sentence “the acetic acid is sour” in Italian, which developed out of Latin, is “l’acido acetico è acido”. I find the way that languages grow and take words from each other fascinating. The English-speaking scientists needed a word for this class of chemicals and they turned to Latin and started calling them acids, but then the Italian-speaking world also needed a word for acids so they just took the English word and Italianized it: Acido (and acidi for the plural).

Bases, like bi-carb soda are bitter tasting. But, tasting a chemical to see whether it’s an acid or a base is obviously not a good idea.

Using what we call indicators is a much better idea. Indicators are chemicals that change colour depending on whether they are in an acid or a base.

For example, this is litmus paper. It comes in two versions: a red version and a blue version. Litmus paper turns red in acid and turns blue in base. However in neutral substances, like water, there’s no colour change. It’s only if it changes to red that you know you have and acid or if it changes to blue that you know you have a base.

Litmus also comes in the form of a solution, a red version and a blue version. Once again, it’s red in acid, it’s blue in base and it stays the same in a neutral substance.

Many indicators in fact, like methyl orange for example, come in solution form. Powdered methyl orange is dissolved in water. In acids, methyl orange turns red and in bases it turns orangey yellow.

Bromothymol blue, another common indicator, is yellow in acids and blue in bases.

Methyl orange and bromothymol blue are artificial but litmus is produced by extracting the natural dyes in certain lichens.

Natural indicators are actually very common in nature.

If I place the petals of these camellia flowers into this beaker, and then bring the water to the boil, all the coloured dyes that give the flower its colour come out into the water. This whole process takes about 5 minutes. I can then pour the coloured solution, once it’s cooled down, into a dropper bottle. When I pour some of the camellia indicator into different acids and bases, the colour of the camellia indicator changes.

In both the strong and weak acid, the indicator is an orangey-pink colour, in the bi-carb soda solution, a weak base, it turns blue, and in the sodium hydroxide solution, a strong base, it turns green.

You can do a similar experiment and make indicators with red cabbage, carrots, other flowers, and anything with a nice rich colour.

A really useful indicator is called universal indicator. It’s made by mixing various other indicators together.

Universal indicator turns red in strong acids like hydrochloric acid, yellow or orange in weaker acids (this is vinegar), green in neutral substances like water, bluey-green in weaker bases (this is bi-carb soda), and dark blue in strong bases like sodium hydroxide. So, universal indicator doesn’t just tell you whether something is an acid or a base, it tells you how strong the acid or base is.

This concept of acid strength or base strength is really important.

In our first episode, we saw that metals have different reactivities. I placed a few different metals in hydrochloric acid and it was pretty obvious that magnesium is very reactive, while the other metals are much less so. Well, acids and bases also have different reactivities; you’ve got your stronger acids and bases and your weaker acids and bases. For example, magnesium reacts very quickly with hydrochloric acid because it’s a strong acid but reacts much more slowly with the acetic acid (in vinegar) because it’s a weaker acid.

So why do we get strong acids and weak acids and strong bases and weak bases? To answer that question, we need to get down to the atomic level, and that’s what we’re going to do in our next episode. See you then.

CREDITS:

https://upload.wikimedia.org/wikipedia/commons/d/d8/Tractus_intestinalis_base.svg by Orem. CC License.

https://commons.wikimedia.org/wiki/File:Sodium_hydroxide_burn.png by Blazius. CC License.

Simulations by PhET: https://phet.colorado.edu/en/simulations/soluble-salts. CC License.