Shedding Light on Acids and Bases Episode 4: pH

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 4, pH, we take an in-depth look at H+ and OH ions and we explain how the pH scale is used to express how strongly acidic or how strongly basic a particular environment is. We examine how the pH of natural environments affects plant and animal life and how the human body has to carefully control the pH of our organs.

A 7-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 H+ Ions
Part C: Bases and OH Ions
Part D: The pH Scale
Part E: pH in Nature

Part A: Introduction

Acids and Bases. They’re everywhere. Our food contains lots of different weak acids and our bodies produce both acids and bases.

Acids and bases are used in the production of steel, fertilizers, soaps and lots of other things. As we’ve seen, some acids are strong (that is, they’re very reactive), and some are weak: they’re much less reactive. Bases can also be strong or weak.

In this program we’re going to look at why acids and bases can be strong or weak, and we’ll cover what’s called the pH scale, the scale that is used to express how strong or weak an acid or a base is. To begin, we’re going to take a deep dive into atoms themselves and look at what makes an acid an acid.

Part B: Acids and H+ Ions

Atoms have a central nucleus, made up of protons, which are positively charged, and neutrons, which have no charge (they’re neutral). The nucleus is surrounded by electrons, which are negatively charged. Atoms have the same number of protons and electrons, three in this case. The size of the positive charge on the protons is equal to the size of the negative charge on the electrons, and so atoms, overall, have a charge of zero, because they have the same number of protons and electrons.

The atomic number, shown on periodic tables, tells you the number of protons in the atom’s nucleus which is the same as the number of electrons that move around the nucleus. Hydrogen atoms have one proton and one electron, helium atoms have two protons and 2 electrons and so on.

Now the electrons occupy only certain fixed energy levels called electron shells and each shell can hold only a fixed number of electrons. The electron configuration of an atom tells you how many electrons are in each shell. Chlorine’s electron configuration, for example, is 2, 8, 7. Chlorine atoms have 2 electrons in their first shell (the first shell is the inner shell), 8 in their second, and 7 in their third (which in this case is also the outer shell).

Since there is a force of attraction between positively charged protons and negatively charged electrons, the electrons tend to stay with their atoms. However (let me stop the electron even though in real life they never stop moving), the protons of one atom can also attract the electrons of another and the two atoms form a bond of attraction called a covalent bond. It’s often described as a sharing of electrons, but it’s more of a tug of war. Each hydrogen atom now has two electrons in its electron shell, rather than just one and this arrangement is stable.

So let’s talk about hydrochloric acid.

When a HCl molecule forms, a chlorine atom’s nucleus attracts the electron of a hydrogen atom into its outer shell and the hydrogen atom’s nucleus attracts one of the chlorine atom’s electrons into its (one and only) shell. This mutual attraction keep the atoms together. Notice how by “sharing”, the chlorine atom now has 8 electrons in its outer shell and the hydrogen atom has 2 electrons in its one and only shell. This is what happens most often in molecules. Apart from hydrogen, non-metal atoms form covalent bonds such that their outer shells have 8 electrons in them. Eight is just a particularly stable number and if the outer shell has 8 electrons in it, it’s called “a full outer shell”. The main exception is hydrogen. A hydrogen atom needs just two electrons in its one and only shell to have a full outer shell, since the inner shell, being smaller, can only hold a maximum 2 electrons.

For more about covalent bonding, you can watch Episode 7 of the Shedding Light on Atoms series which is called Covalent Bonding. This video goes into more detail.

Covalent Bonding

So back to HCl. A Cl atom has 17 protons and 17 electrons which means that it is electrically neutral. The word neutral refers to something that overall doesn’t have a charge AND as we’ve seen to something that is neither an acid or a base, so don’t get confused or anything. A hydrogen atom has one proton and one electron, which means that it’s neutral too. When they bond covalently, the HCl molecule is still neutral having an equal number of protons and electrons.

Now ordinarily, HCl is a gas, but when it dissolves in water, you get hydrochloric acid.

As a gas, you have a H atom and Cl atom bonded together because they’re pulling on each other’s electrons, and there are equal numbers of protons and electrons. Now in water, the H and the Cl atoms separate but they don’t remain neutral. The chlorine atom keeps the electron that had been part of the hydrogen atom and so, with an extra negatively charged electron, it gains a charge of 1-. A chlorine atom with an extra electron is given a name change and is called a chloride ion and is given the symbol Cl. An ion is an atom that has lost or gained electrons. Ions are very common in nature. The hydrogen atom on the other hand is now missing an electron and so, as a single positively charged proton with no electrons, it has a charge of 1+. It’s not a normal hydrogen atom anymore, it’s a hydrogen ion, and its symbol is H+. Positive ions don’t get name changes. Now it’s the presence of these H+ ions bouncing around in water that makes an acid an acid.

The reason the H’s and the Cl’s separate in the first place, by the way, when they dissolve in water, is that the water molecules in water are constantly moving and vibrating wildly, and of course they have their own protons and electrons. The actual collisions and the forces between all the protons and the electrons rips the H+ ions and the Cl ions apart.

So, acids produce H+ ions in water when they dissolve in water and these H+ ions chemically react with metals, bases, and other substances. Strong acids are acids made of molecules that “dissociate” completely in water (that is they break apart completely in water) producing lots of H+ ions. HCl is a strong acid. When HCl molecules dissolve in water, 100% of them dissociate into H+ and Cl ions. Sulfuric acid and nitric acid are also strong acids. Weaker acids are made of molecules that don’t dissociate as much. The chemical formula for acetic acid, the acid in vinegar, is CH3COOH. When acetic acid dissolves in water to form vinegar, it produces H+ ions and CH3COO ions (which are called acetate ions). However only about 0.4% of the acetic acid molecules dissociate into H+ and CH3COO ions. 99.6% remain as CH3COOH molecules. It’s a weak acid because it doesn’t produce many H+ ions.

Most of the foods that we eat are even weaker acids than vinegar. There are H+ ions floating around in the juice of an orange for example, but not that many compared to the amount in vinegar, and far far less than the amount in hydrochloric acid.

So a H atom is made of a proton and an electron, but a H+ ion is basically a single proton. The H+ ions produced by acids typically form a loose attachment to water molecules to produce what are called hydronium ions, which have the chemical formula H3O+. The hydronium ions last for only a tiny fraction of a second because the H+ ions actually bounce around from water molecule to water molecule. However, for convenience, we usually don’t mention hydronium ions, we just say that acids produce H+ ions.

Part C: Bases and OH Ions

Now bases also involve ions. Sodium hydroxide, NaOH, also dissociates when it’s dissolved in water, and forms Na+ ions and OH ions.

Sodium hydroxide is made of sodium ions (that is Na+ ions) and hydroxide ions (OH ions). A sodium ion has 11 protons but only 10 electrons so it has a charge of 1+. A hydroxide ion, made of 1 oxygen atom and 1 hydrogen atom has a total of nine protons (eight in the oxygen atom and 1 in the hydrogen atom) but it doesn’t have just 9 electrons, it has 10 electrons, so its extra electron (this one here) gives it an overall charge of 1-. Hydroxide ions are very reactive. Any chemical that produces hydroxide ions is a base. The more hydroxide ions, the stronger the base.

When sodium hydroxide dissolves in water, there is a 100% dissociation of the Na+ ions and the OH ions, so sodium hydroxide is a strong base. Other bases are weaker.

Bi-carb soda produces OH ions, but it’s a weaker base. When bi-carb soda (NaHCO3) dissolves in water, a very small percentage of the NaHCO3 reacts with the water to form Na+ ions, OH ions and H2CO3. The OH ions here make the solution basic but it’s a very weak base because, as I said, only a small percentage of the NaHCO3 reacts and not many OH ions are formed. Most of the NaHCO3 stays together. So, a strong base produces lots of OH’s and a weaker base produces relatively few.

NaHCO3 + H2O —> Na+ + OH + H2CO3

So, all acids produce H+ ions and all bases produce OH ions when they’re dissolved in water.

A dilute acid or base is one where there’s relatively speaking lots of water (it’s watered down) while a concentrated acid or base means there’s many more H+ or OH ions in a given amount of water.

Now the actual strength of acids and bases can be measured and we have a scale that tells us how strong an acid or a base is. Let’s take a look at it.

Part D: The pH Scale

To express the strength of acids and bases, we use what’s called the pH scale, lower case p, capital H.

The pH scale goes from 0 to 14. At the lower end of the scale, with a pH of about 0 to 1, we have strong acids like sulfuric acid (car battery acid). On the other end of the scale, with a pH of about 14, we have strong bases like sodium hydroxide (found in drain cleaner). In the middle, with a pH of 7, we have neutral substances, like pure water.

As you move below 7, the acidity increases from weaker acids to stronger acids. Milk has a pH of about 6.6 which makes it a very very weak acid. Black coffee is a little more acidic with a pH of about 5, apples and oranges (you’re not supposed to compare apples and oranges but I’m going to…) they’ve got a similar pH to each other of between about 3 and 4 depending on their variety, lemons have a pH of about 2 – 2.6, and vinegar has a pH of about 2.4 – 3.4 depending on the brand and type. Cola drinks have a pH of about 2.3 because they contain both carbonic acid and phosphoric acid.

So a decreasing pH means something is more strongly acidic. Now as you move from a pH of 7 to 14, the basicity increases from weaker bases to stronger bases. Bi-carb soda (dissolved in water) has a pH of about 8-9, soap of about 9-10, cleaning fluids of about 11-12 and the strongest bases have a pH of 14. So, an increasing pH means something is more strongly basic.

Now let me just say again because it sometimes confuses people; on the acidic side of the pH scale, a decrease in pH means a higher acidity. Most scales aren’t like this but this scale is like this.

Anything with a pH of between 4 and 7 is considered a really really weak acid, between about 2 and 4 is considered a medium-strength acid, and strong acids have a pH of less than 2. The same is true on the base side of the scale.

Universal indicator changes colour according to the pH of the chemical that is being tested. The range of colours reflects the colours of a rainbow.

When you buy universal indicator it comes with a colour chart which you just read off. Hydrochloric acid, a strong acid, has a pH of about zero, which means there are lots and lots of H+ ions floating around in it. This watered-down vinegar is showing a pH of about 4, which means that there are far fewer H+ ions floating around in it. Vinegar is far less reactive than hydrochloric acid as we’ve already seen. Water is neutral and has a pH of about 7 which means that there are no H+ ions in it, or very few at least. Bicarb-soda, a weak base, has a pH of about 9. This means that there are OH ions in the water, but very few of them. Weak bases don’t produce many OH ions. Sodium hydroxide, which is a strong base, has a pH of 14. There are lots and lots of OH ions present in the water in this test tube.

The pH scale gives us a very convenient way of expressing how strongly acidic or strongly basic a particular chemical or environment is.

Part E: pH in Nature

Let’s actually quickly go over some of the things that we’ve looked at in previous episodes and look at them from a pH point of view. As we saw in Episode 2, our stomach produces hydrochloric acid.

The pH in our stomach is typically between about 1 and 2, so it’s a very strongly acidic environment. As our partially digested food and the acid that it’s in are then squeezed into our small intestine so that our food can undergo further digestion, sodium hydrogen carbonate produced by the pancreas is added which neutralizes most of the acid. This is the full equation. Since most of the acid goes bye-bye, the pH rises to about 6 in the small intestine, which is pretty much but not quite neutral.

In Episode 2 we also looked at the damage that acid rain was doing in the second half of the 20th century. Let’s use the pH scale to quantify the magnitude of the problem that had to be fixed.

Pure water has a pH of 7, but clean rain water and the water in rivers and creeks has a pH of about 6, so it is very slightly acidic. This is because, as we saw in Episode 2, carbon dioxide in the air reacts with the water in the air to produce carbonic acid AND small amounts of naturally present sulfur dioxide, nitrogen dioxide and nitric oxide in the air also react with the water in the air to produce sulfuric and nitric acids. However, from about the 1950s to the 1990s, the burning of coal in power stations and the burning of fuels in cars and trucks increased the amount of these three gases in the atmosphere, which led to the formation of acid rain, that is, rain with a higher level of acidity or a lower pH. This colour-coded map shows how acidic the rain water got in the United States in 1985. Some parts experienced acid rain with a pH of as low as about 4. We’re talking an acidity that’s similar to orange juice. OJ is nice to drink but it’s hard for fish to live in it and for plants to thrive in it. If you’ve ever had orange juice accidentally get into your eyes, you’ll know exactly what I’m talking about! The acid rain was particularly severe in the heavily industrialized and heavily populated north east of the country where there were more vehicles and more power stations.

This chart shows the tolerance that some aquatic life has to different levels of acidity. The light blue areas show the level of acidity that kills the animals. Clams and snails die off if the pH of the water they live in gets down to about 5.5. This isn’t really all that acidic but it’s much more than what was natural. The increased acidity didn’t just affect their populations but also the populations of animals that fed on them. Perch can handle far higher levels of acidity (that is acidic water with a lower pH), but the fact that their populations were declining meant that the water was often just way too acidic.

Fortunately, thanks to the scrubbers that are now fitted to all coal-fired power stations (which remove sulfur dioxide) and to the catalytic converters that are now fitted to all vehicles (which remove the oxides of nitrogen), acid rain is now much less of a problem.

The pH of soils is a really important factor in the way that plants grow. Farmers, who obviously want to maximize the amount of crops that they grow, have to ensure that the soil pH is just right.

Plants typically grow best in soils with a pH of between about 6 and 7, that is, neutral to very slightly acidic. The pH actually affects how well soil nutrients are absorbed.

This chart shows how well essential nutrients can be absorbed by plants from soil that is at different pH values. A fatter line indicates that more can be absorbed. If we look at the three most important soil nutrients, the ones that plants need the most of—nitrogen, phosphorus, and potassium—there’s plenty of absorption when the soil is more or less neutral, but if the soil pH drops below 6, in other words, if the soil becomes too acidic, then absorption of these three main soil nutrients is significantly depleted and the plants become deficient in them.

The leaves of this corn plant should be a rich green colour but instead they are discoloured and their growth has been stunted due to a phosphorus deficiency. The soil had had a phosphorus fertilizer applied, but the high acidity of the soil adversely affected the absorption of the phosphorus. The cauliflower and the tomato plants pictured are also showing obvious signs of nutrient deficiency.

One reason that soil can become too acidic is if there’s too much rotting plant material in it. The process of rotting is caused by microorganisms that produce lactic acid.

A soil’s pH can be measured by a soil testing kit that contains an indicator and a colour chart that you just read off, in a similar way to universal indicator, or by an electronic pH meter. The soil here has a pH of 7. Well done to the farmer! Now I’ve been using the words base and basic but the pH meter uses the word alkaline, and the soil-testing kit colour chart uses the word alkalinity. Alkaline is just another word for basic, the chemical opposite of acidic.

If the soil pH is too low, that is, if it’s too acidic, farmers (and home gardeners) have to add a base to neutralize some of the acid and to raise the pH.  Crushed limestone (calcium carbonate, CaCO3) is a very common choice. It’s marketed as garden lime. Remember, an acid plus a carbonate produces a salt, water and carbon dioxide. The soil becomes less acidic and its pH goes back up to the near-neutral optimal range. The packaging even says so: corrects soil acidity; raises soil pH. If the soil’s pH is too high, over 7, farmers add an acid. Ammonium sulfate is one option. It’s a weak acid which, as the packet says, lowers soil pH (from basic back to below 7).

Manure and other types of compost are all slightly acidic, so these are useful as well at lowering pH if the soil is too basic.

We can see here one of the reasons that it’s so good to classify things. If you measure the pH of soil and it’s too acidic, you don’t necessarily know which acids are causing the soil to become acidic. But which acid isn’t all that important because all acids produce hydrogen ions that chemically react in a similar way.

And so, in summary, acids produce H+ ions in water, and bases produce OH ions in water. Acids and bases chemically react with each other and with lots of other substances. There are lots and lots of industries that require a good knowledge of acids and bases and they employ millions and millions of people around the world.

And that brings us to the end of the Shedding Light on Acids and Bases series. Hope you liked it, see you next time.

CREDITS:

https://commons.wikimedia.org/wiki/File:Tomate_Blatt_Kalimangel.jpg by Goldlocki. CC License.

https://commons.wikimedia.org/wiki/File:Brassica_oleracea_Cauliflower_Bloemkool_stikstofgebrek.jpg by Rasbak. CC License.

https://commons.wikimedia.org/wiki/File:Soil_pH_effect_on_nutrient_availability.svg by CoolKoon. CC License.

Simulations by PhET Interactive Simulations, University of Colorado Boulder, https://phet.colorado.edu

Additional material produced by the United States Environmental Protection Agency.