Where can we find acids and alkalis in nature?

Image: Richard Bartz, 2007. Bee stings contain formic acid and are slightly acidic. Wasp stings, on the other hand, are slightly alkaline.

What are acids and bases?

Before we delve into the different acids and bases found in nature, we need to be clear about what they actually are. In their simplest definition acids are solutions that have a pH below 7 and react with bases in neutralization reactions which means that the acid effect is cancelled out by the base. A more advanced explanation would add that acids release in hydrogen ions (H+) in water.

Bases are the chemical opposite of acids. Their pH values are above 7 and they react with acids in neutralization reactions. You may also have heard the word alkali being used for bases. Alkalis are bases that are soluble in water and release hydroxide ions (OH-). In this article we will use the word alkalis.

Food and digestion

There is one strong acid that you are carrying around with you all the time, the hydrochloric acid in you stomach. Your stomach acid is quite strong with a pH of 2. Its job is to break down food and kill pathogens that enter the digestive system. Pathogens are microorganisms like bacteria and viruses that can cause diseases.

Most of you will have heard that there is a lot of acid in citrus fruit like lemon. They contain an acid called citric acid which also gives them their sour taste. Lemon juice has a very acidic pH between 2 and 3. However, other fruits and vegetables contain acids too. For example, there is malic acid in apples. Tomatoes and pears contain citric acid as well as malic acid.

A sour taste will tell us if food or drinks are acidic. We can also tell from the taste if they are alkaline. The give-away for alkalis is a bitter taste. Examples of alkaline foods are leafy green vegetables like kale, spinach and parsley.

Insect bites and stings

So far we have only talked about beneficial acids and alkalis in food, drinks or our stomach. However, there are some unpleasant acids and alkalis to be found in nature as well.

Bee and ant venoms contain formic acid making their stings or bites slightly acidic contributing to the pain they cause. In fact, formic acid was first extracted from ants which lead to it being named after the Latin word for ant ”formica”. Nevertheless, we need to be aware that insect poisons are a mixture of different unpleasant substances that work together in a sting. Bees and ants do not rely on the formic acid in their venom alone.

Wasp venom, on the other hand, is slightly alkaline. Just like in bees and ants wasp poison too is a cocktail of various chemicals which contribute to the effect of a sting. Also wasps need other substances apart from the alkali in their venom.

Reading exercise: Acids and Bases

Acids

There are acids in the laboratory, for example hydrochloric acid, sulphuric acid and nitric acid. But there are also natural acids in vinegar, sour fruits like lemon and even your stomach! When an acid is dissolved in lots of water, it is called dilute. The more concentrated an acid, the less water it is dissolved in.

Concentrated acids are corrosive. This means they burn through skin, other living tissue and metal.

Bases and Alkalis

Bases are the chemical opposite of acids. They react strongly with acids in neutralization reactions. In such a neutralization reaction, bases neutralize acids which means that they cancel out the acid effect. If a base is soluble in water it is called an alkali.

Alkalis in concentrated solutions are corrosive like acids. This powerful chemical action is often used in bath and oven cleaners.

Neutralization

Although neutralization may seem like something that only happens in the lab, it is also used at home. Stomach indigestion is usually caused by too much acid in the stomach. The pain from indigestion can be removed if the acid is neutralized. This can be done by swallowing a weak alkali like bicarbonate of soda or milk of magnesia.

Neutralization also occurs when treating wasp or bee stings. A wasp sting is an alkali and can be treated with vinegar which is a weak acid. The vinegar neutralizes the alkali wasp venom. A bee sting is an acid and can be neutralized using baking soda which is a weak alkali.

Farmers use neutralization when too much acid is in the soil is causing it to be “sour”. They normally add lime to their fields. Lime is an alkali that dissolve in rain water and neutralizes the acid in the soil. Crops can now grow better.

Questions

  1. Name three laboratory acids.
  2. Name three acids you have in the home.
  3. What is meant by dilute?
  4. What is meant by concentrated?
  5. What is meant by corrosive?
  6. What are bases? What are alkalis?
  7. What happens in a neutralization reaction?
  8. Where are alkalis used in the home?
  9. How can you treat stomach indigestions? Why does it work?
  10. How can you treat bee and wasp stings? Why does it work?
  11. Why do farmers sometimes add lime to their fields?

Challenge: Why is it important to use a weak alkali when treating stomach indigestion and bee stings?

Why do you have to use a weak acid to treat wasp stings?

Debunking the myth of stinging nettles and dock leaves

Image credit: Copyright by Kenneth Allen. CC BY-SA 2.0. The image shows stinging nettle on the left and dock leaf on the right.

It is the 2020 Corona virus lockdown and I am teaching online from home. Trying to keep things a bit interesting I am putting together small experiments to do at home. While thinking about possible experiments for the topic ”Acids and Alkalis”, I remember something I was taught myself in primary school. The sap of dock leaves is supposed to relieve the symptoms of nettle stings.

Me and many other children around the world were taught that stinging nettles sting because their poison contains acids. The sap of dock leaves is supposed to help because it is alkaline and neutralizes (cancels out) the nettle’s acid.

Perfect! I thought and was very excited to have found a great activity for the children. Finding stinging nettles and dock leaves and investigating their properties at home.

However, I did some further reading and quickly realized that I would not be able to use this activity.

The leaves of stinging nettles are covered in tiny hairs. When you brush against them their tip breaks of and they turn into tiny needles injecting the venom into your skin. It is true that the venom contains acids like formic acid oxalic acid and tartaric acid. Nevertheless, scientists argue that their concentrations are too low to cause any pain.

Today the bad guys of nettle stings are believed to be three compounds that are found in our own bodies as well. Serotonin, acetylcholine and histamine. Serotonin and acetylcholine are produced by our nervous system where they carry messages between nerve cells. But when injected directly into our skin, they cause irritation and pain. Histamine is probably the worst of the trio, causing inflammation and allergic reactions to the skin. The effect or nettle stings is most likely due to a nasty combination of all three. However, in some nettle species tartaric acid and oxalic acid are thought to at least contribute to a longer duration of the pain.

Now we know that the pain and itching of nettle stings is not really caused by acids at all. But what about the alkali part? Is dock leaf sap really alkaline? The answer is no. It has also been suggested that dock leaves may contain antihistamines to cancel out the effect of the histamines, but there is no evidence for this either.

The effect of dock leaves might simply by attributed to the sap cooling the irritated skin or a placebo effect. However, there is some evidence that dock leaves could contain a chemical that reduces the effect of serotonin in the nettle venom.

Reading Exercise: The Halogens

Elements in group 7 of the periodic table are called the halogens. They share some similar properties (= features) and show a pattern in the way their properties change throughout the group.

All halogens exist as diatomic molecules where two atoms are held together by a chemical bond. They are non-metals which makes them poor conductors of heat and electricity. In addition, all halogens are poisonous and corrosive which means they burn through metals and living tissue like skin.

Physical properties

When going down group 7 melting points, boiling points and densities of the halogens increase. At room temperature, fluorine is a pale yellow gas and chlorine a green gas. Bromine is a brown liquid that evaporates easily. Iodine is a purple/black solid.

Chemical properties

Most halogens react with metals to form salts called metal halides, for example:

chlorine + magnesium -> magnesium chloride

fluorine + sodium -> sodium fluoride

The halogens become less likely to react in chemical reactions when going down the group. Their reactivity decreases. Fluorine is most reactive.

Displacement reactions

The order of the reactivity of halogens can be shown using displacement reactions where a more reactive element takes the place of a less reactive element in a salt.  For example, chlorine displaces bromine from sodium bromide in solution because chlorine is more reactive than bromine.

chlorine + sodium bromide à sodium chloride + bromine

However, bromine cannot displace chlorine from sodium chloride because it is less reactive.

Uses

All halogens can be used in disinfectants and bleaches because they kill microorganisms like bacteria and remove colour from materials. Chlorine is used as a disinfectant in swimming pools and in bleaches. Iodine is used to clean wounds.

Test for chlorine

If a damp, blue litmus paper is placed in chlorine gas, it first turns red then bleaches white.

Questions

  1. In which group of the periodic table are the halogens found?
  2. Halogens exist as diatomic molecules. What is meant by “diatomic”?
  3. Halogens are also corrosive. What is meant by “corrosive”?
  4. Describe what fluorine, chlorine, bromine and iodine look like at room temperature.
  5. What is formed when halogens react with metals?
  6. Chlorine reacts with iron. What is the name of the product formed in this reaction? Write the word equation.
  7. What is the trend in reactivity for halogens when going down the group?
  8. What happens in a displacement reaction?
  9. Why can chlorine displace bromine from sodium bromide?
  10. Give two uses for chlorine and one use for iodine.
  11. Describe the chemical test for chlorine.
  12. Fluorine is mixed with lithium iodide Can fluorine displace iodine from lithium iodide? Why? Write the word equation.

Testing Acids and Alkalis in the Kitchen

Background

In this experiment you will investigate the properties of three substances in the kitchen to determine if they are acids or alkalis.

Acids have a sour taste. In high concentrations acids can burn your skin and other living tissue. But the acids you are working with in the kitchen have very low concentrations and safe to touch. Examples of laboratory acids are hydrochloric acid, sulphuric acid and nitric acid.

Alkalis have a bitter taste and feel slippery when you touch them. Alkalis in high concentrations can burn your skin or other living tissue too. But the alkalis in your kitchen have low concentrations and are safe to touch. One common alkali in laboratories is sodium hydroxide.

You will need:

  • 1 lemon or lemon juice
  • vinegar
  • dish washing soap (liquid soap for hand washing works too)

What to do:

  1. Copy the table below.
Substance Look Feel Taste Acid or alkali?
Lemon juice
Vinegar
Dish washing soap

 

  1. If you have a lemon instead of lemon juice you need to squeeze it now and collect some juice from it for your experiment.
  2. Look at the lemon juice, vinegar and dish washing soap and record in your table what they look like.
  3. In turns drop a bit of each of the substances on your hand and test what they feel like. Record it in your table.
  4. Now taste the lemon juice and the vinegar. Record what they taste like in your table. You do not have to taste the dish washing soap.

Questions

  1. Based on your data decide which substances are acids and which are alkalis and record it in your table. Use the text in the introduction to help you. It gives you information about the properties of acids and alkalis.
  2. What do you expect the dish washing soap would taste like? Why?
  3. Name two properties of acids.
  4. Name three properties of alkalis.
  5. Name two laboratory acids.
  6. Name one laboratory alkali.
  7. Which other acids and alkalis do you know that you have in the kitchen or the home?

The Science of Chocolate – Investigating the States of Matter

You will need:

  • Some pieces of chocolate (dark chocolate works best, but you can use milk chocolate too)
  • Pan
  • Bowls
  • Water

What to do:

  1. Put water in the pan and place it on the hob.
  2. Break the chocolate into small pieces and place it in a bowl over the pan with the water. (See image above.) Be careful not to mix any water with the chocolate.
  3. Turn on the hob and gently heat the water with the chocolate and bowl on top.
  4. Once the chocolate has melted, turn off the hob.
  5. Place half of the molten chocolate in a freezer to cool. If you do not have a freezer, you can use the fridge.
  6. Let the rest of the chocolate cool slowly at room temperature.
  7. Once both chocolates have frozen, compare what they look like. In addition, test how they taste differently and how they feel to the touch.
  8. Although both chocolates freeze and become solid again, they will be very different depending which temperature they are freezing at.

Questions

  1. A) Draw a diagram showing the arrangement of the chocolate particles in a solid.  B) Draw a diagram showing the arrangement of the chocolate particles in a liquid.
  2. What happens to the energy of the chocolate particles when it melts?
  3. What happens to the energy of the chocolate particles when it freezes?
  4. How is the chocolate that was cooled in the freezer different from the chocolate cooled at room temperature? Compare how they look, taste and feel when you touch them.
  5. The process of melting and freezing chocolate is quite important in food industry and chocolate making. Based on your experiment which temperature do you think is better for freezing chocolate, room temperature or the freezer/fridge? Why?

How to make an atomic model with sweets

You will need:

  • String
  • Scissors
  • Coloured sweets, for example smarties or skittles

What to do:

  1. Choose three colours that you want to use in your atom. Once colour for the protons, one for the neutrons and one for the electrons.
  2. Cut some string to make the electron shell.
  3. Put the sweets that represent the protons and the neutrons in the middle. The middle of the atom is called the nucleus.
  4. Arrange the string in a circle around the nucleus. It will form the electron shell.
  5. Put the sweets that represent the electrons on the electron shell (string). Make sure that the amount of protons and electrons is the same.
  6. Your atomic model is now complete. If you wish you can label the atom and add the charges for protons, electrons and neutrons.

You can watch this experiment on YouTube:

Active reading exercise: Isotopes

Isotopes

In 1932 James Chadwick, a British scientist, discovered the neutron. His discovery explains how isotopes are formed. Isotopes have the same number of protons, but different numbers of neutrons.

We can also say that the atomic number is the same, but the mass numbers are different. Isotopes are the same element because their atomic number does not change.

We refer to an isotope by adding its mass number to the element’s name. The isotope in the diagram below is called carbon-12.

carbon

Carbon isotopes

Carbon can occur as three different isotopes. They are called carbon-12, carbon-13 and carbon-14.

Carbon dating is used to identify the age of very old objects, for example the remains of extinct animals such as mammoths. The amount of the carbon isotope carbon-14 in an object is examined to determine its age.

Things to do

  1. Fill in the missing words and numbers to describe the similarities and differences between isotopes of the same element.                                                                                 a) Isotopes are atoms with the same            number but different                   number. b) They have the same number of protons and electrons but different numbers of                      .
  2. Name the scientist who discovered the neutron.
  3. Why are isotopes the same element?
  4. How do we refer to isotopes?
  5. Name the three isotopes of carbon.
  6. Why is the isotope carbon-14 useful?
  7. Using your knowledge about isotopes, fill in the gaps in the table a, b and c.
Isotope name Atomic number Mass number
 Tin-116  50 a
b c 118

Remember that the atomic number is the same for isotopes of the same element.

Active reading exercise: The Atom

A bit of history

In 1805 the English Chemist John Dalton published his atomic theory that said:

  • Everything is made up from tiny particles called atoms
  • Atoms are tiny hard spheres (= balls) that cannot be broken down into smaller parts
  • Atoms in one element are all identical

This helped to explain many properties of materials. However, later experiments showed that atoms contained even smaller particles. In 1897 another British scientist, JJ Thomson, discovered the electron. The nucleus which makes up the middle of an atom was discovered by Ernest Rutherford in 1913.

The Structure of the atom

Today we know that atoms are made from three subatomic particles: proton, neutron and electron.

Protons and neutrons are found in the centre of the atom which is called the nucleus. Both have a mass of 1. Protons have a positive (+) charge and neutrons are neutral (= no charge).

Electrons have a negative (-) charge and have almost no mass at all. They are found on the electron shells on the outside of the atom, circling the nucleus.

What you need to remember

  • Atoms are made from protons, neutrons and electrons called subatomic particles
  • Protons: found in nucleus, positive (+) charge and a mass of 1
  • Neutrons: found in nucleus, neutral (no charge) and a mass of 1
  • Electrons: found on electron shells, negative (-) charge and almost no mass

 

Things to do

  1. Name the scientist who first introduced atomic theory.
  2. Name the scientists who discovered the electron and the nucleus.
  3. State the names of the three subatomic particles as well as their masses and charges.
  4. State where in the atom protons and neutrons are found.
  5. State where in the atom electrons are found.
  6. Copy and label the image of the atom. Words: electron, proton, neutron, shellLithium atom
  7. Describe in your own words what an atom looks like. Include information about the charges and masses of the subatomic particles.

Reading Exercise: The History of the Atomic Model

Democritus’ atoms

Around 440 BC, Greek philosopher Democritus was the first to suggest the existence of atoms, tiny particles that make up all matter. The word atom comes from the Greek ”atomos” meaning indivisible.

However, most of his colleagues, especially Aristotle, did not agree with Democritus. Instead, they thought that matter was made up of the four ”elements” fire, water, wind and earth.

Dalton’s spheres

It took over 2000 years until another scientist would challenge Aristotle’s ”element” theory.

In 1803, John Dalton, an English teacher from Manchester, carried out experiments proving that all matter is made up of tiny particles. He chose to use Democritus’ name and called them atoms.

In Dalton’s model atoms were tiny, hard spheres that vary in size and mass, but cannot be split into smaller pieces.

Thomson’s plum pudding

It took a much shorter time to reach the next step in the discovery of the atomic model.

J.J. Thomson, another English scientist, discovered the electron in 1897 and developed the plum pudding model of the atom for which he received the Nobel prize in 1906.

The plum pudding model said that the tiny negative electrons were distributed in a positive mass inside the atom. The electrons were like negative raising in a positive plum pudding dough.

Rutherford’s gold foil experiment

The next experiment to develop the atomic model even further were carried out by one of Thomson’s former students, Ernest Rutherford, who was originally from New Zealand.

In 1909, Rutherford and his team conducted one of the most important experiments in the history of science. They used a gold foil which was bombarded with alpha particles (= Helium nuclei which have 2 protons and 2 neutrons).

If Thomson’s plum pudding model were true, you would expect all alpha particles to punch holes through the positive ”dough” and pass straight through the foil.

The results looked somewhat different. Most alpha particles did pass straight through the gold foil. However, some particles did not pass through and bounced back. This suggested that there were parts in the foil where mass was very concentrated, while the rest seemed to be empty space.

As a consequence, Rutherford introduced the modern planetary model of the atom where the electrons circulate around a nucleus. The nucleus is small, but contains most of the atom’s mass and is where the alpha particles bounced back in his experiment. The major part of the atom is empty space where the alpha particles could pass through.

By the way, Ernest Rutherford received a Nobel prize as well. However, it was for discovering the concept of half-life for radioactive substances rather than his work on the atomic model.

Tasks:

  1. Draw a timeline including the four stages of the atomic model’s development.
  2. Describe what the word ”atom” means.
  3. Describe what atoms were like in Dalton’s atomic model.
  4. State what Thomson discovered.
  5. Describe what atoms were like in Thomson’s atomic model.
  6. Describe the gold foil experiment Rutherford conducted.
  7. Explain how the gold foil experiment showed that Thomson’s theory was wrong.
  8. Describe what atoms are like in Rutherford’s model.
  9. Challenge: Find out how the atomic model developed further. You could look at the work of Niels Bohr, Werner Heisenberg and James Chadwick.