Lemon Volcano

You will need

  • 1 plate or tray
  • 1 lemon
  • 1 cutting knife (to cut the lemon)
  • 1 butter knife
  • Baking soda
  • 1 table spoon
  • Food colouring (1 to 4 colours of your choice)

What to do

  1. Cut your lemon in half. Then cut of a bit at the bottom, so that the lemon can now stand up.
  2. Place the lemon on the plate or tray with the wide side facing up.
  3. Use the butter knife to poke holes in the flesh.
  4. Add 8 to 10 drops of food colouring spread out over the lemon.
  5. Add 1 heaped table spoon of baking soda.
  6. Poke with the butter knife to mix the baking soda with the lemon juice.

The acid in the lemon reacts with the baking soda to form salt, water and carbon dioxide gas. The carbon dioxide gas will cause fizzing. We also call this effervescence.

You can watch this experiment on YouTube:

Coke and Mentos Fountain

Background

Catalysts are substances that speed up chemical reactions. However, they do not directly take part in the reaction and are not used up themselves.

Cars contain catalysts in catalytic converters that split toxic substances released by the car’s engine into less harmful ones.

The gas bubbles inside coke are the result of a chemical reaction where carbonic acid decomposes to water and carbon dioxide gas. The bubbles you feel when drinking coke are carbon dioxide. The word equation for this reaction is:

Carbonic acid → Water + Carbon dioxide

Carbonic acid is the reactant. Water and carbon dioxide are the products.

Mentos can act as a catalyst and increase the speed of carbon dioxide production. This causes the foaming you can see when adding Mentos to coke. The scientific word for bubbles, fizzing or foaming is effervescence.

You will need:

  • 1 bottle with coke or diet coke (Normal coke and diet coke both work, but diet coke is less sticky and easier to clean up afterwards.)
  • 1 pack of Mentos

What to do:

  1. Go outside to do this experiment.
  2. Put the coke bottle on the floor and remove the lid.
  3. Put about 5 pieces of Mentos inside at the same time.
  4. Step back and watch.
  5. You should see a lot of foaming due to the increased carbon dioxide production.

Questions

  1. What is meant by a “catalyst”?
  2. What is the catalyst in this reaction?
  3. Is the Mentos used up in this reaction or not? Why?
  4. What is meant by the “reactant” in a reaction? What is the reactant in this reaction?
  5. What is meant by the “product” in a reaction? What are the products in this reaction?
  6. Where are catalysts used in our everyday lives?
  7. What is meant by “effervescence”?

You can watch this experiment on YouTube:

Reacting Vinegar and Baking Soda

You will need:

  • Baking soda (= alkali)
  • Vinegar (= acid)
  • 1 empty, clean bottle
  • 1 spoon
  • 1 balloon

What to do:

  1. Fill the bottle about one third with vinegar (your acid).
  2. Use the spoon to fill the balloon with baking soda (your alkali). You might need another person to help you and hold the balloon open.
  3. Put the opening of the balloon over the opening of the bottle. (See image above.)
  4. Now tip the baking soda from the balloon into the bottle.
  5. A neutralization reaction will take place where the acid and alkali react to form the products salt, water and carbon dioxide. All three products are neutral substances. Observe what happens.

Questions

  1. Which signs do you see that tell you a chemical reaction is happening?
  2. What happens generally in a neutralization reaction?
  3. Which two products are formed in the neutralization reaction?
  4. Which colour would the following have when adding universal indicator?                   A) Vinegar          B) Baking soda       C) The products after the reaction

You can watch this experiment on YouTube:

Disappearing Egg Shell

Background

In this experiment you will use vinegar to dissolve the shell of a raw egg. Vinegar is an acid and egg shells are made from calcium carbonate. Acids react with metal carbonates like calcium carbonate to form salt, water and carbon dioxide gas. The metal carbonate is dissolved in the process. You can find the word equation below.

Acid + Metal carbonate → Salt + Water + Carbon dioxide

You will be able to observe gas bubbles because carbon dioxide gas is formed. We also call this “effervescence”. The egg shell will dissolve leaving behind the raw egg in its membrane.

You will need:

  • Vinegar (= acid)
  • 1 raw egg
  • 1 glass

What to do:

  1. Place the raw egg carefully in the glass.
  2. Fill the glass with vinegar until the egg floats.
  3. Observe what happens on the egg shell. You should see effervescence.
  4. Leave your experiment for three days in a cool, safe place.
  5. After three days remove the egg from the vinegar and carefully dry it with kitchen roll paper.
  6. Take the egg in your hand and squeeze it gently. What does it feel like?

You can watch this experiment on YouTube:

Colourful Milk Experiment

You will need:

  • 1 plate
  • milk
  • food colouring (2 to 4 colours)
  • washing-up liquid
  • 1 cotton bud

What to do:

  1. Pour the milk on the plate.
  2. Choose 2 to 4 colours of food colouring that you want to use.
  3. Add 8 to 10 drops of each food colour to the milk in different spots. (See image above.) Do NOT stir or mix.
  4. Put one drop of washing-up liquid on the end of your cotton bud.
  5. Hold the end of the cotton bud into the middle of the milk with the food colouring.
  6. Observe what happens.
  7. You can move the cotton bud around the plate to different places and observe what happens.

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?