Acids and bases

What is an acid?

Ancient civilisations had already identified acid as a sour-tasting substance that corroded metal, but confirmation about the exact nature of acid eluded chemists until the 20th century. Early in the 20th century, a number of chemists developed specific chemical definitions for the term 'acid', although many of these definitions refer to subatomic processes, going into much greater depth than required here.

The simplest, most general definition is that an acid is a substance that contains hydrogen and which can release hydrogen cations (H+) during a reaction. The strength of an acid depends on its ability to release hydrogen ions - stronger acids release hydrogen ions more readily.

Some of the properties of acid are that they:

• Dissolve in water to form excess hydrogen ions
• Are highly reactive and will corrode most metals
• Conduct electricity
• Have a sour taste (strong acids are dangerous and should not be taste-tested)
• Produce a stinging sensation (as above, strong acids should not be handled)

There are some common edible acids such as citric acid, which is found in fruits like oranges, lemons and limes, acetic acid, found in vinegar, carbonic acid, which is the 'fizz' in soft drinks and dairy products, which contain lactic acid. Examples of other acids include: sulphuric acid, present in batteries; and hydrochloric acid, which breaks down food in your stomach. See image 1.

Acids like vinegar are used to preserve food because many organisms cannot live in an acidic environment. Similarly, fermentation of food can also produce an acidic environment for preservation purposes - vinegar is an acetic acid formed from grapes, lactic acid comes from fermentation of milk.

What is a base?

Bases are substances with the opposite properties to acids, that is, a base is a substance that accepts hydrogen ions in a reaction. Strong bases will accept more hydrogen ions than weak ones. Alkalis are soluble bases that contain hydroxide ions (OH-).

Some properties of bases include that they:

• Dissolve in water to absorb excess hydrogen ions
• Neutralise the effect of acid
• Denature (change the molecular structure) of proteins
• Have a bitter taste (strong bases are dangerous and should not be taste-tested)
• Feel soapy (as above, strong bases should not be handled)

Basic substances in everyday use include sodium hydrogen carbonate, also known as sodium bicarbonate, used in baking to help bread rise, sodium carbonate, used to make soap, and magnesium hydroxide, commonly used in indigestion remedies. Because of an ability to denature proteins, basic substances break down grease and make good cleaners. Considering that the human body is made up of proteins,
this makes bases more dangerous for humans than acids.

Clarification of terms

Before proceeding, it is important to clarify some terms used in experiments with acids and bases. Strong substances are either acids that readily lose hydrogen cations or bases that readily gain hydrogen ions; weak substances less readily lose or gain hydrogen ions. For clarity, concentrated acids and bases are either pure or come dissolved in very little water, while dilute substances are dissolved in a lot of water. Therefore, strong and weak refer to the chemical reactivity of an acidic/basic substance while dilute and concentrate refer to the ratio of water into which the substance dissolves.

Indicators

It is also important to learn about some of the ways in which to test the strength of acidic and basic substances, since it is not permitted to taste or touch chemicals in a laboratory environment.

Chemical substances are classified as acidic (containing acid), basic (containing base) or neutral (containing neither acid nor base). Chemists have developed a number of methods to test the acidity or alkalinity of a substance using chemical indicators. These indicators use the pH scale, with measurements from one to 14 based on the activity of hydrogen ions in the solution. Substances with a low pH are acidic. Substances with a reading of seven are neutral while basic solutions will elicit a higher reading.

Developed by Danish scientist Søren Sørensen, the pH scale may have come from the German word 'potenz' (meaning power or potency) and 'H', the chemical symbol for hydrogen. It is also possible the term is derived from the Latin 'pondus hydrogenii', which translates to 'weight of hydrogen'.

Most indicators have only two colours. The universal indicator is an instrument that mixes several types of indicators and colours in order to show whether a substance is acidic, basic or neutral. Universal indicators have a colour scale that corresponds to the numbered pH scale. After testing, the colour of the paper is matched to a number on the scale for a more exact reading of acidity or alkalinity. See image 2.

Reactions

Since acids and bases are more or less opposite substances, they tend to cancel each other out in a process called neutralisation. This reaction produces a salt and water.

acid + base salt + water

Neutralisation is commonly used in a number of remedies, such as the treatment of bites and stings. Bluebottles inject a basic substance when they sting, so a weak acid like vinegar (acetic acid) will neutralise a bluebottle sting. Conversely, bee stings are slightly acidic, so a bee sting would be neutralised with a weak base, such as sodium bicarbonate. Seafood gives off an odour due to the basic amines it contains. An acidic acid substance such as lemon juice is squeezed over it to neutralise the smell. Excess acid in the stomach causes indigestion, so it can be neutralised with a weak base called an antacid.

An example of an equation using this format is when hydrochloric acid meets sodium hydroxide to form sodium chloride and water:

HCl + NaOH NaCl + H₂O

Adding an acid to a base does not necessarily mean that the product is automatically neutralised. The strength of each of the reactants must be matched so that all the ions released by the acid find a place with the base. A strong acid with a weak base will result in an acidic salt, a weak acid with a strong base will result in a basic salt, while acids and bases of the same strength will neutralise completely.

Both acidic and metallic substances are highly reactive, which is why acid reacts aggressively in the presence of metal, corroding the metal much faster than moisture and air. The combination of an acid and a metal produces a metallic salt and hydrogen gas in an equation represented like this:

acid + metal metallic salt + hydrogen

The hydrogen ions are easily lost and replaced by the metallic ions, forming a metallic salt. The hydrogen then forms molecules with itself, resulting in hydrogen gas.

An example of this is sulphuric acid and magnesium producing magnesium sulphate salt and hydrogen gas:

H₂SO₄ + Mg MgSO₄ + H₂