What Is an Acid?

An acid is a substance that produces hydrogen ions, H+, when dissolved in water.

For example, hydrochloric acid forms hydrogen ions when it dissolves in water:

HCl_(aq) → H⁺_(aq) + Cl⁻_(aq)

The presence of hydrogen ions gives acidic solutions many of their characteristic properties.

Acids are found in laboratories, industries, foods, drinks, and even inside the human body.

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Acids in Everyday Life

Many familiar substances contain acids.

Citric Acid

Citric acid occurs naturally in citrus fruits such as:

• Lemons
• Limes
• Oranges
• Grapefruits

Citric acid contributes to their sour taste.

Acetic Acid

Acetic acid is the main acid found in vinegar.

Vinegar used in cooking is a dilute solution containing acetic acid.

Carbonic Acid

Carbonic acid forms when carbon dioxide dissolves in water.

It contributes to the acidity of carbonated drinks.

Hydrochloric Acid

Hydrochloric acid is found in the stomach.

It helps:

• Create acidic conditions for digestive enzymes.
• Break down food.
• Destroy many harmful microorganisms entering with food.

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Common Laboratory Acids

Several acids are commonly encountered in chemistry.

Hydrochloric acid, sulfuric acid, and nitric acid are especially important industrial acids.

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Properties of Acids

Acids have several characteristic properties.

Acids Have a pH Below 7

The pH scale is used to describe how acidic or alkaline a solution is.

A typical pH scale runs from:

0 ⟶ 14​

In general:

pH below 7 = acidic​

pH above 7 = alkaline​

The lower the pH, the more acidic the solution.

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Acids Can Be Corrosive

Some acids are corrosive.

This means they can damage materials and living tissue through chemical reactions.

Concentrated acids must therefore be handled carefully in laboratories and industry.

However, not every acidic substance is extremely dangerous. Foods such as oranges and vinegar are acidic but contain relatively low concentrations of acid.

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Acids React with Metals

Many acids react with certain metals.

The general reaction is:

acid + metal → salt + hydrogen​

For example:

2HCl + Mg → MgCl₂ + H₂

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When magnesium is placed in hydrochloric acid, bubbles of hydrogen gas are produced.

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Acids React with Bases

Acids can react with bases in a process called neutralisation.

The general reaction is:

acid + base → salt + water​

For example:

HCl + NaOH → NaCl + H₂O

Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water.

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Acids React with Carbonates

Acids also react with carbonates.

The general reaction is:

acid+carbonate→salt+water+carbon dioxide​

This reaction usually produces visible bubbling because carbon dioxide gas is released.

For example:

2HCl + CaCO₃ ​→ CaCl₂ + H₂O + CO₂​

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How Can We Identify an Acid?

We should never identify an unknown chemical by tasting it.

Instead, chemists use substances called indicators.

An indicator changes colour depending on the pH of a solution.

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Litmus

Litmus is a simple acid-base indicator.

In an acidic solution:

Blue litmus → Red​

Red litmus remains red in an acid.

Therefore, turning blue litmus paper red is evidence that a solution is acidic.

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Universal Indicator

Universal indicator can show a range of colours depending on pH.

Acidic solutions usually produce colours such as:

• Red
• Orange
• Yellow

Neutral solutions are usually green.

Alkaline solutions usually produce blue to purple colours.

Universal indicator therefore gives more information than litmus because it can provide an approximate pH value.

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Indicators and pH

Suppose three unknown solutions are tested.

Solution A can be identified as acidic because:

pH < 7​

and the universal indicator shows an acidic colour.

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Strong and Weak Acids

Not all acids behave in exactly the same way.

Some acids are described as strong acids, while others are weak acids.

A strong acid ionises almost completely when dissolved in water.

Examples include:

• Hydrochloric acid
• Nitric acid
• Sulfuric acid

A weak acid only partially ionises.

Examples include:

• Acetic acid
• Carbonic acid
• Citric acid

Strong and weak do not mean the same thing as concentrated and dilute.

Strength describes how much an acid ionises, while concentration describes how much acid is present in a certain volume of solution.

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Why Are Acids Important?

Acids are extremely important in both science and industry.

They are used to manufacture a huge variety of materials and chemicals.

Fertiliser Production

Nitric acid and sulfuric acid are important in the manufacture of fertilisers.

These fertilisers provide plants with nutrients needed for growth.

Batteries

Sulfuric acid is used as the electrolyte in traditional lead-acid car batteries.

It helps chemical reactions occur that allow the battery to supply electrical energy.

Manufacturing

Acids are used in the production and processing of:

• Metals
• Plastics
• Medicines
• Dyes
• Detergents
• Explosives
• Fertilisers

Food Industry

Acids are also used in food production.

For example, citric acid may be used to:

• Add a sour flavour.
• Control acidity.
• Help preserve some foods.

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Acids in the Human Body

Acids are also important in biology.

Hydrochloric acid in the stomach creates strongly acidic conditions.

These conditions allow enzymes such as pepsin to function effectively during protein digestion.

The acid also helps destroy many microorganisms that enter the digestive system with food.

Other acids are involved in important chemical reactions throughout living organisms.

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Worked Example

A student tests an unknown liquid.

The results are:

• Blue litmus paper turns red.
• Universal indicator turns orange.
• The pH is approximately 3.

Is the liquid acidic?

Yes.

There are several pieces of evidence:

1. Its pH is below 7.
2. It turns blue litmus red.
3. Universal indicator produces an acidic colour.

The solution can therefore be classified as an acidic solution.

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Common Mistakes

Mistake 1: All acids are extremely dangerous

Some concentrated acids are highly corrosive, but many everyday foods and drinks contain relatively dilute or weak acids.

Mistake 2: Acid means pH 7 or below

A solution with a pH of exactly 7 is neutral.

Acidic solutions have:

pH < 7​

 

Mistake 3: Strong means concentrated

These terms describe different properties.

Strong/weak describes the degree of ionisation.

Concentrated/dilute describes the amount of acid present in a given volume.

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Key Vocabulary

Acid – A substance that produces hydrogen ions when dissolved in water.

Hydrogen ion (H+) – The ion associated with acidic solutions.

pH – A measure used to describe how acidic or alkaline a solution is.

Indicator – A substance that changes colour depending on pH.

Litmus – An indicator that can distinguish acidic and alkaline solutions.

Universal indicator – An indicator that produces different colours across a range of pH values.

Corrosive – Able to chemically damage materials or living tissue.

Neutralisation – A reaction between an acid and a base.

Strong acid – An acid that ionises almost completely in water.

Weak acid – An acid that only partially ionises in water.

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Key Takeaways

• An acid produces H+ ions when dissolved in water.
• Acidic solutions have a pH below 7.
• Common acids include hydrochloric, sulfuric, nitric, acetic, and citric acids.
• Acids can react with metals, bases, and carbonates.
• Blue litmus paper turns red in an acidic solution.
• Universal indicator can be used to estimate the pH of a solution.
• Strong and weak acids differ in how completely they ionise in water.
• Acids have important uses in industry, food production, batteries, manufacturing, biology, and scientific research.