Properties of Acids

Acids are an important group of chemicals with characteristic physical and chemical properties. They are found in foods, the human body, laboratories, batteries, and many industrial processes.

Some acids are relatively weak and safe to encounter in everyday life, while others are highly corrosive and must be handled carefully.

What Is an Acid?

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

For example, hydrochloric acid produces hydrogen ions and chloride ions:

HCl → H⁺ + Cl⁻

It is the presence of hydrogen ions that gives acidic solutions many of their characteristic properties.

Common acids include:

Physical Properties of Acids

Acidic solutions share several common properties.

Sour Taste

Many weak acids found naturally in foods have a sour taste.

For example:

• lemons contain citric acid
• vinegar contains ethanoic acid
• some soft drinks contain carbonic or phosphoric acid

However, chemicals in a laboratory should never be tasted to determine whether they are acidic.

They Conduct Electricity

Acidic solutions can conduct electricity because they contain mobile ions.

The H⁺ ions and other ions in the solution can move and carry electrical charge.

pH Below 7

Acidic solutions have a pH below 7.

A lower pH generally indicates a greater concentration of H⁺ ions.

Acids and Indicators

An indicator is a substance that changes colour depending on whether a solution is acidic, neutral, or alkaline.

Indicators provide a quick way of identifying acidic solutions.

A simple rule to remember is:

Acids turn blue litmus paper red.

Universal indicator gives more information because its colour can be compared with a pH scale.

Strongly acidic solutions usually appear red, while less acidic solutions may appear orange or yellow.

Chemical Properties of Acids

Acids undergo several characteristic chemical reactions.

Acids React with Metals

Many acids react with reactive metals to produce a salt and hydrogen gas.

acid + metal → salt + hydrogen

For example:

2HCl + Mg → MgCl₂ + H₂

Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas.

You may observe:

• bubbles or fizzing
• the metal gradually disappearing
• an increase in temperature

Acids React with Bases

Acids react with bases in neutralisation reactions.

acid + base → salt + water

For example:

2HCl + MgO → MgCl₂ + H₂O

The base neutralises the acid.

Acids React with Carbonates

Acids also react with carbonates to produce a salt, water and carbon dioxide.

acid + carbonate → salt + water + carbon dioxide

For example:

2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

The production of carbon dioxide causes visible fizzing or bubbling.

Corrosive Acids

Some acids are corrosive.

A corrosive substance can chemically damage materials it contacts, including living tissue.

Concentrated acids such as:

• sulfuric acid
• nitric acid
• hydrochloric acid

can cause serious burns to the skin and severe damage to the eyes.

Corrosiveness should not be confused with simply being acidic. Many weak or dilute acidic solutions, such as fruit juice, can be handled safely under normal conditions.

Hazards Associated with Acids

When working with acids in the laboratory, several hazards must be considered.

Acids may:

• cause chemical burns
• damage the eyes
• damage clothing or surfaces
• produce harmful vapours
• react strongly with other chemicals
• react with metals and produce flammable hydrogen gas

Appropriate laboratory precautions include wearing safety goggles, using small quantities and following instructions for handling and dilution.

Acid spills should always be dealt with according to laboratory safety procedures.

Strong and Weak Acids

Not all acids behave in exactly the same way.

Acids can be classified as strong or weak depending on how completely they ionise in water.

Strong Acids

A strong acid ionises almost completely in water.

Examples include:

• hydrochloric acid (HCl)
• nitric acid (HNO₃)
• sulfuric acid (H₂SO₄)

A simplified representation is:

HCl → H⁺ + Cl⁻

Most of the HCl particles separate into ions.

Weak Acids

A weak acid only partially ionises in water.

Examples include:

• ethanoic acid
• carbonic acid
• many organic acids

For ethanoic acid:

CH₃COOH ⇌ H⁺ + CH₃COO⁻

The reversible arrow indicates that only some of the acid molecules have formed ions.

Strength Is Not the Same as Concentration

This is an important distinction.

Strength describes how completely an acid ionises.

Concentration describes how much acid is present in a certain volume of solution.

Therefore, it is possible to have:

• a dilute strong acid
• a concentrated strong acid
• a dilute weak acid
• a concentrated weak acid

For example, hydrochloric acid remains a strong acid even when it has been diluted with a large amount of water.

Comparing Acids

Consider equal-concentration solutions of hydrochloric acid and ethanoic acid.

Hydrochloric acid ionises almost completely:

HCl → H⁺ + Cl⁻

Ethanoic acid only partially ionises:

CH₃COOH ⇌ H⁺ + CH₃COO⁻

Therefore, the hydrochloric acid solution contains a greater concentration of H⁺ ions and generally has a lower pH.

This is why pH measurements can help us compare acidic solutions, provided factors such as concentration are also considered.

Did You Know?

Your stomach contains hydrochloric acid. This creates the strongly acidic conditions needed for digestion and helps destroy many microorganisms entering the digestive system.

The stomach itself is protected by a layer of mucus. Without this protective barrier, the acidic contents could damage the stomach tissue.

Key Terms

Acid: A substance that produces H⁺ ions in water.

Indicator: A substance that changes colour depending on pH.

Corrosive: Able to chemically damage materials or living tissue.

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

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

Concentration: The amount of dissolved substance present in a given volume of solution.

Neutralisation: A reaction between an acid and a base.

Key Takeaways

• Acids produce H⁺ ions in aqueous solution.
• Acidic solutions have a pH below 7.
• Acids turn blue litmus paper red.
• Acids have characteristic reactions with metals, bases and carbonates.
• Some acids are corrosive and can cause serious chemical burns.
• Strong acids ionise almost completely, while weak acids ionise only partially.
• Acid strength and acid concentration are not the same thing.
• Hydrochloric, sulfuric and nitric acids are common strong acids, while ethanoic acid is a common weak acid.