Safety with Acids, Bases and Salts

Introduction

Acids and alkalis are among the most useful substances in a laboratory and in the home, but they can also cause serious injuries. A splash of concentrated alkali in the eye can cause lasting damage within seconds. Knowing the hazard labels, understanding why substances are dangerous and following sensible principles turns hazardous chemistry into safe chemistry. This page focuses on understanding hazards rather than memorising rules.

Core explanation

Hazard pictograms. Chemical containers carry the Globally Harmonised System (GHS) pictograms: black symbols in a red-bordered diamond. The ones you will meet most with acids, bases and salts are:

Pictogram Meaning Typical examples --- --- --- Corrosion (liquid eating into a hand and a surface) Causes severe burns and eye damage; attacks metals Concentrated sulfuric, nitric and hydrochloric acids; sodium hydroxide solution above about 0.5 mol/dm³ Exclamation mark Irritant, harmful, may cause skin or eye irritation Dilute acids and alkalis at moderate concentration; many salts Skull and crossbones Acutely toxic Soluble barium salts Environment (dead fish and tree) Harmful to aquatic life Copper(II) sulfate Flame over circle Oxidising; can make fires more intense Nitrates such as ammonium nitrate, concentrated nitric acid

Why concentration matters. Hazard depends strongly on concentration. Very dilute hydrochloric acid may carry no hazard label, a moderate concentration is an irritant, and concentrated acid is corrosive. This links to the idea that concentration and strength are different: a weak acid can still be hazardous if concentrated, and a strong acid can be mild if very dilute.

Why alkalis are especially dangerous to eyes. Acids tend to coagulate proteins at the surface, which can partly limit how deep they penetrate. Strong alkalis break down fats and proteins in tissue, so they keep penetrating deeper. This is why eye protection is essential with sodium hydroxide, even at concentrations that feel harmless on the skin.

Heat on dilution. Mixing concentrated sulfuric acid or solid sodium hydroxide with water is strongly exothermic. The principle " add acid to water, never water to acid " follows from this. If water is poured onto concentrated acid, the small amount of water can boil instantly at the surface and spit hot, concentrated acid out of the container. If acid is added slowly to a large volume of water, the water absorbs the heat and the acid is diluted as it enters. Dilution of concentrated acids is a job for trained staff with proper equipment, not for students.

Neutralisation also releases heat. Neutralisation is exothermic, so neutralising a concentrated spill with a strong base would release a lot of heat and create a second hazard. Spills are dealt with using mild neutralisers, such as sodium hydrogencarbonate for acid spills, and plenty of water.

Safe practice principles.

- Wear eye protection whenever acids, alkalis or their solutions are in use; wear gloves when the risk assessment says so. - Use the lowest concentration that does the job. - Keep containers labelled and stoppered; store acids away from alkalis and from oxidising substances. - Never taste chemicals or smell them directly. - Report all spills and splashes to the person in charge.

First aid principles. For splashes on skin or in eyes, the key action is to flush with plenty of cool running water for a prolonged time (at least 10 minutes for the eye) and seek help. Attempting to "neutralise" a splash on the body with another chemical is not done, because it wastes time and the reaction releases heat.

Step-by-step reasoning

Reading a hazard label before using a chemical:

1. Read the name and concentration. 2. Identify each pictogram and what it means. 3. Read the hazard statements, such as "causes severe skin burns and eye damage". 4. Decide what protection and precautions are needed. 5. Check where to find the eyewash and how to deal with spills.

Visual explanation

Picture two beakers. In the first, water is being poured onto a thin layer of concentrated acid; a small cloud of steam and droplets bursts upward. In the second, acid trickles slowly down a glass rod into a large beaker of water; a thermometer shows only a small rise and nothing splashes. Captions read "wrong way" and "right way".

Real-world analogy

Adding acid to water is like pouring a cup of boiling water into a full bath: the bath barely warms. Adding water to acid is like pouring a teaspoon of water onto a hot frying pan: it spits violently, because the small amount of water cannot absorb the heat.

Real-world example

Drain cleaners often contain concentrated sodium hydroxide or sulfuric acid. Accidents happen when people mix two different drain products or pour water onto the solid granules in a confined drain, causing spitting and burns. Labels therefore warn users never to mix products and to wear gloves and eye protection.

Why?

Why does a spill of concentrated acid need a mild neutraliser rather than a strong alkali? Neutralisation releases heat. A mild base such as sodium hydrogencarbonate reacts steadily and cannot itself cause burns if used in excess, whereas a strong alkali could leave a new corrosive hazard and heat the spill.

Common misconception

"Weak acids are safe to handle." Strength describes how much an acid ionises, not how hazardous it is. Concentrated ethanoic acid (glacial acetic acid) is a weak acid but is corrosive and flammable.

Worked example

Question: A bottle shows the corrosion pictogram and is labelled 2.0 mol/dm³ sodium hydroxide. State two precautions and explain why eye protection is especially important.

Reasoning: The pictogram means it destroys tissue. Alkalis penetrate tissue by breaking down fats and proteins.

Answer: Wear eye protection and gloves, and use it in small volumes with the container stoppered when not in use. Eye protection is vital because alkali keeps penetrating the eye and can cause permanent damage.

Quick check

1. Which should be added to which when diluting a concentrated acid? Answer: The acid should be added slowly to water, never water to acid.

Exam focus

You may be shown GHS pictograms and asked what they mean: learn corrosive, irritant/harmful, toxic, oxidising and environmental hazard. Explain "add acid to water" using the idea of an exothermic process. Emphasise that hazard depends on concentration as well as on identity.

Advanced insight

Hydrofluoric acid is a weak acid but is one of the most dangerous acids in industry. Fluoride ions pass through skin and bind calcium in tissues, so injuries may not hurt at first but can become severe. It is a striking example of why the hazard of a substance cannot be predicted from acid strength alone.

Summary

GHS pictograms warn of corrosive, irritant, toxic, oxidising and environmental hazards. Concentrated acids and alkalis are corrosive; strong alkalis are particularly damaging to eyes. Hazard depends on concentration, not just strength. Diluting concentrated acid is exothermic, so acid is added to water. Spills use mild neutralisers, and splashes on the body are flushed with plenty of water.

Practice questions

1. What does the corrosion pictogram warn about? Answer: The substance can destroy living tissue, causing severe burns and eye damage, and can attack metals. 2. Explain why water should not be added to concentrated sulfuric acid. Answer: Dilution is very exothermic; a small amount of water can boil instantly on contact and spit hot concentrated acid out of the container. 3. Why is sodium hydrogencarbonate suitable for treating small acid spills? Answer: It is a mild base that neutralises the acid steadily without releasing excessive heat, and excess of it is not corrosive. 4. A student says dilute hydrochloric acid is harmless because it is dilute. Evaluate this statement. Answer: It is less hazardous than concentrated acid but may still be an irritant, especially to eyes, so eye protection is still needed.