Acids, Alkalis and Corrosive Substances

Introduction

Acids and alkalis are among the most useful substances in chemistry and daily life. Vinegar, lemon juice and the acid in your stomach are acids; soap, baking soda solution and household cleaners are often alkaline. Yet in the laboratory, stronger or more concentrated acids and alkalis can burn skin and permanently damage eyes. This page explains why they can be hazardous, how concentration changes that hazard and how to work with them safely.

Core explanation

What makes them hazardous? Acids and alkalis react with many materials, including the proteins and fats that make up skin and eyes. Concentrated acids such as sulfuric, nitric and hydrochloric acid can cause severe chemical burns, and some release choking fumes. Concentrated alkalis such as sodium hydroxide are especially dangerous to the eyes because they damage tissue quickly and deeply, and at first the skin may only feel slippery or soapy, so people may not realise they have been harmed.

The pH scale. Acidity and alkalinity are measured on the pH scale, which usually runs from 0 to 14. Solutions with pH below 7 are acidic, pH 7 is neutral and pH above 7 is alkaline. Indicators such as litmus and universal indicator change colour to show pH. You will study pH in detail later; for safety it is enough to know that the further a solution's pH is from 7, the more likely it is to cause harm.

Concentration and hazard. The hazard of an acid or alkali depends strongly on its concentration. Very dilute hydrochloric acid may only irritate the skin, while concentrated hydrochloric acid causes burns and gives off irritating fumes. Labels therefore state the concentration, and the pictogram may change from an exclamation mark (irritant) at low concentration to the corrosion symbol at high concentration. Schools use the most dilute solution that works for each experiment.

Safe handling.

- Always wear eye protection; goggles are best for corrosive solutions. - Use the lowest concentration and smallest volume that will do the job. - Carry bottles with care, holding the base, and keep them upright. - Dilute concentrated acids by adding the acid slowly to water, never water to acid, because the process releases heat. - Neutralise and clean small spills as instructed by your teacher. - If acid or alkali touches the skin, rinse with plenty of cold running water immediately; for eye splashes, rinse at an eye-wash station for at least ten minutes and tell the teacher at once.

Neutralisation. Acids and alkalis react with each other to form a salt and water. This reaction is used to treat spills (using a suitable, weak neutralising substance chosen by the teacher) and in many everyday applications, such as antacid tablets that neutralise excess stomach acid.

Step-by-step reasoning

Before using an acid or alkali:

1. Read the label: name, concentration, pictograms and signal word. 2. Ask whether a more dilute solution could be used instead. 3. Put on eye protection before opening the bottle. 4. Use small volumes in stable containers. 5. Know where the eye-wash station and running water are.

Visual explanation

Substance Typical pH Hazard at school concentrations Key precaution --- --- --- --- Lemon juice About 2–3 Stings cuts and eyes Avoid eye contact Dilute hydrochloric acid Below 2 Irritant to corrosive (depends on concentration) Goggles; rinse splashes Pure water 7 None — Baking soda solution About 8–9 Mild Avoid eyes Dilute sodium hydroxide 13–14 Can seriously damage eyes Goggles essential

Real-world analogy

Acids and alkalis are like hot water. Warm water is pleasant to wash with, but boiling water scalds. It is the same substance at a different "strength". Similarly, a very dilute acid is harmless enough to flavour food, while a concentrated one can burn — so we always check concentration before handling.

Real-world example

Car batteries contain sulfuric acid, which is why they carry corrosion warnings and why mechanics wear eye protection when working with them. Similarly, drain cleaners containing concentrated sodium hydroxide can cause serious injuries if splashed, so their labels instruct users to wear gloves and eye protection.

Why?

Why are alkali splashes in the eye treated so seriously? Alkalis react with the fats in cell membranes and can penetrate deeper into the eye than many acids, continuing to cause damage until they are washed away. Immediate, prolonged rinsing with water removes the alkali and limits injury, which is why eye-wash stations exist in laboratories.

Common misconception

Many students think only acids are dangerous and that alkalis are safe because they feel soapy. Concentrated alkalis can be just as dangerous as acids, and often more dangerous to the eyes. The soapy feeling is actually the alkali reacting with fats in your skin.

Worked example

Question: A method says "use 1 mol/dm³ hydrochloric acid", but only 5 mol/dm³ is available. What should be done?

Reasoning: The available acid is more concentrated and more hazardous; it should be diluted safely by the teacher or technician.

Answer: Do not use the stronger acid directly. Ask the teacher; the acid can be diluted by adding one volume of 5 mol/dm³ acid slowly to water to make five volumes in total, with eye protection and stirring, which gives 1 mol/dm³ acid.

Quick check

1. What should you do immediately if a dilute acid splashes onto your hand? Answer: Rinse it with plenty of cold running water and tell the teacher.

Exam focus

Exam questions on corrosive substances usually expect: eye protection (goggles), using dilute solutions, adding acid to water, rinsing splashes with plenty of water and reporting to the teacher. Explain how concentration changes the hazard and pictogram.

Advanced insight

The difference between a "strong" and a "concentrated" acid is important later in chemistry. Strength describes how completely an acid releases hydrogen ions in water; concentration describes how much acid is dissolved in a volume. A weak acid can be concentrated, and a strong acid can be dilute. Both properties affect hazard and pH.

Summary

Acids (pH below 7) and alkalis (pH above 7) can damage skin and eyes, and concentrated alkalis are especially dangerous to eyes. Hazard increases with concentration, so the most dilute suitable solution is used. Always wear eye protection, handle small volumes carefully, dilute by adding acid to water and rinse splashes immediately with plenty of water. Acids and alkalis neutralise each other to form a salt and water.

Practice questions

1. What pH values indicate an alkaline solution? Answer: pH values above 7. 2. Why might an alkali splash on skin go unnoticed at first? Answer: It may only feel slippery or soapy at first, even though it is reacting with and damaging the skin. 3. Why do schools use dilute rather than concentrated acids? Answer: Dilute acids are less hazardous, reducing the harm if a spill or splash happens. 4. What are the products when an acid reacts with an alkali? Answer: A salt and water.