Common Laboratory Acids and Bases

Introduction

A school laboratory stocks only a handful of acids and alkalis, yet they turn up again and again in experiments, textbooks and exam papers. Knowing their names and formulas lets you read chemical equations with confidence. Just as important is recognising the warning symbols printed on their bottles. Those small red diamonds tell you, at a glance, what could happen if a substance is mishandled, and they are the first step in working safely with any chemical.

Core explanation

The three common mineral acids. These are strong acids made from mineral sources rather than living things:

Name Formula Negative ion left in water --- --- --- Hydrochloric acid HCl chloride, Cl⁻ Sulfuric acid H₂SO₄ sulfate, SO₄²⁻ Nitric acid HNO₃ nitrate, NO₃⁻

A common weak acid. Ethanoic acid, CH₃COOH, is the acid in vinegar. It releases far fewer hydrogen ions than the mineral acids at the same concentration, so it is milder, although concentrated ethanoic acid is still corrosive. Citric acid is also used, often as a solid.

The common alkalis.

Name Formula Notes --- --- --- Sodium hydroxide NaOH strong alkali, supplied as pellets or solution Potassium hydroxide KOH strong alkali, similar to sodium hydroxide Calcium hydroxide Ca(OH)₂ only slightly soluble; its solution is limewater Ammonia solution NH₃(aq) weak alkali with a sharp smell

Insoluble bases. Copper(II) oxide (CuO), a black powder, and magnesium oxide (MgO), a white powder, are used to make salts. Calcium carbonate (CaCO₃), found as marble chips, also neutralises acids.

Hazard pictograms. Chemical containers carry internationally agreed pictograms: a black symbol inside a white diamond with a red border. The ones you meet most often with acids and alkalis are:

- Corrosive — liquid dripping from two test tubes onto a hand and a metal bar, eating into both. Found on concentrated acids and alkalis, and on sodium hydroxide solution above a fairly low concentration. - Exclamation mark — used for irritants and other lower-level hazards. Many dilute acids and alkalis carry this symbol instead of the corrosive one. - Skull and crossbones — acutely toxic ; harmful in small amounts if swallowed, breathed in or absorbed through skin. - Flame over a circle — oxidising ; can make other materials burn more fiercely. Concentrated nitric acid carries this.

Concentration changes the label. Hydrochloric acid might be labelled corrosive when concentrated but only irritant when dilute. The hazard depends on how much acid or alkali is in each cubic decimetre of solution, so always read the label on the specific bottle you are using.

Step-by-step reasoning

To identify a laboratory chemical and its dangers:

1. Read the name and formula on the label. 2. Decide whether it is an acid, an alkali or an insoluble base. 3. Check whether it is concentrated or dilute. 4. Look at the pictograms and identify each hazard. 5. Choose protective measures, such as eye protection, to match.

Visual explanation

Picture a shelf of brown and clear bottles. Each has a label showing the name, formula and concentration, and one or more red-bordered diamonds. The concentrated sulfuric acid bottle shows the corrosive symbol; the dilute hydrochloric acid bottle shows an exclamation mark; the concentrated nitric acid bottle shows both corrosive and flame-over-circle symbols.

Real-world analogy

Hazard pictograms are like road signs. You do not need to read a paragraph to understand a sign showing a sharp bend or falling rocks; one glance tells you to take care. Chemical pictograms do the same job for anyone handling a bottle, whatever language they speak.

Real-world example

The same pictograms appear on household products. Drain cleaners containing sodium hydroxide carry the corrosive symbol, while many bathroom sprays carry the exclamation mark. Learning the symbols in the laboratory helps you read labels safely at home.

Why?

Why are pictograms used rather than written warnings alone? They are understood across languages and can be recognised instantly, even from a distance or in an emergency. A single worldwide system, the Globally Harmonized System, means a bottle made in one country is labelled the same way everywhere.

Common misconception

"Alkalis are safe because they are not acids." Concentrated sodium hydroxide is highly corrosive and can cause permanent eye damage very quickly. Strong alkalis must be treated with the same respect as strong acids.

Worked example

Question: A bottle is labelled "Sulfuric acid, 0.5 mol/dm³" with an exclamation-mark pictogram. Give its formula, the negative ion it forms in water and the hazard shown.

Reasoning: Sulfuric acid has the formula H₂SO₄; removing the hydrogen ions leaves the sulfate ion. The exclamation mark indicates an irritant at this dilute concentration.

Answer: H₂SO₄; sulfate ion, SO₄²⁻; irritant.

Quick check

1. What is the formula of nitric acid? Answer: HNO₃.

Exam focus

Learn the names and formulas of hydrochloric, sulfuric and nitric acids and of sodium hydroxide, calcium hydroxide and ammonia. Be able to recognise the corrosive, irritant (exclamation mark), toxic and oxidising pictograms and explain what each means.

Advanced insight

Sulfuric acid has two hydrogens that can be released, so it can form two kinds of salt: sulfates, such as Na₂SO₄, and hydrogensulfates, such as NaHSO₄. Hydrochloric and nitric acids have only one releasable hydrogen each. Ethanoic acid has four hydrogen atoms, yet only the one in the –COOH group is released as H⁺.

Summary

The main laboratory acids are hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃), with ethanoic acid (CH₃COOH) as a common weak acid. The main alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂) and ammonia solution. Hazard pictograms warn of corrosive, irritant, toxic and oxidising dangers, and the label depends on concentration.

Practice questions

1. Give the names of the acids with formulas HCl and H₂SO₄. Answer: Hydrochloric acid and sulfuric acid. 2. Describe the corrosive hazard pictogram and state what it means. Answer: It shows liquid from test tubes eating into a hand and a metal surface; it means the substance can destroy skin and damage materials on contact. 3. Why might the same acid carry a corrosive symbol on one bottle and an exclamation mark on another? Answer: The concentration differs; the concentrated solution is corrosive, while the dilute solution is only an irritant. 4. Name an insoluble base used in the laboratory and give its formula. Answer: Copper(II) oxide, CuO (magnesium oxide, MgO, is also acceptable).