Acids Reacting with Metals

One of the classic chemistry experiments is dropping a piece of metal into an acid and watching it fizz. The bubbles are hydrogen gas, the metal slowly disappears, and a new chemical called a salt is formed. This reaction is one of the simplest and most useful in chemistry. It explains why iron railings rust faster near acid rain, why a pickled coin gets shiny when dropped in vinegar and why the inside of your stomach is built to handle some seriously powerful chemistry.

  • What happensAcid + metal -> salt + hydrogenBubbles, fizzing, metal dissolves
  • Gas given offHydrogen (H2)Lights with a squeaky pop
  • Useful metalsMagnesium, zinc, ironReact with acids
  • Quiet metalsCopper, silver, goldDo not react with most acids
  • Speed of reactionDepends on metal reactivitySee the reactivity series
  • Salt formedAcid sets the typeChloride from HCl, sulfate from H2SO4

The basic reaction

The general word equation is:

Acid + metal -> salt + hydrogen

For example, hydrochloric acid plus magnesium produces magnesium chloride plus hydrogen gas:

  • 2 HCl + Mg -> MgCl2 + H2

The magnesium ribbon fizzes vigorously, dissolves and is replaced by a clear, slightly warm solution. The bubbles that float to the surface are pure hydrogen gas. Light them with a flame and they make a satisfying squeaky pop.

The reactivity series

Not all metals react with acids. Chemists rank metals by how easily they give up electrons (how reactive they are) in something called the reactivity series:

  • Potassium (most reactive)
  • Sodium
  • Calcium
  • Magnesium
  • Aluminium
  • Carbon (not a metal, included as a reference)
  • Zinc
  • Iron
  • Tin
  • Lead
  • Hydrogen (not a metal, included as a reference)
  • Copper
  • Silver
  • Gold (least reactive)

Metals above hydrogen in this list will react with dilute acid. Metals below hydrogen will not. That is why iron rusts but gold rings stay shiny forever. Gold is so unreactive that it survives nearly anything chemistry can throw at it.

How fast?

The higher up the reactivity series, the more vigorously the metal reacts:

  • Potassium and sodium: react so violently with even a weak acid that they would explode. Never tried in school labs.
  • Magnesium: bubbles vigorously, dissolves quickly, gets very warm.
  • Zinc: bubbles steadily, dissolves more slowly than magnesium.
  • Iron: bubbles slowly, takes a while to dissolve completely.
  • Tin and lead: very slow.
  • Copper, silver, gold: no reaction at all with dilute hydrochloric or sulfuric acid.
Fact Some metals appear unreactive but are actually protected by a thin invisible layer of oxide on the surface. Aluminium is one. In theory it should react vigorously with acid, but the protective oxide layer keeps the acid away from the metal. Scratch the layer off and pour acid on the fresh metal underneath, and it will react quickly. The protective oxide is also why aluminium does not rust like iron does.

What salts are made

The salt produced depends on which acid was used:

  • Hydrochloric acid (HCl) makes a chloride salt (sodium chloride, zinc chloride, iron chloride etc.)
  • Sulfuric acid (H2SO4) makes a sulfate salt (sodium sulfate, magnesium sulfate, copper sulfate etc.)
  • Nitric acid (HNO3) makes a nitrate salt (sodium nitrate, magnesium nitrate, copper nitrate etc.)
  • Phosphoric acid (H3PO4) makes a phosphate salt.

Many of these salts are useful chemicals. Copper sulfate is sky-blue and is used in fungicides and electroplating. Magnesium sulfate (Epsom salts) is sold for soothing baths and as a laxative. Iron chloride is used to make printed circuit boards.

Where you see this reaction

  • Cleaning copper coins: vinegar (a weak acid) reacts very slowly with the dark copper oxide layer on old coins, leaving them shiny again. This is more of an acid-oxide reaction than an acid-metal one, but it follows similar chemistry.
  • Acid rain on iron railings: traces of sulfuric and nitric acid in the rain slowly react with the iron, speeding up rusting.
  • Indigestion and dental erosion: hydrochloric acid in your stomach acts on metals in your food (in tiny amounts, intentionally) to help release minerals. Fizzy drink acids slowly attack metal fillings in teeth over many years.
  • Battery acid leaks: when an old battery leaks, the acid attacks any metal it touches, eating away contacts and wires.
  • Pickling steel: factories dip new steel sheets into dilute acid to dissolve the dark oxide layer, leaving a clean shiny surface ready for painting or galvanising.
Did you know? The squeaky pop test for hydrogen is one of the oldest and most reliable chemistry tests. Light a wooden splint, hold it at the mouth of a tube containing hydrogen, and you will hear a loud, sharp pop. The pop is the sound of the hydrogen burning in air, combining with oxygen so quickly that the rapid expansion of hot water vapour pushes the air aside, like a tiny explosion. Astronauts use exactly the same reaction (only much bigger) to power some rockets.

Why no reaction with copper or gold?

For an acid to react with a metal, the metal must be reactive enough to push hydrogen out of the acid. Copper and gold sit below hydrogen in the reactivity series, so they cannot do this. They simply sit there, untouched, when dropped into dilute acid. This is one of the reasons gold has been so prized through history. It does not tarnish, rust or dissolve. Ancient gold coins and jewellery look almost exactly as they did when they were made thousands of years ago.

Very strong oxidising acids (like concentrated nitric acid, or "aqua regia", a mix of nitric and hydrochloric acid) can attack copper and even gold. These are special cases used in industry and chemistry labs, not normal acid-metal reactions.

Try this Drop a clean iron nail into a small cup of vinegar (with adult permission). After a few hours, look carefully: tiny bubbles will be forming on the surface of the nail. Those are hydrogen bubbles. Leave the nail for a few days and you will see the vinegar slowly turn pale green as iron from the nail dissolves into the acid as iron acetate. If you then put a clean copper coin into the green solution, you will see a thin layer of orange-brown copper deposit on the iron (the more reactive iron displaces the copper from the solution). One simple experiment, three different reactions.
Deeper dive: how acid-metal reactions made the Industrial Revolution

Acid-metal chemistry helped power the early Industrial Revolution. In the 1700s, factories needed large amounts of clean iron and zinc, and the chemistry of acids made them possible.

One of the most important early uses was making hydrogen gas for balloons. The first manned hydrogen balloon flight, in Paris in 1783, was made possible by dripping sulfuric acid onto iron filings to produce hydrogen on a huge scale. Whole rooms full of barrels were used to fill a single balloon. The same chemistry was used in the early 1900s to fill the Hindenburg and other passenger airships, until safer helium replaced hydrogen after several disasters.

Another use was extracting metals from their ores. Iron, copper and zinc could all be purified by dissolving impure metal in acid, then either evaporating to get pure salts or running an electric current through the solution to plate out pure metal on an electrode. This last process, called electroplating, became hugely important in industry and is still used today to coat metal objects with thin layers of decorative or protective metal (silver-plated cutlery, gold-plated electrical contacts, chromium-plated car parts).

Modern industries still rely on acid-metal reactions. Steel-makers wash new sheets in acid before painting them. Mobile phone makers etch tiny circuit patterns onto copper using acid. Even space rockets use catalysed acid-metal reactions inside fuel cells to generate clean electrical power. The simple bubbling reaction you can demonstrate in five minutes with a school flask underpins industries worth hundreds of billions of pounds.

For more, see what is an acid and oxidation and rusting.