More reactive metals form positive ions more readily

1. When a metal reacts with water or a dilute acid, its atoms lose electrons and become positive ions.
   1. Magnesium atoms become ${Mg}^{2+}$ ions, while the hydrogen given off comes from the water or the acid.
2. A metal whose atoms form positive ions very readily, such as potassium, reacts vigorously.
3. A metal whose atoms form positive ions only with difficulty, such as copper, reacts slowly or not at all.
4. Placing metals in order of this tendency gives the reactivity series: potassium, sodium, lithium, calcium, magnesium, aluminium, zinc, iron, copper, silver and gold.
   1. Hydrogen is often placed between iron and copper as a reference point, because metals above it react with dilute acids to release hydrogen.

Reactions with water rank the most reactive metals

1. A reactive metal reacts with water to form a metal hydroxide and hydrogen: $$2K(s)+2H_{2}O(l)\to 2KOH(aq)+H_2(g)$$
2. Potassium reacts so vigorously that the hydrogen catches fire, and sodium fizzes rapidly as it melts into a ball.
3. Lithium and calcium fizz steadily, and calcium forms a cloudy solution because calcium hydroxide is only slightly soluble.
4. Magnesium reacts only very slowly with cold water, giving a few bubbles over several days.
5. Zinc, iron and copper show no visible reaction with cold water.
6. Water therefore separates the most reactive metals well but cannot rank the less reactive ones, which need another test.

Reactions with dilute acids rank the middle metals

1. A metal above hydrogen reacts with a dilute acid to form a salt and hydrogen: $$Mg(s)+2HCl(aq)\to {MgCl}_2(aq)+H_2(g)$$
2. With the same dilute acid, magnesium fizzes vigorously, zinc fizzes steadily and iron gives slow bubbles.
   1. The faster the bubbles of hydrogen form, the more readily the metal forms positive ions.
3. Zinc reacts with dilute sulfuric acid in the same way: $$Zn(s)+H_2{SO}_4(aq)\to {ZnSO}_4(aq)+H_2(g)$$
4. Copper, below hydrogen, shows no reaction with dilute hydrochloric or sulfuric acid.
5. The gas can be confirmed as hydrogen because it burns with a squeaky pop when a lit splint is held at the mouth of the tube.

More reactive metals displace less reactive metals from salts

1. A metal that forms positive ions more readily takes the place of a less reactive metal in its compound.
2. Magnesium displaces copper from copper(II) sulfate solution: $$Mg(s)+{CuSO}_4(aq)\to {MgSO}_4(aq)+Cu(s)$$
   1. The blue colour fades, a pink-brown coating of copper forms on the magnesium and the solution gets warmer.
3. Copper does not react with magnesium sulfate solution, because copper is less reactive than magnesium.
4. Magnesium also displaces iron from iron(II) sulfate solution, and the iron that forms can be picked up with a magnet wrapped in cling film.
5. The further apart two metals are in the series, the larger the temperature rise when they react.
6. Displacement also ranks metals that do not react with dilute acids, since copper displaces silver from silver nitrate solution.

Deducing a reactivity order from fair comparisons

1. Use the same size and form of each metal, such as equal-mass strips or powders, because surface area also changes how fast a metal reacts.
2. Keep the volume and concentration of the acid or solution, and the starting temperature, the same for every metal.
3. Change only the metal, so that any difference in the results comes from its reactivity.
4. Rank the metals using a measured result, such as the temperature rise, the volume of hydrogen collected in the first minute or the number of displacement reactions each metal gives.
5. The metal with the most reactions, the largest temperature rise or the fastest gas production goes at the top of the order.