A Metal Can Only Push Out One That Is Weaker Than Itself
Learn to recognise combination reactions, sort decomposition into thermal, electrolytic and photochemical, use the reactivity series to predict whether a displacement will happen at all, and write double displacement equations.
Why does an iron nail turn copper-coloured but a copper wire never turns silvery in iron solution?
Because displacement only works one way — a more reactive metal can push out a less reactive one, and never the reverse.
Stand an iron nail in blue copper sulphate solution and within minutes it is coated red-brown with copper. Stand a copper wire in green iron sulphate solution and nothing at all happens, however long you wait.
Iron is above copper in the reactivity series, so it can take copper's place. Copper is below iron, so it cannot take iron's. The reactivity series is what lets you predict this before touching a test tube. This page covers the second part of the ICSE Class 8 Chemistry chapter on chemical reactions: the four main types and how to write each one.
Stand an iron nail in blue copper sulphate solution and within minutes it is coated red-brown with copper. Stand a copper wire in green iron sulphate solution and nothing at all happens, however long you wait.
Iron is above copper in the reactivity series, so it can take copper's place. Copper is below iron, so it cannot take iron's. The reactivity series is what lets you predict this before touching a test tube. This page covers the second part of the ICSE Class 8 Chemistry chapter on chemical reactions: the four main types and how to write each one.
What is a combination reaction?
A reaction in which two or more substances combine to form a single product. It is also called a synthesis reaction, and its pattern is .
Element with element:
Compound with compound:
How to recognise one instantly. Count the products. A combination reaction has exactly one product, and more than one reactant. If you can see two products, it is not a combination.
Most combination reactions are exothermic. Magnesium burning, carbon burning and quicklime slaking all pour out heat, because forming new bonds releases more energy than breaking the old ones cost. The ammonia synthesis is the notable one needing pressure and a catalyst to proceed at a useful rate.
The everyday case worth knowing. Adding water to quicklime to make slaked lime is a combination reaction used on every building site for whitewash and mortar — and the heat it gives out is enough to make the water hiss and steam. It is the same reaction that makes handling quicklime hazardous.
And its exact opposite is the next section: a decomposition reaction runs the pattern backwards, splitting one substance into several.
Element with element:
Compound with compound:
How to recognise one instantly. Count the products. A combination reaction has exactly one product, and more than one reactant. If you can see two products, it is not a combination.
Most combination reactions are exothermic. Magnesium burning, carbon burning and quicklime slaking all pour out heat, because forming new bonds releases more energy than breaking the old ones cost. The ammonia synthesis is the notable one needing pressure and a catalyst to proceed at a useful rate.
The everyday case worth knowing. Adding water to quicklime to make slaked lime is a combination reaction used on every building site for whitewash and mortar — and the heat it gives out is enough to make the water hiss and steam. It is the same reaction that makes handling quicklime hazardous.
And its exact opposite is the next section: a decomposition reaction runs the pattern backwards, splitting one substance into several.
How do you classify a decomposition reaction?
By what supplies the energy to break the substance apart. A decomposition follows the pattern , and there are three kinds.
Thermal decomposition — brought about by heat.
Limestone giving quicklime and carbon dioxide, used to make lime in a kiln.
The lead nitrate reaction gives brown fumes of nitrogen dioxide and a crackling sound, which makes it a memorable demonstration.
Electrolytic decomposition — brought about by electricity, and always called electrolysis.
Hydrogen collects at the cathode and oxygen at the anode, in a **volume ratio of ** — a ratio you can read straight off the balanced equation.
for molten sodium chloride, which is how sodium metal is obtained.
Photochemical decomposition — brought about by light.
White silver chloride turns grey in sunlight as metallic silver is set free, which is why silver salts are kept in dark bottles and why they were the basis of photographic film.
Nearly all decomposition reactions are endothermic, which follows directly from the previous part: energy has to be put in to break a stable substance apart. This is why a decomposition stops the moment you remove the heat, the current or the light.
Combination and decomposition as a pair. is combination and exothermic; is decomposition and endothermic. Recognising one type tells you a good deal about the energy flow before you do anything else.
Thermal decomposition — brought about by heat.
Limestone giving quicklime and carbon dioxide, used to make lime in a kiln.
The lead nitrate reaction gives brown fumes of nitrogen dioxide and a crackling sound, which makes it a memorable demonstration.
Electrolytic decomposition — brought about by electricity, and always called electrolysis.
Hydrogen collects at the cathode and oxygen at the anode, in a **volume ratio of ** — a ratio you can read straight off the balanced equation.
for molten sodium chloride, which is how sodium metal is obtained.
Photochemical decomposition — brought about by light.
White silver chloride turns grey in sunlight as metallic silver is set free, which is why silver salts are kept in dark bottles and why they were the basis of photographic film.
Nearly all decomposition reactions are endothermic, which follows directly from the previous part: energy has to be put in to break a stable substance apart. This is why a decomposition stops the moment you remove the heat, the current or the light.
Combination and decomposition as a pair. is combination and exothermic; is decomposition and endothermic. Recognising one type tells you a good deal about the energy flow before you do anything else.
How does the reactivity series predict a displacement reaction?
A single displacement reaction is one in which a more reactive element displaces a less reactive one from its compound. Its pattern is .
The reactivity series of metals, most reactive first:
K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au
Hydrogen is placed in the list even though it is not a metal, because it marks the important boundary for reactions with acids.
The rule. A metal higher in the series displaces one lower in the series. A metal lower down can do nothing to one above it.
Worked predictions:
Iron is above copper, so this happens — blue solution turns pale green and red-brown copper deposits.
Zinc is above copper, so this happens too.
Copper is below iron, so nothing occurs. Writing a product here is a genuine error, not a near miss.
Copper is above silver, so copper can displace silver — which shows that copper is unreactive is too crude a statement. It is unreactive relative to iron and reactive relative to silver.
Displacement from acids, where the hydrogen position decides everything:
Zinc is above hydrogen, so it displaces hydrogen from a dilute acid.
Copper is below hydrogen, so it cannot displace hydrogen from a dilute acid — which is why copper vessels are safe for acidic foods and zinc ones are not.
Non-metals displace too. A more reactive halogen displaces a less reactive one, chlorine being more reactive than bromine and bromine than iodine.
The question to ask every time. Is the free element above or below the one in the compound? Above means a reaction with products you can write; below means no reaction, and saying so plainly is the correct answer.
The reactivity series of metals, most reactive first:
K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au
Hydrogen is placed in the list even though it is not a metal, because it marks the important boundary for reactions with acids.
The rule. A metal higher in the series displaces one lower in the series. A metal lower down can do nothing to one above it.
Worked predictions:
Iron is above copper, so this happens — blue solution turns pale green and red-brown copper deposits.
Zinc is above copper, so this happens too.
Copper is below iron, so nothing occurs. Writing a product here is a genuine error, not a near miss.
Copper is above silver, so copper can displace silver — which shows that copper is unreactive is too crude a statement. It is unreactive relative to iron and reactive relative to silver.
Displacement from acids, where the hydrogen position decides everything:
Zinc is above hydrogen, so it displaces hydrogen from a dilute acid.
Copper is below hydrogen, so it cannot displace hydrogen from a dilute acid — which is why copper vessels are safe for acidic foods and zinc ones are not.
Non-metals displace too. A more reactive halogen displaces a less reactive one, chlorine being more reactive than bromine and bromine than iodine.
The question to ask every time. Is the free element above or below the one in the compound? Above means a reaction with products you can write; below means no reaction, and saying so plainly is the correct answer.
What happens in a double displacement reaction?
Two compounds exchange their radicals, so both parts swap partners. The pattern is .
These reactions almost always take place in solution, because the radicals must be free to move and meet — the solvent condition of the previous part. There are two important kinds.
Precipitation reactions produce an insoluble solid, the precipitate, which settles out and drives the reaction forward.
Barium sulphate is a white precipitate. Both starting solutions were colourless, so a white solid appearing from nowhere is striking evidence of reaction.
Lead iodide is a yellow precipitate.
Silver chloride is a white precipitate, and this reaction is the standard laboratory test for a chloride.
Neutralisation reactions are double displacements between an acid and a base, giving a salt and water.
Every neutralisation is exothermic, which is why a beaker of acid and alkali warms as they are mixed.
This is also the chemistry of taking an antacid: the base neutralises excess acid in the stomach, and of spreading lime on acidic soil.
How to tell single from double displacement. Look at the reactants. One element and one compound means single displacement. Two compounds means double. That single check settles almost every classification question.
And the driving force. A double displacement runs forward when one product leaves the solution — as a precipitate, as a gas, or as the very stable molecule water. If all four possible products stay dissolved, nothing effectively happens, which is why these reactions are so often accompanied by a visible solid.
These reactions almost always take place in solution, because the radicals must be free to move and meet — the solvent condition of the previous part. There are two important kinds.
Precipitation reactions produce an insoluble solid, the precipitate, which settles out and drives the reaction forward.
Barium sulphate is a white precipitate. Both starting solutions were colourless, so a white solid appearing from nowhere is striking evidence of reaction.
Lead iodide is a yellow precipitate.
Silver chloride is a white precipitate, and this reaction is the standard laboratory test for a chloride.
Neutralisation reactions are double displacements between an acid and a base, giving a salt and water.
Every neutralisation is exothermic, which is why a beaker of acid and alkali warms as they are mixed.
This is also the chemistry of taking an antacid: the base neutralises excess acid in the stomach, and of spreading lime on acidic soil.
How to tell single from double displacement. Look at the reactants. One element and one compound means single displacement. Two compounds means double. That single check settles almost every classification question.
And the driving force. A double displacement runs forward when one product leaves the solution — as a precipitate, as a gas, or as the very stable molecule water. If all four possible products stay dissolved, nothing effectively happens, which is why these reactions are so often accompanied by a visible solid.
Exam tip
Exam tip: write no reaction when the series says so
If the free metal sits below the one in the compound, the answer is no reaction — write those words and give the reason. Inventing products for loses the mark that saying no reaction would have earned.
Learn the reactivity series in order, and remember hydrogen's position: metals above it displace hydrogen from dilute acids, metals below it do not.
Classify by counting. One product means combination. One reactant means decomposition. One element plus one compound means single displacement. Two compounds means double displacement.
Name the type of decomposition by its energy source — thermal, electrolytic or photochemical. A bare decomposition is an incomplete answer.
Give the colour of a precipitate: barium sulphate and silver chloride white, lead iodide yellow. Colours are marked.
State the energy direction where it is asked: most combination reactions are exothermic, most decomposition reactions endothermic, and every neutralisation is exothermic.
Balance every equation and check it element by element.
And for electrolysis of water, quote the volume ratio of hydrogen to oxygen, read from the balanced equation.
Learn the reactivity series in order, and remember hydrogen's position: metals above it displace hydrogen from dilute acids, metals below it do not.
Classify by counting. One product means combination. One reactant means decomposition. One element plus one compound means single displacement. Two compounds means double displacement.
Name the type of decomposition by its energy source — thermal, electrolytic or photochemical. A bare decomposition is an incomplete answer.
Give the colour of a precipitate: barium sulphate and silver chloride white, lead iodide yellow. Colours are marked.
State the energy direction where it is asked: most combination reactions are exothermic, most decomposition reactions endothermic, and every neutralisation is exothermic.
Balance every equation and check it element by element.
And for electrolysis of water, quote the volume ratio of hydrogen to oxygen, read from the balanced equation.
Did you know
Why does a silver ornament left in a copper dish tarnish oddly?
The reactivity series is usually met as a rule about which reaction will work. Turn it round and it becomes a rule about which metal will be protected.
Put copper and silver in contact in a solution and it is the copper that is attacked, because copper sits above silver and will give up its place first. The silver is left alone. The more reactive metal takes the damage, and the less reactive one is spared.
This is exactly why iron objects are coated with zinc rather than with tin or copper. Zinc sits above iron, so if the coating is scratched and moisture gets in, the zinc corrodes in preference and the iron underneath survives. The coating works even when it is damaged, which no ordinary layer of paint manages.
So a single ordered list predicts three different things at once: which displacement reactions occur, which metals dissolve in acids, and which metal in a pair will be sacrificed to save the other.
Put copper and silver in contact in a solution and it is the copper that is attacked, because copper sits above silver and will give up its place first. The silver is left alone. The more reactive metal takes the damage, and the less reactive one is spared.
This is exactly why iron objects are coated with zinc rather than with tin or copper. Zinc sits above iron, so if the coating is scratched and moisture gets in, the zinc corrodes in preference and the iron underneath survives. The coating works even when it is damaged, which no ordinary layer of paint manages.
So a single ordered list predicts three different things at once: which displacement reactions occur, which metals dissolve in acids, and which metal in a pair will be sacrificed to save the other.
Key takeaways
Types of chemical reaction: quick revision
- Combination — , one product: , , , . Mostly exothermic.
- Decomposition — , one reactant, mostly endothermic, classified by energy source.
- Thermal: , , .
- Electrolytic: — hydrogen and oxygen in a **** volume ratio; when molten.
- Photochemical: — white silver chloride turns grey in sunlight.
- Single displacement — one element plus one compound; a metal higher in the reactivity series displaces one lower.
- The series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au.
- Works: , , .
- No reaction: , and with dilute acid — copper is below both iron and hydrogen.
- Double displacement — two compounds exchange radicals, in solution.
- Precipitation: (white) ; (yellow) ; (white) .
- Neutralisation: acid base **salt water, always exothermic** — , .
- Classify by counting reactants and products, and a double displacement proceeds when one product leaves the solution.
Predict the products for a dozen displacement pairs using the series alone, writing no reaction where it applies — that is precisely the form the question takes.
- Decomposition — , one reactant, mostly endothermic, classified by energy source.
- Thermal: , , .
- Electrolytic: — hydrogen and oxygen in a **** volume ratio; when molten.
- Photochemical: — white silver chloride turns grey in sunlight.
- Single displacement — one element plus one compound; a metal higher in the reactivity series displaces one lower.
- The series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au.
- Works: , , .
- No reaction: , and with dilute acid — copper is below both iron and hydrogen.
- Double displacement — two compounds exchange radicals, in solution.
- Precipitation: (white) ; (yellow) ; (white) .
- Neutralisation: acid base **salt water, always exothermic** — , .
- Classify by counting reactants and products, and a double displacement proceeds when one product leaves the solution.
Predict the products for a dozen displacement pairs using the series alone, writing no reaction where it applies — that is precisely the form the question takes.