Chemical Reactions
ICSE Class 8 · Chemistry · Chapter 6
What this chapter covers
- What happens in a chemical reaction
- Combination reactions
- Decomposition reactions
- Reactivity series of metals
- Displacement reactions
- Double displacement reactions
- Exothermic and endothermic reactions
- Neutralisation
- Formation of oxides
- Classification of oxides
Key definitions
Reactants are the substances that take part in a chemical reaction and are used up as it goes on.
Products are the new substances formed as a result of a chemical reaction.
A precipitate is an insoluble solid that separates out from a solution during a chemical reaction.
Neutralisation is the reaction between an acid and a base to form a salt and water only.
Key points
- A chemical reaction changes reactants into new products; bonds break and new bonds form, with energy given out or taken in.
- Combination: two or more substances form one product, for example C + O2 → CO2.
- Decomposition: one compound breaks into simpler substances; heating the carbonates, nitrates and sulphates of many metals, such as lead, copper and iron, leaves metal oxides, but sodium and potassium compounds do not behave this way.
- Oxides of less reactive metals, such as mercury(II) oxide, break down on heating to give the metal and oxygen.
- The reactivity series lists metals from most to least reactive; from magnesium downwards, a metal displaces a metal below it from a salt solution, while potassium, sodium and calcium react with the water instead.
- Displacement: a more reactive element replaces a less reactive one, for example zinc with copper sulphate.
- Double displacement: two compounds exchange parts, often forming a precipitate.
- Exothermic reactions give out heat; endothermic reactions take in heat.
- Neutralisation: acid + base → salt + water; it is exothermic and has many uses in daily life.
- Oxides form when elements are heated in air and when some compounds are decomposed.
- Metal oxides are basic, non-metal oxides are acidic, ZnO, PbO and Al2O3 are amphoteric, and CO, N2O and water are neutral.
Questions answered
Define a chemical reaction and name two signs that one has taken place.
A chemical reaction is a process in which substances change into new substances with different properties. Signs include a change of colour, a gas given off, a precipitate forming, a change in temperature, or light being given out.
Name the type of reaction: CaO + H2O → Ca(OH)2. Is it exothermic or endothermic?
Combination; it is exothermic, as a lot of heat is given out.
What do you see when lead carbonate is heated strongly? Write the equation.
A gas that turns lime water milky is given off, and a residue that is reddish-brown when hot and yellow when cold remains. PbCO3 → PbO + CO2.
Why does the blue colour of copper sulphate solution fade when zinc is added?
Zinc is more reactive than copper, so it displaces copper from copper sulphate. Colourless zinc sulphate forms and copper is deposited.
Will copper react with iron(II) sulphate solution? Give a reason.
No. Copper is below iron in the reactivity series, so it cannot displace iron.
Arrange Cu, Ag, Al, Mg and Fe in decreasing order of reactivity.
Mg, Al, Fe, Cu, Ag.
What is a precipitate? Give one double displacement reaction that forms a white precipitate.
A precipitate is an insoluble solid that separates out from a solution during a reaction. NaCl + AgNO3 → AgCl + NaNO3 gives white silver chloride.
Dissolving ammonium chloride in water makes the test tube cold. What kind of process is this?
An endothermic process, because heat is taken in from the surroundings.