Physical and Chemical Changes Worked ExamplesIGCSE Chemistry 0620

In a physical change no new substance is formed — the particles stay the same and only their arrangement or spacing changes. In a chemical change at least one new substance is formed.

B: steam (gaseous water) becomes liquid water. It is still water, so this is a physical change. The answer is B.

A makes ethanol and carbon dioxide; C makes a salt and water; D thermally decomposes calcium carbonate into calcium oxide and carbon dioxide. All three form new substances, so they are chemical changes.

Change 1 is physical: the molecules after are the same molecules as before — they have only moved further apart. No new substance forms.

Change 2 is chemical: the atoms have been rearranged into molecules of a new substance (each containing one white and one pink atom). The number of each type of atom is unchanged (6 white, 6 pink).

(b) The particles go from close together and random (liquid) to far apart (gas): boiling (or evaporation).

(a) P: melting. Q: boiling (or evaporation). R: condensation. S: freezing. T: dissolving.

(b) When sugar dissolves, its molecules spread out between the water molecules. They are still sugar molecules — no new substance is formed — so dissolving is a physical change. The solution tastes sweet, and the total mass is unchanged.

To show it: evaporate the water (or leave the solution to crystallise). Sugar is recovered — the change is easily reversed and the same substance comes back.

(a) The flat sections are the changes of state. First flat section: melting point $= 80\,^{\circ}\text{C}$. Second: boiling point $= 218\,^{\circ}\text{C}$.

(b) B to C: Z is melting — solid and liquid. D to E: Z is boiling — liquid and gas.

(c) The thermal energy taken in is used to overcome the forces of attraction between the particles (letting them move out of the fixed lattice), not to increase their kinetic energy, so the temperature does not rise until all the solid has melted.

(d) A pure substance condenses at the same temperature at which it boils: $218\,^{\circ}\text{C}$. Melting and boiling are physical because the particles of Z are unchanged — no new substance forms — and both changes are reversed simply by cooling.

1 chemical — a gas forms (bubbles) and magnesium is used up to make a new substance (magnesium chloride); energy is released.

2 physical — melting: the wax is still wax and the change reverses on cooling. 4 physical — dilution: the copper(II) ions are simply spread through more water; no new substance.

3 chemical — a precipitate (a new, insoluble substance, silver chloride) forms. 5 chemical — the anhydrous salt combines with water to form a new substance, hydrated copper(II) sulfate, with a colour change and energy released.

(b) An observation is what you see, hear, feel or smell. You cannot see that the gas is hydrogen — that is a conclusion from a test. The observation is fizzing / bubbles.

(a) Hydrated. (Anhydrous means containing no water.)

(b) The solid changes from blue to white as the water of crystallisation is driven off.

(c) Chemical test: add X to anhydrous copper(II) sulfate — it turns from white to blue (or anhydrous cobalt(II) chloride turns from blue to pink). Purity: pure water boils at exactly $100\,^{\circ}\text{C}$ (and freezes at $0\,^{\circ}\text{C}$).

(d) Chemical: the decomposition of hydrated copper(II) sulfate into anhydrous copper(II) sulfate and water (new substances). Physical: the steam condensing to liquid water in the cooled tube.

(e) If the boiling tube cools while the delivery tube is still in the liquid, the gas inside contracts and cold water is sucked back into the hot tube, which could crack it.

(a) Experiment 1 is physical: the same substance (white sodium chloride, same mass) is recovered — dissolving is reversed by evaporation.

Experiment 2 is chemical: evaporating does not give back black copper(II) oxide; instead a new substance with different properties (blue crystals) forms.

(b) Moles of copper(II) oxide:

Each mole of CuO gives one mole of $\text{CuSO}_4$, which crystallises as one mole of $\text{CuSO}_4\cdot 5\text{H}_2\text{O}$.

Physical changes of simple molecular substances (melting, boiling, dissolving) overcome the weak forces between molecules. The $\text{H}_2\text{O}$ molecules themselves stay intact.

In C, water is decomposed into new substances — hydrogen and oxygen — so the O–H bonds within the water molecules must be broken. This is a chemical change. The answer is C.

A and D are changes of state: steam and ice are still $\text{H}_2\text{O}$. B separates sugar molecules and water molecules from each other, but no bonds inside the molecules break.

(a) 1: fractional distillation. 2: cracking. 3: addition polymerisation.

(b) 1 physical — the mixture is separated by boiling point (evaporation and condensation); the hydrocarbon molecules are unchanged.

2 chemical — large alkane molecules are broken down into smaller molecules, including ethene (new substances). 3 chemical — ethene molecules join together to form a new substance, the polymer.

4 physical — the pellets are melted and moulded into a new shape; the substance is still poly(ethene).

(a) The iron filings are attracted to the magnet and separate from the yellow sulfur. The iron still has its own properties, so no new substance formed: mixing is physical.

(b) The mixture keeps glowing after the heat is removed — energy is given out (exothermic). A black solid forms with different properties: it is not attracted by a magnet, unlike iron.

(c) Moles of each reactant:

They react in a $1:1$ ratio, so they react exactly. Mass is conserved in a chemical change.

(a) The water produced leaves the flame as steam (water vapour). The ice cools it so it condenses to liquid and collects.

(b) Add P to anhydrous copper(II) sulfate: it turns white to blue (or anhydrous cobalt(II) chloride turns blue to pink).

(c) The limewater turns milky, showing that carbon dioxide is produced. The carbon comes from the hydrocarbons in the wax.

(d) Physical: the solid wax melts; the liquid wax evaporates up the wick (also: steam condensing in the U-tube). Chemical: the wax vapour burns (combustion) in oxygen.

The combustion is chemical because new substances — carbon dioxide and water — are formed (shown by the tests), and a large amount of energy is released as heat and light. Melted wax, by contrast, is still wax and solidifies again on cooling.

(e) The pump draws the gases from the flame through the U-tube and the limewater (and supplies air to the flame).

Rule 1: heating blue hydrated copper(II) sulfate gives white anhydrous copper(II) sulfate and water; adding water reverses it. It is reversible but chemical (new substances form each way).

Rule 2: diluting blue copper(II) sulfate solution makes the colour paler, but no new substance forms — a colour change in a physical change.

Rule 3: freezing water (or condensing steam) gives out energy, but it is still water — a physical change.

(b) In a chemical change at least one new substance is formed (atoms are rearranged by breaking and making bonds). In a physical change no new substance forms.

(c) Burning hydrogen releases about $\dfrac{286}{6.0} \approx 48$ times as much energy per mole as freezing water. Chemical changes usually involve much larger energy changes than physical changes — but this is a guide, not a rule.