Physical and Chemical Changes

1Two changes in one candle

Light a candle and watch it closely. Near the wick, the solid wax melts into a clear liquid pool; if you blow the candle out, the pool sets back into solid wax. But the wax that goes up the wick burns away: it becomes gases you cannot see, and it can never be turned back into a candle. One candle shows two very different kinds of change. How can you tell them apart?

2Experiments: three changes

A. Heat a few small pieces of candle wax gently in a test tube, then let it cool. B. Heat a spatula of sugar in a dry test tube, gently at first, then strongly. C. (Teacher demonstration) Hold a 2 cm strip of magnesium ribbon with tongs in a Bunsen flame, over a heat-proof mat. Record what you see in each case.

Figure 4.1: Heating wax, heating sugar and burning magnesium.

Table 4.1: Results

3Explaining the results

In A, the wax only changed state: it was still wax, and cooling brought back the solid. This is a physical change. In B and C, new substances formed. Sugar broke down into carbon and water:

$${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}(\mathrm{s})\to 12\mathrm{C}(\mathrm{s})+11{\mathrm{H}}_2\mathrm{O}(\mathrm{g})\backslash mathrm\{C\_\{12\}H\_\{22\}O\_\{11\}(s)\; \backslash rightarrow\; 12C(s)\; +\; 11H\_2O(g)\}$$

and magnesium combined with oxygen from the air to form magnesium oxide:

$$2\mathrm{M}\mathrm{g}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to 2\mathrm{M}\mathrm{g}\mathrm{O}(\mathrm{s})\backslash mathrm\{2Mg(s)\; +\; O\_2(g)\; \backslash rightarrow\; 2MgO(s)\}$$

These are chemical changes (chemical reactions).

Figure 4.2: In a physical change the particles stay the same; in a chemical change the atoms are rearranged into new particles.

4Telling the difference

Table 4.2: Physical and chemical changes compared

Signs that a chemical change has happened: a gas is given off (fizzing); the colour changes; heat or light is given out, or the mixture gets cold; a solid (precipitate) appears when two solutions are mixed; a new smell appears. One sign alone is not proof (boiling also gives off a gas), so look for a new substance.

5Changes around you in Rwanda

Rust is a chemical change that costs Rwandan households money every year. Iron reacts slowly with oxygen and water to form brown, flaky hydrated iron(III) oxide:

$$4\mathrm{F}\mathrm{e}(\mathrm{s})+3{\mathrm{O}}_2(\mathrm{g})+2\mathrm{x}{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\cdot \mathrm{x}{\mathrm{H}}_2\mathrm{O}(\mathrm{s})\backslash mathrm\{4Fe(s)\; +\; 3O\_2(g)\; +\; 2xH\_2O(l)\; \backslash rightarrow\; 2Fe\_2O\_3\; \backslash cdot\; xH\_2O(s)\}$$

Roofing sheets are therefore coated with zinc (galvanised) or painted, which keeps air and water away from the iron. Fermentation is a useful chemical change: yeast in banana juice turns glucose into ethanol and carbon dioxide,

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6(\mathrm{a}\mathrm{q})\to 2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}(\mathrm{a}\mathrm{q})+2\mathrm{C}{\mathrm{O}}_2(\mathrm{g})\backslash mathrm\{C\_6H\_\{12\}O\_6(aq)\; \backslash rightarrow\; 2C\_2H\_5OH(aq)\; +\; 2CO\_2(g)\}$$

while drying cassava, grinding it into flour and mixing the flour with water are all physical changes.