Chemical reactions, Year 8: more practice

4

提示Atoms are rearranged, never made or destroyed.

为什么The same atoms are present before and after the reaction. They are just joined to different partners.

4.4

提示Nothing is lost: the missing mass left as a gas.

为什么Mass is conserved, so the gas must account for the difference: 10.0 − 5.6 = 4.4 g.

20

提示Add the masses of the reactants.

为什么In a closed system the total mass stays the same, so 12 + 8 = 20 g.

Carbon dioxide

提示Carbonates lose a gas when they are strongly heated.

为什么Calcium carbonate → calcium oxide + carbon dioxide. This is how quicklime is made.

It is made of only one kind of atom, so there is nothing simpler to break into

提示Think about what decomposition has to produce.

为什么Thermal decomposition splits a compound into simpler substances. An element contains only one kind of atom.

Copper oxide

提示The metal gains the -ide ending to show oxygen has joined it.

为什么Oxidation is the gain of oxygen. Copper turns black as copper oxide forms.

The energy stored in the reacting chemicals

提示Think about what a battery holds before it is used.

为什么Reactants hold stored chemical energy. In an exothermic reaction some of it is released as heat.

Sodium, zinc, iron

提示The soft metal that is kept under oil is the most reactive here.

为什么The order in the series is sodium, then zinc, then iron. Most of the reactions are quicker the higher up the series a metal is.

It is very unreactive

提示Most metals are found joined to other elements in ores.

为什么Unreactive metals do not combine easily with oxygen or other elements, so they can survive uncombined for millions of years.

Yes, because zinc is more reactive than copper

提示Check which metal is higher in the reactivity series.

为什么A more reactive metal takes the place of a less reactive one in its compound: zinc + copper sulfate → zinc sulfate + copper.