1 · Identifying a chemical change

Physical changes

A change in which no new substance is formed is a physical change. Freezing and boiling are the standard examples — ice, liquid water and steam are all still water. Dissolving table salt in water is another: the salt is still salt, just spread through the liquid, and you can get it back by evaporating the water.

Because nothing new has been made, physical changes are usually reversible, and you can usually get the original substances back using separation techniques.

Chemical changes

A change in which one or more new substances are formed is a chemical change, and we say a chemical reaction has taken place. The starting substances are the reactants; the new substances formed are the products.

Chemical changes are usually not easily reversed. Once you have burnt a piece of wood you cannot get the wood back — you would need to supply a large amount of energy, and even then not easily.

Two groups of chemical change Your textbook splits every chemical change into one of two groups:

Combining — two or more substances chemically combine to form new substances. For example, carbon burns in oxygen to form carbon dioxide.
Breaking down — one substance breaks down into two or more new substances. For example, calcium carbonate is heated and breaks down into calcium oxide and carbon dioxide.

Table 11.1 — the comparison you need to know

Physical changes	Chemical changes
No new substances are formed.	One or more new substances are formed.
The changes can usually be reversed. We can get the constituents back using separation techniques.	The changes are usually not easily reversed. We cannot get the reactants back easily.

Watch out Energy is sometimes taken in or given out during a chemical reaction — a lot of heat is released when carbon burns, and heat is taken in when calcium carbonate is heated. But a temperature change on its own does not prove a chemical change: boiling water takes in heat and is still purely physical. Always come back to the real test — has a new substance formed?

Signs that suggest a chemical change

A colour change — green copper carbonate turning black on heating.
A gas given off — bubbles fizzing from a carbonate, or a new smell.
A solid forming in a clear solution (a precipitate) — limewater turning chalky.
A temperature change the mixture causes by itself.
Light being produced — a sparkler, or magnesium burning with a brilliant white flame.

Any one of these is a clue rather than proof. Taken together they are a strong signal that new substances have been made.

Sort these fourteen changes

Decide for each one: does the substance merely change form, or has something new been made?