Question

Classify each substance as a pure substance or a mixture.

Answer 1

Answer:

• 1) Copper is a pure substance

• 2) A solution of sugar dissolved in water is a mixture

• 3) Carbon dioxide is a pure substance.

• 4) Seawater is a mixture

• 5) Table salt is a pure substance.

Explanation:

1) Copper, which consists of only one kind of atom

There are two kind of pure substances: elements and compounds.

Substances made up of one kind of atom, like copper, are elements.

There are 118 known elements, and you can find them in any periodic table.

Copper is the element with atomic number 29.

Thus, copper is a pure substance, because it is consituted by one kind of atoms.

2) A solution of sugar dissolved in water

A solution consists of solute and solvent, which can be in any proportion. The solutions of sugar and water may be 1%, 2%, 15%, 98%, or any percent. Thus, a solution does not have a fixed composition and is not a pure substance. Pure substances have definite compositions.

It is a mixture of sugar (solute) and water (solvent) which constitutes a solution.

A mixture is the physical combination of two or more substances which can be in any proportion.

Therefore, a solution of sugar and water is a mixture.

3) Carbon dioxide, which has two atoms of oxygen for each atom of carbon

Carbon dioxide has a definite composition of two atoms of oxygen for each atom of carbon. Every molecule of carbon dioxide has the same composition.

The atoms of carbon and oxygen are chemically bonded. They cannot be separated by any physical media.

Two or more atoms chemically bonded constitute a compound, which will have a definite composition and is a pure substance.

Thus, this is a pure substance.

4) Seawater, which can be purified by filtering it.

Mixtures can be purified by physical media, such as filtering (among many others like evaporation or uisng magnets, if one of the substaces has magnetic properties).

Pure substaces, elements or compounds, cannot be separated by physical media.

Seawater can be purified, i.e. separated from sand grains and algae residue, for instance, by fitering because seawater is a mixture.

Yet, the "pure" seawater, i.e. the liquid that remains after filtering, will be a mixture, because it is a solution of several salts (including sodium chloride, among others) and water.

You cannot separate the pure substances from seawater by filtering. That would require a chemical process.

Therefore, seawater is a mixture.

5) Table salt, which can be separated into sodium and chlorine only by chemical processes

The fact that the table salt can be separated into sodium and chlorine only by chemical processes indicates that it is a pure substance.

Table salt is the ionic compound NaCl

NaCl has a definite composition: every crystal of salt will have many NaCl units. Every NaCl unit is equal to any other NaCl unit: one atom of Na is chemically bonded to one atom of Cl.

The chemical bonds cannot be broken by physical media; a chemical process is required, and the formed substances will have completely different properties than the original compound.

Hence, table salt is a pure substance (a compound).