If someone takes a material that looks pure, and then they do something to it that results in two separate materials, how can they know if they originally had a pure-looking mixture of two things that they simply separated into its parts, or if they originally had a pure material that was a compound that was chemically broken down into new compounds or elements?

In order to determine whether the original material was a uniform/pure-looking mixture or a chemical compound broken into two new compounds, the following points must be noted;

1. If the two new materials are separated by physical means without any  chemical change occurring, the original material is a pure-looking mixture, but if a chemical reaction was involved in separating the material, then it is a compound.

2. If the individual properties of properties of the constituents were retained in the original material, the substance is a mixture, but if the properties of the constituents differed entirely from that of the material, then the material is a compound.

3. If the constituents of the material are not present in a fixed ratio, the material is a mixture but if they are present in a fixed ratio, the material is a compound.

Explanation:

When a substance is being separated into its components, the nature of the substance can be determined by the components obtained.

Matter can be classified into pure substances and mixtures.

Pure substances include elements and compounds which have distinct chemical properties, whereas mixture are composed of two or more constituents physically joined together.

In order to determine whether the original material was a uniform/pure-looking mixture or a chemical compound broken into two new compounds, the following points must be noted;

1. If the two new materials are separated by physical means without any  chemical change occurring, the original material is a pure-looking mixture, if the a chemical reaction was involved in separating the material, then it is a compound.

For example, a sugar and water mixture can be easily separated by evaporation and crystallization to obtain sugar and water respectively, but water which is composed of the elements hydrogen and oxygen cannot be separated by any physical means but by means of a chemical reaction, electrolysis.

2. If the individual properties of properties of the constituents were retained in the original material, the substance is a mixture, but if the properties of the constituents differed entirely from that of the material, then the material is a compound.

For example, a pure-looking mixture of sugar and water has the sweet taste of sugar and the liquid properties of water. However, water, a compound composed of hydrogen and oxygen which are gases, is a liquid and gas properties entirely different from either two.

3. If the constituents of the material are not present in a fixed ratio, the material is a mixture but if they are present in a fixed ratio, the material is a compound.

For example, sugar and water can be mixed together in any ratio to produce a sugar solution, but hydrogen and oxygen are always in a fixed ratio of 2:1 in water