Read the descriptions below of two substances and an experiment on each. Decide whether the result of the experiment tells you t
he substance is a pure substance or a mixture, if you can. Sample A is a solid yellow cube with a total mass of 50.0g. The cube is put into a beaker filled with 250.mL of water. The cube collapses into a small pile of orange powder at the bottom of the beaker. When this powder is filtered out, dried and weighed, it has a total mass of 29.9g If the experiment is repeated with 500.mL of water, the powder that's left over has a mass of 30.0g
Sample B is 100.mLof a clear liquid. The liquid is heated in a flask until it boils, which starts to happen at 66.2°C. As the liquid boils, the temperature continues to rise, until the last of the liquid boils away at 76.0°C..
Determine whether each is a pure substance, mixture, or can't decide.
For sample A, we are told that the originally yellow solid was dissolved and we obtained an orange powder at the bottom of the beaker. Subsequently, only about 30.0 g of solid was recovered out of the 50.0g of solid dissolved. This implies that the solid is not pure and must be a mixture. The other components of the mixture must have remained in solution accounting for the loss in mass of solid obtained.
For sample B, we are told that boiling started at 66.2°C and continued until 76.0°C. The implication of this is that B must be a mixture since it boils over a range of temperatures. Pure substances have a sharp boiling point.
Mixing water and sugar is a physical change. A physical change is just a change in the form of the substance, whereas a chemical change is when the substance itself changes… chemically. An easy way to figure out which is which is trying to "undo" the change. If it can be undone, it is physical.
