Mixtures, pure substances and alloys

Question 1

What’s the difference between a pure substance and a mixture?

Answer 1

Pure substances always have a definite composition, they can be made of anything but must have a definite composition. (H2O, He, O2, H2)
A mixture is basically a combination of multiple pure substances that doesn’t necessarily have a definite composition. (H2O + NaCl, N2 + O2)

Question 2

What’s the difference between an element and a compound?

Answer 2

An element is only one element (Helium, H2, etc)
Compounds are multiple elements (H2O, CO2, etc)

Question 3

If we want to separate mixtures, we use a different type of process than if we want to separate pure substances. What are those processes?

Answer 3

When separating mixtures, we can use physical processes to separate them (ie. boiling salt water to get salt and water vapour). But to separate pure substances, we need to use chemical processes (ie. electrolysis to convert water into H2 and O2)

Question 4

Define alloy

Answer 4

An alloy is composed of many metals (bit like compound but it’s metals)

Question 5

What’s the difference between a homogeneous and a heterogeneous mixture?

Answer 5

A homogeneous mixture has virtually indistinguishable parts, all you can see is one clear solution (salt water).
In a heterogeneous mixture, you can see multiple distinguishable parts. (Oil and water)

Question 6

What’s the difference between a physical change and a chemical change?

Answer 6

A physical change is when you technically changes something about the substance, but the substance itself doesn’t change.
A chemical change is when the chemical identity of a substance does change.

Question 7

What is an equation you can use to calculate concentration and volume from mixing two solutions together?

Answer 7

C1V1 + C2V2 = C3(V1 + V2)
C = Concentration
V = Volume

Question 8

You mix a solution with a known concentration of a substance with a solution with a known concentration of the same substance to get a known concentration AND a known volume. Find the volume of one of them.

Note: concentration is in percentage.

Answer 8

C1V1 + C2V2 = C3V3
C1 * V1 + C2 * V2 = Cm * Vm
Note: C1, C2, Cm and Vm are known.
Assume the volume of the one you need to find is X.
C1 * X + C2 * (Vm - X) = Cm * Vm
C1X + C2Vm - C2X = CmVm
(C1 - C2)X + C2Vm = CmVm
Solving for X should be easy now.

M means mixture because we mixed two solutions.