You wake up in the morning and make yourself a cup of coffee. You pour the coffee in your cup, add milk, add sugar, and stir everything together. The result is a uniform cup of caffeinated goodness. Each sip should taste and look the same.
This is an example of a homogeneous mixture. In this type of mixture, the components are evenly distributed throughout and the mixture has a uniform appearance. You shouldn't be able to pick out the individual components. Additionally, you can describe a homogeneous mixture as having only one phase. A phase is any part of the mixture that has the same chemical properties and uniform distribution of the components.
Heterogeneous
When you're eating a bowl of cereal, you are consuming a heterogeneous mixture. You can easily see the different things that make up this mixture: the milk and the cereal. Each spoonful will have different distributions of the components.
A heterogeneous mixture has components that are not evenly distributed. This means that you can easily distinguish between the different components. A heterogeneous mixture has two or more phases. This doesn't necessarily mean phases of matter, like liquid or solid. For example, a mixture of water and oil has two liquid phases. Each phase has its own distinct chemical properties.
Average atomic mass is the weighted average atomic masses with regard to the relative abundance of the isotopes average atomic mass of Li = relative abundance of Li-6 x mass of Li-6 + relative abundance of Li-7 x mass of Li-7 average atomic mass of Li = (7.42% x 6.0151 a.m.u) + (92.58% x 7.0160 a.m.u) = 0.446 + 6.495 = 6.941 amu average atomic mass of Li is 6.941 amu