"Small change" can refer to coins of low value, or to paper money of low value, or metaphorically to anything of low relative worth; a couple of $50 bills is "small change" to someone looking for a $200,000 payment, for example.
"Loose change" specifically refers to all of the coins that you happen to have in your pockets or purse that are not rolled or bundled up; if you got two quarters back from a purchase and just dropped them into your front pocket, for example, that's loose change. If you put the quarters into a sleeve for making a "roll of quarters", then they aren't loose anymore. (In a coin purse or a wallet pouch, they're probably still able to rattle about and make noise, which is a key element of being considered "loose", I think.)
In many contexts, the meanings are pretty much the same, but you might hear finished more often than completed in casual conversation. For example:
I've finished my shopping.
She finished the song.
He finished the race.
I could use completed in those sentences – the meaning wouldn't change, but the register might sound off.
The word completed can convey some sense of accomplishment. In the context of a race, it might work when the race is a major achievement:
He completed his first marathon last year.
Homework, though, is not really a major achievement, so I think you'd hear finished more often in casual conversation:
“Joey, where are you going? Did you finish your homework?”
That said, you might see completed in more formal contexts, such as a paper on education, or a course syllabus:
Students must complete six homework assignments during the semester.
Best Answer
If used of a person, both expressions mean that he or she has left the mortal coil; the difference is in how long ago. Dead and gone always means "long dead", while dead and buried could mean that the death was recent, or it could be used as a variant of dead and gone, i.e. long dead.
If used of an object or idea, dead and gone again emphasizes that it's not just gone/obsolete/archaic/out-of-fashion, but that it has been so for a long time. With dead and buried, the emphasis is on the fact that it's out-of-date, not on how long it's been so: it might not yet be forgotten, but perhaps it ought to be.
If you disapprove of an attitude, then you'd want it "dead and buried". If you've never even heard of anyone actually believing an obsolete idea, then you could call it "dead and gone".