What is the weakest rock there is?
Depends on what you mean by "weak." The softest minerals are talc and gypsum, so any rock composed of one of these minerals would be pretty weak. Rocks that have been heavily altered by hydrothermal fluids are also weak. They are composed of many clay minerals and they break apart very easily. If you mean chemically weak, any rock that has minerals that weather easily or dissolve easily in water would be the weakest.
Which rock breaks up the fastest? Thanks.?
Sandstone breaks up the most easily, but I notice that one's not on your list. If a rock is easy to scratch, I'd guess it's also easier to break.
Can a igneous rock break? (for homework)?
yes, but some can be extremely hard to break. need a good sledgehammer and some muscle to break them. Sedimentary rocks tend to break easily by comparison. And if you ever do try to use a sledge on such a rock, be careful, especially of your eyes, because little splinters tend to fly off and can cut you pretty good. Voice of much experience.
What type of rock is most easily broken down by carbonic acid?
Limestone reacts to acids -- the standard test is to pour vinegar on it and see if it bubbles. Carbonic acid (carbonated water) should be similar