Southern Magnolia is the common name of Magnolia grandiflora. Magnolia del sur is Spanish!
Yes, Southern Magnolia is hardy to 0ºF, but has a lot of trouble with frozen earth, windy winters, and prolonged periods below freezing (ie. days/weeks). They won't thrive for you in Chicago and without significant protection, they likely won't survive.

Simon

Yep,

Sounds like you feel victim to the Box Stores' practice of stocking plants that won't survive the winters where they're located -- they just send the same plant varieties to their stores all over -- but don't give up hope! If you're in a fairly built-up urban area, and can plant the trees close to a south-facing wall with some shelter from winds (yes, I know it's the Windy City and all!) and afternoon sun, they could do fine for you. Do the tags give any further name like "Edith Bogue" or what nursery they came from? Some varieties of mag. grandiflora can survive up to zone5b in good locations and are much hardier than the straight species.

Hope this helps, it's always good to check gardening books or knowledgable friends about winter hardiness and growing conditions -- the chance of an average store clerk knowing much about gardening is pretty slim.

Jim

Some growers are putting out plants with bilingual name tags on them, so Spanish names will often appear with the English. Southern magnolia is a variable species that varies in hardiness, in your area I would only bother with one of the most hardy and ornamental, vegetatively propagated named varieties. If your plants do not have (hardy) cultivar names attached, deep green leaves with heavy coating of reddish or brown indumentum (fuzz) beneath they should be taken back.

'Edith Bogue' is of unexceptional appearance and is not the most hardy (minor leaf burn at -13F reported), one like 'Bracken's Brown Beauty' (unfazed by -20F) would be a better choice. Even then death in an extra hard winter may occur sometime, as Chicago area has been known to get well below -20F.