Cracks in the cement before tiling.

[Hassin Bin Sober]

6. You can apply a crack isolation membrane either liquid or mat.

Home Depot uses Custom Building Products as their tile products supplier for grout, thinset and liquid isolation membrane called "Redgard".

Redgard acts as a waterproofing membrane AND crack isolation membrane. It should be applied thicker for crack isolation - with a 3/16th trowel if I remember correctly. You can do the whole floor or there is a formula for width over the crack based on tile size. Read the directions AND call the mfg.

I like Redgard because it s relatively easy to apply, acts as a water proofing membrane AND has the extra added benefit of giving you extra working time when working over porous surfaces like concrete or cement board. It keeps the concrete from sucking the thinset dry before you set your tile - this helps the thinset cure slower and stronger. Make sure you dampen the concrete so the Redgard doesn't dry too quickly per mfg instructions. READ instructions!

As mentioned up thread, use a good quality modified/fortified with latex thinset like Custom's flexbond in the mid to high 20 dollar range for a 50 pound bag. Don't cheap out with 8 dollar a bag stuff.

This is all predicated on the fact your slab is suitable for tile and not subject to hydrostatic water pressure. Is it above grade? Or a basement below grade?