Reading the information stored in DNA requires unzipping the double stranded helix. In our body, this strand separation is achieved by motor proteins applying mechanical stresses (force and torque) at constant temperature and solution conditions. However, the DNA double helix is also denatured in the laboratory by exposure to elevated temperatures or non-physiological solvents. Here, we use magnetic tweezers to directly stretch and twist single DNA molecules under variation of temperature and solvent. Our aim is to determine the thermal, mechanical, and chemical conditions for denaturation of the DNA double helix. At high tensions we found that the DNA double helix exhibits a mechanical transition accompanied by a considerable extension in length, consistent with previous measurements. We also saw evidence for strand separation by adding a controlled number of rotations to unwind the helix. The strands separated more easily at low salt concentrations. In addition to varying the stability of DNA, we also observed that raising the salt concentration of the solution results in a decrease of the helical pitch. These measurements demonstrate the feasibility of our approach to determine the limits of DNA stability. In the long term, this project will allow us to establish a phase diagram of DNA stability as a reference for further studies.