What is magnesium used for in its pure form

The pure metal has low structural strength, magnesium is mainly  used in the form of alloys- principally with 10% or less of aluminum, zinc, and manganese- to improve it's hardness, tensile strength, and ability to be cast, welded, and machined.

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answer: the rate of diffusion increases with increase in temperature.

increase in temperature, increases the molecular/particles speed (kinetic energy. diffusion means movement of particles through the material.

generally, the heavier the particle the more it interacts with the environment and more the temperature matters. so higher temperature results in higher diffusion.