Everything I have read says that when mixing aqua regia, one must add Nitric acid to Hydrochloric acid and not the other way around. Why is this? If one were to mix it backwards would it have different properties? I know acid poured into water as a safety precaution, is there a similar rationale behind the combination of two acids?
Re: Does order matter when mixing acid (aqua regia)
Rather than rewrite an entire article, let me direct you to this link that has an excellent discussion of the preparation, use, and safety of handling aqua regia. : http://sciabc.us/LUnfl
The only thing not explained there is the order of addition, but that can be readily surmised from any other acid dilution. A smaller volume of the nitric acid is used compared to the hydrochloric acid. So, from the perspectives of both the amounts of transfer needed and the general principle of adding concentrated acid to more dilute solutions (either water or acids), it makes sense to add the nitric acid to the hydrochloric acid instead of the other way around.
Always wear appropriate Personal Protective Equipment, understand the procedures, and work SAFELY!