A1 mm solution of a substance has a ph of 3. what must be true of this substance?

To have a PH of 3, it means that the substance must be acidic since a PH of less than 7  less means that the tested substance is acidic. If the PH is greater than 7 then it will be  a base or the opposite of acidic. PH of a solution can be tested with litmus paper ie qualitatively to see if it is an acid or a base. Aof an example of a base is NaOH sodium hydroxide and of an acid is H2SO4 or sulfuric acid.

May be this question have some options to choose the right answer.

Nevertheless, I can tell you a real true about the nature of that substance.

Take into account that the pH reveals whether a solution iis acid, basic or neutral.

The scale of pH goes from 0 to 14.

You can use this table:

pH value          kind of substance

7 < pH < 14      basic

pH = 7              neutral

0 < pH < 7        acid

So, the pH of that substance reveals that it is an acid.

Yes you are correct use the scientific method

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.