An unknown substance turns blue litmus paper to red. which additional test could be used to confirm the substance identity

The additional test is that it should turn methyl orange to red because it is an acid.  

The mentioned substance given in the test is an acid, as the bases turn red litmus paper blue, while an acid turns blue litmus paper red. The acid-base indicators are generally weak bases or acids and they function as a chemical detector for hydronium and hydrogen cations.  

Phenolphthalein is an example of an acid-base indicator, it becomes pink in basic solutions and colorless in acidic solutions. Methyl orange is another example, which becomes yellow in basic solutions and red in acidic solutions.  

methyl orange to red because its an acid.

Explanation:

edge

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.