Suggest a reason why it is desirable to use an excess of barium ion in the precipitation.

Hello your question is poorly written hence i will give you a more general answer

answer :  The reason why it is desirable to use an excess of barium ion in the precipitation is to form the compound needed,

Explanation:

Natural Precipitation occurs when condensed atmospheric water vapor falls due to the gravitational pull exerted on it by the clouds. precipitation can be in various forms like ; rain , ice pellets and snow . e.t.c

Hence the reason why it is desirable to use an excess of barium ion in the precipitation is to form the compound needed,

Also in order to form the precipitation, we will also add excess of barium ion which will help form the required precipitate and also  filter out the mass of Sulphate.

pure substance: iron,copper

impure substance: water,charcoal

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.