Why does an aqueous solution of an acid conduct electricity?

You have been provided with three test tubes. One of them contains distilled water,and the other two contain an acidic solution and a basic solution,respectively. If you are given only red litmus paper,how will you identify the contents of each test tube?

$A$ milkman adds a very small amount of baking soda to fresh milk.
$(a)$ Why does he shift the $pH$ of the fresh milk from $6$ to slightly alkaline?
$(b)$ Why does this milk take a long time to set as curd?

Five solutions $A, B, C, D$ and $E$ when tested with universal indicator showed $pH$ values of $4, 1, 11, 7$ and $9$,respectively. Which solution is:
$(a)$ neutral?
$(b)$ strongly alkaline?
$(c)$ strongly acidic?
$(d)$ weakly acidic?
$(e)$ weakly alkaline?
Arrange the $pH$ values in increasing order of hydrogen-ion concentration.

Write word equations and then balanced chemical equations for the reactions taking place when:
$(a)$ Dilute sulphuric acid reacts with aluminium powder.
$(b)$ Dilute hydrochloric acid reacts with iron filings.

How is the concentration of hydroxide ions $(OH^-)$ affected when excess base is dissolved in a solution of sodium hydroxide?