When testing the acidity of a solution with litmus paper, do not immerse the entire strip — only dip the tip.
Literal
Litmus-paper [with-で] solution [possessive-の] acidity (acidic, neutral, basic) [object-を] test-when [topic-は], litmus-paper [object-を] entirely solution [to-に] immerse [not-X-のではなく], paper [possessive-の] tip [possessive-の] part [only-だけ] [object-を] immerse.
A precise lab instruction. ~のではなく corrects: 'don't immerse all of it; rather, only the tip.' 液性 (acidity/alkalinity) is a chemistry term with the parenthetical (酸性、中性、塩基性) spelling out the three categories. ~ときは sets the condition for the instruction.