Litmus paper is a simple pH indicator that changes color depending on how acidic or basic a solution is. In acidic solutions, blue litmus turns red. In basic (alkaline) solutions, red litmus turns blue. Neutral solutions don’t cause dramatic color changes for either paper, so blue stays blue and red stays red. So, when you’re distinguishing acids and bases, acids make blue litmus red, and bases make red litmus blue. Using both papers helps you confirm: a blue paper turning red indicates acidity, while a red paper turning blue indicates basicity.

Standardizing chemical symbols and formulas means creating a clear, universal way to write elements and compounds so scientists can share and compare results. That system was developed by Jöns Jakob Berzelius. He introduced the modern chemical symbol system—one or two letters for each element—and used subscripts to show how many atoms of each element are present in a compound. This laid the foundation for the notation we still use today. Dmitri Mendeleev is famous for organizing the periodic table and predicting properties of undiscovered elements. Antoine Lavoisier helped shape chemical naming conventions and promoted the law of conservation of mass. Marie Curie made pivotal discoveries in radioactivity.

When an ionic compound dissolves in water, it splits into charged particles and those particles become surrounded by water molecules, forming hydrated ions in the solution. The standard way to name them in chemistry is aqueous ions, meaning they are ions present in water (often written with the (aq) notation). This term highlights both their charged nature and their state in a dissolved, water-based solution. Free radicals are unrelated here because they’re species with unpaired electrons, not simply ions in solution. Gas ions refer to ions in the gas phase, which isn’t what happens when a salt dissolves in water. So the best description for the ions formed in an aqueous solution is aqueous ions.

Balanced chemical equations follow the law of conservation of mass: the same number of atoms of each element must appear on both sides of the equation. That is why this option is the best description—the counts of each type of atom match on the reactant and product sides, so mass and matter are preserved. To balance, you adjust the coefficients in front of formulas (not the subscripts inside a molecule), so you’re just showing more or fewer molecules involved in the reaction. For example, H2 + O2 -> H2O isn’t balanced because there are four hydrogen atoms and two oxygen atoms on the left, but only two hydrogen and one oxygen on the right. Balancing gives 2 H2 + O2 -> 2 H2O, which has the same number of each atom on both sides (four hydrogen and two oxygen). If total mass changed, that would contradict the conservation law, which is why that choice isn’t correct. An equation with only products on one side wouldn’t represent a real balanced reaction, and the idea that an equation cannot be solved isn’t accurate—most reactions can be balanced by adjusting coefficients.