Write an unbalanced formula equation for each of the following. Include symbols for physical states in the equation. $$\begin{array}{c}{\text { a. solid zinc sulfide + oxygen gas } \longrightarrow} \\ {\text { solid zinc oxide + sulfur dioxide gas }} \\\ {\text { b. aqueous hydrochloric acid + solid }} \\ {\text { magnesium hydroxide } \longrightarrow \text { aqueous }} \\ {\text { magnesium chloride + liquid water }}\end{array}$$

Understanding the process of balancing chemical equations is vital to grasp the fundamentals of chemistry. What does it actually mean to balance an equation? In essence, equalizing the number of each type of atom on both sides of the equation is the key. According to the Law of Conservation of Mass, we cannot create or destroy matter during typical chemical reactions. As such, the atoms that start a reaction are always present at the end, just rearranged into different compounds or elements.

To balance an equation, start by counting the number of atoms of each element in the reactants and products. Use coefficients (whole numbers placed in front of the compound symbols) to multiply the number of atoms in each compound as needed so that both sides equate. For instance, in the exercise, the unbalanced equation for the reaction of solid zinc sulfide with oxygen gas is: \( \text{ZnS (s) + O}_2 \text{(g)} \rightarrow \text{ZnO (s) + SO}_2 \text{(g)} \). Balancing this would require ensuring the number of zinc, sulfur, and oxygen atoms is the same on both sides. Remember, while balancing, never alter the subscripts of a chemical formula because doing so would change the compound's identity.

Chemical reaction notation involves the use of symbols and formulas to represent substances involved in chemical processes. Each symbol and formula has specific meanings. The reactants (substances initially present) are listed on the left side of a reaction arrow, with the products (substances formed during the reaction) on the right side. Additional symbols are also commonly used to indicate the states of matter of the reactants and products: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous, which means dissolved in water.

To write a comprehensive reaction notation, you should include the physical states of the substances as done in the exercise. For example, \( \text{HCl (aq) + Mg(OH)}_2 \text{(s)} \rightarrow \text{MgCl}_2 \text{(aq) + H}_2 \text{O(l)} \) clearly communicates not just what happens during the reaction, but how each substance is physically present. This notation helps chemists understand conditions for the reaction and anticipate the physical nature of the reaction's environment.

The states of matter are distinct forms that different phases of matter take on. Four fundamental states are familiar in daily life: solid, liquid, gas, and plasma. In the context of chemistry, we usually consider the first three since plasma is a high-energy state not often encountered in basic chemical reactions.

Solids have a defined shape and volume with particles arranged in a fixed, rigid structure. Liquids have a definite volume but no fixed shape; they take up the shape of their container as the particles can flow freely past one another. Gases have neither definite shape nor volume, filling up any available space as their particles move randomly at high speeds.

In chemical reactions, the state of matter can affect the rate in which the reaction occurs, so it's crucial to note these when writing a formula equation. For instance, solid reactants may react more slowly than those in the aqueous phase due to less molecular mobility and interaction.