Use the following equations to answer the next four questions: i. \(\mathrm{H}_{2} \mathrm{O}(s) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)\) ii. \(\mathrm{Na}^{+}(a q)+\mathrm{Cl}^{-}(\mathrm{aq})+\mathrm{Ag}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q) \longrightarrow \mathrm{AgCl}(s)+\mathrm{Na}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q)\) ii. \(\mathrm{CH}_{3} \mathrm{OH}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) iv. \(2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g)\) v. \(\mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q) \rightarrow \mathrm{H}_{2} \mathrm{O}(l)\) (a) Which equation describes a physical change? (b) Which equation identifies the reactants and products of a combustion reaction? (c) Which equation is not balanced? (d) Which is a net ionic equation?

When we talk about a physical change in chemistry, we're referring to a process where the form of matter may alter, but not its chemical identity. This includes changes in state (such as solid to liquid or gas to liquid), texture, or appearance. The key is that the substance undergoing a physical change doesn't become a different substance.

For instance, when ice melts into water, represented by the equation \(\mathrm{H}_2\mathrm{O}(s) \longrightarrow \mathrm{H}_2\mathrm{O}(l)\), the chemical formula remains the same—indicating that only its physical form has changed from solid to liquid. This is why equation (i) from the given exercise exemplifies a physical change.

Understanding physical changes is essential, especially when distinguishing them from chemical changes where new substances are formed, and the properties of the original substance are completely different.

A combustion reaction is a high-temperature, exothermic reaction where a fuel, typically containing carbon and hydrogen (a hydrocarbon), reacts with an oxidizer (commonly oxygen from the air) to form carbon dioxide and water.

Following equation (iii) \(\mathrm{CH}_3\mathrm{OH}(g) + \mathrm{O}_2(g) \longrightarrow \mathrm{CO}_2(g) + \mathrm{H}_2\mathrm{O}(g)\), we see that methanol gas (\(\mathrm{CH}_3\mathrm{OH}\)) reacts with oxygen (\(\mathrm{O}_2\)) to produce carbon dioxide and water in gaseous forms, fitting the criteria of a combustion reaction perfectly. This reaction is important not only in chemistry but also in real-world applications, such as in engines and power generation. Students must learn to identify combustion reactions because they can involve complex balancing and show the interplay of energy within chemical processes.

Balancing chemical equations is a foundational skill in chemistry that maintains the law of conservation of mass, ensuring that atoms are neither created nor destroyed in a chemical reaction. To balance an equation, one adjusts the coefficients (numbers before elements and compounds) to get the same number of each type of atom on both sides of the arrow, representing the reactants and products.

Using equation (iv) \(2\mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{H}_2(g) + \mathrm{O}_2(g)\) as an example, note that it is not balanced regarding oxygen atoms. There are two oxygen molecules on the left but only one on the right. Correcting this discrepancy, we'd achieve a balanced equation \(2\mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{H}_2(g) + \mathrm{O}_2(g)\), where each side of the reaction contains two hydrogen and two oxygen atoms. Balancing equations is crucial for correctly interpreting and predicting the outcomes of chemical reactions.

Diving into the realm of solutions and ionic compounds, the net ionic equation provides a streamlined representation of a chemical reaction. It excludes the spectator ions—those entities that appear identically on both the reactant and product sides of an equation. This focus on the substances that actually participate in a chemical change yields a simpler and more informative view of the reaction.

For equation (ii) \(\mathrm{Na}^+(aq) + \mathrm{Cl}^-(aq) + \mathrm{Ag}^+(aq) + \mathrm{NO}_3^-(aq) \longrightarrow \mathrm{AgCl}(s) + \mathrm{Na}^+(aq) + \mathrm{NO}_3^-(aq)\), the net ionic equation would filter out the sodium \(\mathrm{Na}^+\) and nitrate \(\mathrm{NO}_3^-\) ions, as they don't undergo any change. The resulting net ionic equation simplifies to \(\mathrm{Ag}^+(aq) + \mathrm{Cl}^-(aq) \longrightarrow \mathrm{AgCl}(s)\), highlighting the reaction that leads to the formation of solid silver chloride. Learning to write net ionic equations can help students understand the actual chemistry happening in a reaction.