Introduction to Chemical Reactions and Equations

Chemical reactions occur frequently in daily life. Here are some common examples:

  • Milk: When left out in hot weather, milk changes and can sour.

  • Iron: Iron objects rust when exposed to humidity.

  • Grapes: Grapes undergo fermentation to make wine.

  • Food: Cooking and digesting food involve chemical changes.
    In these cases, original substances change into new substances, indicating a chemical change.

Experimental Activity 1.1: The Combustion of Magnesium

This activity helps us explore chemical reactions. We burn a magnesium ribbon. Start by cleaning a piece of magnesium ribbon (around 3−4 cm3-4\,cm long) with sandpaper to remove any impurities. Using tongs, hold the ribbon over a flame (from a spirit lamp or burner) and collect the ash in a watch-glass. Caution: Keep the burning ribbon away from your eyes.

You'll notice that it burns with a bright light, forming a white powder called magnesium oxide from the reaction with oxygen in the air. The reaction can be represented as:
Mg+O2→MgOMg + O_2 \rightarrow MgO.

Experimental Activities 1.2 and 1.3: Chemical Interactions and Gas Evolution

In Activity 1.2, mix lead nitrate and potassium iodide solutions in a test tube to observe a reaction. Activity 1.3 involves metal reactions with acids. Place zinc granules in a conical flask, then add dilute hydrochloric acid or dilute sulfuric acid (H<em>2SO</em>4H<em>2SO</em>4). Warning: Handle acids with care. You will see gas bubbles forming around the zinc and feel a temperature change in the flask, showing a reaction. This interaction can be described with the word equation:
Zinc+Sulfuric acid→Zinc sulfate+Hydrogen\text{Zinc} + \text{Sulfuric acid} \rightarrow \text{Zinc sulfate} + \text{Hydrogen}.

Core Characteristics of Chemical Reactions

Here are four key signs that indicate a chemical reaction has taken place:

  1. Change in state (solid, liquid, gas).

  2. Change in color.

  3. Gas evolution (often observed as bubbling).

  4. Temperature change (indicating if the reaction is exothermic or endothermic).

Understanding Chemical Equations

Describing chemical reactions can be lengthy, but using word equations simplifies this. For instance, the reaction of magnesium burning in oxygen can be expressed as:
Magnesium+Oxygen→Magnesium oxide\text{Magnesium} + \text{Oxygen} \rightarrow \text{Magnesium oxide}.
Here, magnesium and oxygen are the reactants, while magnesium oxide is the product. Reactants are listed on the left side of the equation, and products on the right, with an arrow indicating the direction of the reaction.

Writing and Balancing Chemical Equations

To make chemical equations more concise, we can use formulas. The reaction of magnesium with oxygen is noted as:
Mg+O<em>2→MgOMg + O<em>2 \rightarrow MgO. To determine if an equation is balanced, compare the number of atoms of each element on both sides. If they match, the equation is balanced. If they don’t, it’s unbalanced, violating the Law of Conservation of Mass. For example, in the equation $$Zn + H2SO4 \rightarrow ZnSO4 + H_2