Chemical reactions Study Guide

When substances like acids and alkalis dissolve in water, they form ions (electrically charged particles) that give them their characteristic properties. Acids are substances that release hydrogen ions (H⁺) when they dissolve in water. It is the presence and concentration of these hydrogen ions that give acidic solutions their properties, such as a sour taste (do not taste in a lab setting) and the ability to turn blue litmus paper red. Common examples of acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). When hydrochloric acid dissolves in water, it dissociates to form H⁺ and Cl⁻ ions.

Alkalis are a specific type of base that are soluble in water. When alkalis dissolve in water, they release hydroxide ions (OH⁻). These hydroxide ions are responsible for the distinct properties of alkaline solutions, such as a soapy feel and the ability to turn red litmus paper blue. Examples of alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂). When sodium hydroxide dissolves in water, it dissociates to form Na⁺ and OH⁻ ions. The concentration of these hydroxide ions determines how strong the alkali is. Understanding the roles of H⁺ and OH⁻ ions is fundamental to grasping acid-base chemistry, including neutralisation reactions where these ions combine to form water.

Chemical formulae are a concise way to represent the composition of elements and compounds using chemical symbols and numbers. Each element has a unique chemical symbol, typically one or two letters, with the first letter always capitalised. For example, 'H' for hydrogen, 'O' for oxygen, 'Fe' for iron. When writing the formula for an element that exists as individual atoms, like helium (He) or iron (Fe), the symbol is simply used. However, some elements exist naturally as molecules made of two or more atoms bonded together, such as oxygen (O₂), nitrogen (N₂), chlorine (Cl₂), and hydrogen (H₂). In these cases, a small subscript number indicates how many atoms are in one molecule.

For compounds, which are substances made of two or more different elements chemically bonded together, the formula shows the ratio of each type of atom. This involves understanding the valency or combining power of each atom. In simple covalent compounds, such as water (H₂O) or carbon dioxide (CO₂), the formula indicates the exact number of atoms of each element in one molecule. For example, water has two hydrogen atoms and one oxygen atom. For ionic compounds, such as sodium chloride (NaCl) or magnesium oxide (MgO), the formula represents the simplest whole number ratio of ions in the giant ionic lattice, reflecting the overall electrical neutrality of the compound. For example, in sodium chloride, one sodium ion combines with one chloride ion. In magnesium chloride (MgCl₂), one magnesium ion combines with two chloride ions to balance the charges. The sum of the charges of the positive and negative ions must be zero in the overall formula. Polyatomic ions, like the sulfate ion (SO₄²⁻), are enclosed in brackets if there is more than one present in the formula, with the subscript placed outside the bracket, e.g., Ca(OH)₂ for calcium hydroxide.