Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define and give examples of physical and chemical changes.
• Distinguish between the two types of changes.
• Write word equations for reactions.
• Write skeleton equations using chemical formulas.
• Balance simple chemical equations.

Lesson summary

Chemical changes are fundamental to understanding the world around us. From the burning of firewood in a braai to the rusting of a corrugated iron roof, chemical reactions are constantly occurring and shaping our environment. In South Africa, with its rich natural resources and diverse industries, understanding chemical changes is crucial for fields like mining, agriculture, and environmental management. Knowing how to represent these changes using chemical equations is essential for predicting and controlling these processes. For example, understanding the chemistry of fertilizer production is critical to improving crop yields, a major concern for food security in many parts of the country.

Lesson notes

2.1 Physical vs. Chemical Changes A physical change alters the form or appearance of a substance but does not change its chemical composition. The substance remains the same; its molecules are just rearranged. Examples include melting ice (H₂O(s) → H₂O(l)), boiling water (H₂O(l) → H₂O(g)), dissolving sugar in water (sugar(s) → sugar(aq)), and crushing a rock. Key indicators of a physical change are changes in state (solid, liquid, gas), shape, or size, without the formation of new substances. A chemical change (also known as a chemical reaction) involves the formation of new substances with different chemical properties. The chemical composition of the original substance(s) is altered. Examples include burning wood (cellulose + oxygen → carbon dioxide + water), rusting of iron (iron + oxygen → iron oxide), cooking an egg (protein denaturation), and the reaction between baking soda and vinegar (sodium bicarbonate + acetic acid → sodium acetate + water + carbon dioxide).

Indicators of a chemical change: Formation of a gas: Bubbles are produced (e.g., adding acid to a metal).

Formation of a precipitate: A solid forms when two solutions are mixed (e.g., mixing silver nitrate and sodium chloride solutions).

Change in color: A permanent color change occurs (e.g., burning magnesium ribbon).

Change in temperature: Heat is either released (exothermic reaction) or absorbed (endothermic reaction).

Odor change: A new odor is produced (e.g., food rotting).

Light is produced: Emission of light (e.g., burning methane gas). Important

Note: It's essential to remember that not every change in temperature or color indicates a chemical change. Dissolving salt in water, for instance, might slightly change the water's temperature, but it's still a physical change. 2.2 Representing Chemical Changes Chemical changes can be represented using different types of equations: Word Equation: Describes the reaction using the names of the reactants and products.

Example: Hydrogen + Oxygen → Water Skeleton Equation (Unbalanced Chemical Equation): Uses chemical formulas to represent the reactants and products.

Example: H₂ + O₂ → H₂O Balanced Chemical Equation: A skeleton equation that has been balanced to satisfy the Law of Conservation of Mass. The number of atoms of each element must be the same on both sides of the equation.

Example: 2H₂ + O₂ → 2H₂O 2.3 Balancing Chemical Equations Balancing chemical equations is crucial to ensure that the number of atoms of each element is the same on both sides of the equation, adhering to the law of conservation of mass. The following steps are generally followed: Write the skeleton equation: Identify the reactants and products and write their correct chemical formulas.

Count the atoms: Count the number of atoms of each element on both sides of the equation.

Balance the elements one at a time: Start with elements that appear in only one reactant and one product. Use coefficients (numbers placed in front of chemical formulas) to adjust the number of atoms.

Balance hydrogen and oxygen last: Hydrogen and oxygen often appear in multiple compounds, so it's generally easier to balance them after other elements.

Reduce to simplest whole number ratio: Ensure the coefficients are in the simplest whole-number ratio.

Check your work: Recount the atoms to verify that the equation is balanced.