Grade 9 Chemistry Review: Matter, Chemical Change, and Atomic Theory

Laboratory Safety and the Fundamentals of Matter

• Grade 9 Review: Unit A – Energy and Matter in Chemical Change

  • The core understanding of chemistry is based on the principle that particles constitute the underlying structure of all matter, a concept that has directly facilitated significant advancements in modern technology.

• A1.1 – Safety in the Laboratory

  • Strict adherence to safety protocols is mandatory.

  • Students must be intimately familiar with WHMIS (Workplace Hazardous Materials Information System) symbols.

  • Reference: Lab Safety Activity and Workbook pages 2-4.

• A1.2 – Properties and Classification of Matter

  • The Purpose of Classification: Scientists utilize classification systems to organize and manage the millions of chemical compounds that have been discovered in nature or synthesized in laboratories.

  • Definition of Matter: Matter is defined as anything that possesses mass and occupies physical space (volume).

Objective 1: Classifying Substances via Properties

• Physical Properties

  • Defined by the physical appearance and the composition of a substance.

  • Examples include boiling point and general appearance.

• Chemical Properties

  • Defined by the reactivity of a substance when it interacts with other matter.

  • Example: A substance's specific reaction when introduced to water.

• Detailed Catalog of Properties (Physical vs. Chemical)

  • Colour: Physical; the visual perception of light reflected by the substance.

  • Solubility: Physical; the ability of a substance to dissolve in a solvent.

  • Ability to Burn: Chemical; refers to combustibility or flammability.

  • Reaction to Litmus: Chemical; indicates the acidity or alkalinity (pH) of a substance.

  • Flash Point: Chemical; the lowest temperature at which a liquid gives off enough vapor to ignite.

  • State: Physical; the phase of matter (solid, liquid, or gas) at a given temperature and pressure.

  • Ductility: Physical; the ability of a material (usually metal) to be stretched into a wire.

  • Magnetism: Physical; the force exerted by magnets when they attract or repel each other.

  • Reaction to Heat: Chemical; how a substance changes its chemical identity when heated (e.g., decomposition).

  • Crystal Formation: Physical; the geometric structural arrangement of atoms in a solid.

  • Boiling/Condensation Point: Physical; the temperature at which liquid turns to gas or gas turns to liquid.

  • Melting/Freezing Point: Physical; the temperature at which solid turns to liquid or liquid turns to solid.

  • Malleability: Physical; the ability of a substance to be hammered or rolled into thin sheets.

  • Conductivity: Physical; the ability to transmit heat or electricity.

  • Behaviour in Air: Chemical; reactions such as oxidation or tarnishing when exposed to the atmosphere.

  • Reaction with Water: Chemical; the transformation of substances upon contact with $H_2O$.

  • Reaction with Acids: Chemical; the transformation or gas production when a substance interacts with acidic solutions.

Objective 2: The Classification of Matter (Pure Substances and Mixtures)

• Pure Substances

  • Characterized by having identical particles throughout the substance.

  • The physical and chemical properties of a pure substance remain constant.

  • Elements: A pure substance that cannot be broken down into simpler parts by chemical means. It consists of only one type of atom (e.g., Helium ($He$)).

  • Compounds: A chemical combination of two or more elements in a specific, fixed ratio (e.g., pure water, $H_2O$).

• Identifying Pure Substances

  • Identified by observing characteristic properties. For instance, pure water has a fixed melting point of $0^{\circ}\text{C}$.

  • Physical Change vs. Identity: Changing the state of a substance (e.g., melting ice into liquid water) does not change its chemical identity or composition; it remains a pure substance.

  • Identification Test: If melting a solid produces two distinct substances (a gas and a liquid), the original substance was likely a mixture, not a pure substance.

• Mixtures

  • A combination of two or more pure substances in which the components retain their individual identities.

  • Properties of a mixture vary depending on the composition (the ratio of the substances involved).

• Two Primary Types of Mixtures

  1. Homogeneous Mixtures (Solutions): The separate components are not visible to the naked eye. One substance (the solute) is completely dissolved into another (the solvent). Example: Apple juice.

     • Solute: The substance being dissolved (e.g., sugar).

     • Solvent: The substance doing the dissolving (e.g., water).

  2. Heterogeneous Mixtures:

     • Mechanical Mixtures: Different substances are clearly visible (e.g., soil).

     • Suspensions: Components exist in different states (solid/liquid/gas). Example: Mud (solid dirt particles suspended in liquid water).

     • Colloids: The suspended substances cannot be easily separated from the mixture. Example: Fresh milk, which contains fat, water, and cream that do not settle out easily.

Chemical Changes and Reactions

• Definition of Chemical Change/Reaction: A process occurring when substances react to form new materials with different characteristic physical and chemical properties. This may include changes in state at room temperature, melting point, colour, or density.

• General Characteristics of Chemical Reactions:

  • Energy is always either absorbed (endothermic) or released (exothermic), usually resulting in a temperature change.

  • Phase changes may occur, such as the release of a gas (bubbles) or the formation of a precipitate.

• Diagnostic Tests for Chemical Reactions:

  • Change in Appearance: Often seen when metals transform into ionic compounds.

  • Change in Colour: This can indicate the presence of specific aqueous ions.

  • Change in Odour: Look for the distinct scents of Ammonia ($NH_3$), Acetic Acid ($CH_3COOH$), or Chlorine gas ($Cl_2$).

  • Change in State:

    • Formation of a precipitate (a solid ionic compound appearing in Single Replacement (SR) or Double Replacement (DR) reactions).

    • Production of bubbles (gas forming within an aqueous environment).

  • Change in Mass of Solid Substances: Frequently used to monitor reactions involving solid metals.

  • Change in pH: The solution becomes more or less acidic.

  • Change in Energy: The environment feels hot or cold. While this indicates whether a reaction is exothermic or endothermic, it does not definitively prove a specific reaction occurred without other data.

• Demonstration: Cobalt (II) Nitrate and Sodium Carbonate

  • Reactant Properties: Initial physical states and appearances must be noted before mixing.

  • Evidence of Reaction: Observation of diagnostic tests (e.g., precipitate formation or colour change).

  • Energy Transfer: Detection of temperature changes to determine energy flux.

A1.3 – Developing Ideas about Matter: Atomic Theory

• Definition of a Scientific Theory: A major idea that explains a vast amount of data. It is founded on numerous experiments and is subject to revision or change when new data is collected.

• Atomic Theory: Explains the fundamental structure and nature of an atom.

• Historical Timeline of Atomic Models

  1. Dalton: "Billiard Ball Model"

     • Proposed that atoms are small, solid spheres.

     • Stated that atoms differ in size, mass, and colour depending on the element type.

  2. Thomson: "Plum-Pudding Model"

     • Discovered the electron via experiments with particle beams in vacuum tubes.

     • Found that all elements produced identical beams of negative charges.

     • Postulated that atoms are made of smaller subatomic particles.

     • The model featured negative electrons embedded within a sphere of positive charge.

  3. Rutherford: "Bee-hive" or Nuclear Model

     • Discovered the atomic nucleus through the Gold Foil Experiment.

     • Experiment: Shot positively charged alpha particles at a thin sheet of gold foil.

     • Expected Result: Particles would pass straight through.

     • Actual Result: Most passed through, but some were deflected at sharp angles and some reflected directly back.

     • Metaphor: It was as shocking as firing a cannonball at a piece of tissue paper and having it bounce back.

     • Conclusions:

       • The atom contains a strong, dense, positively charged core called the nucleus.

       • The nucleus is extremely small, approximately $\frac{1}{10,000}$ the size of the entire atom.

       • The majority of an atom is empty space.

  4. Bohr: "Solar System Model"

     • Proposed that electrons orbit the nucleus in specific, fixed energy levels.

     • Evidence: Based on the specific patterns of light released by hydrogen atoms (emission spectra).

     • Mechanics: When an electron falls from a higher energy level to a lower one, it releases a specific colour of light.

     • Discovery: Every element has a unique pattern of light, implying a unique atomic structure.

• Modern Theory: Quantum Mechanical Model

  • Based on quantum mechanics rather than classical orbits.

  • The atom features an "electron cloud" surrounding the nucleus.

  • Electrons occupy the whole space of their energy level simultaneously.

  • The Nucleus contains Nucleons:

    • Protons: Positively charged particles.

    • Neutrons: Particles with no electrical charge.