Chemistry Review on Matter and Molecules

Chemistry Review

• Chemistry is the study of matter
• Matter is anything that takes up space
• Matter can be categorized as either pure substances or mixtures
  • Pure substances have a constant composition
    • Elements are pure substances that cannot be broken down into simpler units (e.g., Hydrogen (H), Calcium (Ca))
    • Compounds are pure substances that contain two or more different elements in a fixed proportion (e.g., Water (H₂O), Carbon dioxide (CO₂), Sugar (C₆H₁₂O₆))
  • Mixtures have a variable composition
    • Solutions are homogeneous mixtures (e.g., salt in water). Only one type of substance is visible
    • Mechanical mixtures are heterogeneous mixtures (e.g., oil and water) where more than one substance is visible

Molecules

• Molecules are formed when two or more atoms join together (e.g., O₂ - oxygen molecule, NH₃ - ammonia molecule, H₂O - water molecule)

Homogeneous and Heterogeneous Matter

• Homogeneous matter contains only one type of substance (e.g., salt in milk, oil)
• Heterogeneous matter contains more than one type of substance (e.g., mixtures of salt and pepper or oil and water)

Mixtures

• Mixtures combine two or more substances
• Substances in a mixture can be combined in different ways

Properties of Matter

• Properties of matter are used to identify different types of matter
• Two types of properties exist: physical and chemical
• Physical properties can be observed or measured without changing the substance (e.g., temperature, colour, volume, texture, mass, taste, clarity, hardness)
• Chemical properties describe how a substance reacts with another substance to produce a new substance (e.g., flammability, reactivity, corrosiveness)

Characteristic Physical Properties

• Characteristic physical properties are properties unique to a particular substance that distinguish it from other substances.
  • Example: Boiling point
    • Boiling point of a substance is usually unique. (e.g., water boils at 100°C)

Chemical Properties

• Chemical properties describe how a substance reacts with another substance during a chemical reaction to form a new substance (e.g., Oxygen burns something, reacting to form rust, reacting with Chlorine to form salt)

Changes in Matter

• Physical change is a change in the state of matter, but no new substance is formed (e.g., melting, dissolving, freezing)
• Chemical change is a change in matter where a new substance is formed with different properties than the original substance(s). (e.g., burning, rusting)

Clues to Identify Chemical Changes

• Clues that a chemical change has happened:
  • Temperature change (hot or cold)
  • Colour change
  • Production of gas bubbles
  • Reactants are used up
  • Precipitate formation (a new substance that doesn't dissolve and settles out of the mixture)