Chemical Reactions Study Guide

Identifying Substances by Their Properties

  • A property is something you can observe about a substance

  • Physical properties can be observed without changing what the substance is.

  • Examples: color, shape, mass, texture, and density

  • Chemical properties describe how a substance can react with other substances

  • Examples: Flammability and ability to rust

  • The formula for density is mass/volume

  • A substance’s density is unique for that material and helps identify it.

  • Flammability describes how easily a substance can burn.

Law of Conservation of Matter

  • The Law of conservation of matter says matter cannot be created or destroyed.

  • In a chemical reaction, atoms are simply rearranged to form new substances.

  • The total number of each type of atoms is the same before and after the reaction.

What Happens During a Chemical Reaction

  • During a chemical reaction, atoms are rearranged to make new substances.

  • The new substance has new properties from the original ones.

  • Signs of a chemical reaction include:

    • color change

    • gas formation

    • temperature change

    • light or sound production

Atoms in Reactants and Products     

  • Atoms in the reactants must be found in the products.

  • No atoms are lost or gained — they are just rearranged.

  • This conservation of mass is fundamental to understanding how chemical reactions occur, ensuring that the total number of each type of atom remains constant throughout the process.

Balancing Chemical Equations

  • You CANNOT change the subscripts of a chemical equation to balance it.

  • You CAN change the coefficients of a chemical equation to balance it.

Steps to balance:

  • Count how many atoms of each element there are before balancing.

  • Write the correct subscripts for reactants and products.

  • Re-count how many atoms of each element there are.

  • Add coefficients (numbers in front of formulas) to make both sides equal.

  • Check your work that you have the same number of each atom on both sides of your equation.

Counting Atoms in a Molecule

  • Record the number of atoms by examining the chemical formula and tallying each type of atom present.

  • Use the distributive property for any coefficients to calculate the total number of atoms for complex molecules.

  • Ensure to account for all atoms, including those in polyatomic ions, during your count.

    For example, in the chemical formula 3(Ca(NO3)2), you would calculate the total as follows:

    • Calcium (Ca): 3 x 1 = 3

    • Nitrogen (N): 3 x 2 = 6

    • Oxygen (O): 3 x 6 = 18

      Thus the total atom count would yield 3 Ca, 6 N, and 18 O.

Substances vs Mixtures

  • Substance - Made of only one kind of material with definite properties. (Water)

  • Mixture - Made of two or more substances physically combined. (Air)

Determining a Mystery Substance

  • Observe physical properties (like color, state, or density).

  • .Test chemical properties (like flammability or reactivity).

  • Compare results to known data.

  • Use a controlled experiment — change only one variable at a time.

Setting Up an Experiment to Test Properties

  • Question: What do I want to test?

  • Hypothesis: What do I think will happen?

  • Variables:

    • Independent = what you change

    • Dependent= what you measure

    • Controlled= what stays the same

  • Record careful observations and data.

Chemical vs. Physical Changes

  • Physical changes result in the appearance of a substance changing. Example: Ice melting (Usually reversible).

  • Chemical changes result in creating a new substance. Example: Wood burning (Usually not reversible).

Lab Safety

Always:

  • Wear Goggles and gloves.

  • Read all directions before starting.

  • Report accidents or spills right away.

  • Keep chemicals closed and labeled.

Never:

  • Taste or smell unknown chemicals.

  • Mix substances unless instructed.

  • Run or play in the lab.