Skip to main content
  2. Unit Two

Unit Two

 Investigating Chemical Formulas and Reactions

Chemical formulas reveal the composition of a substance. Properties of matter change when substances are chemically combined, separated, or rearranged. All the atoms in a chemical reaction can be accounted for before and after. Students learn how formulas reveal the composition of substances and indicate the number of atoms of each element in a substance. They analyze chemical formulas to determine the number of atoms of each element present in a compound. Students use scientific practices and various tools to investigate how evidence of chemical reactions indicates that new substances with different properties are formed and the relationship between chemical reactions and the law of conservation of mass. Students construct models of chemical formulas and chemical reactions and identify the advantages and limitations of models.

Investigations of chemical reactions and the conservation of mass will be the most familiar to students since they are directly related to the experiences students can have in the classroom. Students can determine the total mass in grams of the reactants, observe evidence of chemical reactions, and determine the mass in grams of the products. Students can verify that the total mass of reactants and products are equal in reactions occurring in a closed system to demonstrate the law of conservation of mass. Students can account for lost or gained mass in reactions occurring in an open system. For example, if a gas is produced and released into the atmosphere, a student can mass the remaining solid or liquid product and determine the mass of the released gas.

Students use models of chemicals and demonstrate changing bonds between atoms. Students can be expected to understand that the numbers of atoms in a chemical reaction will not change and only the arrangement of the atoms and bonds change in the chemical reaction to adhere to the law of conservation of mass.

An understanding of the scientific conventions used to illustrate chemical reactions and the law of conservation of mass is only partially expected in the streamlined TEKS. Students are no longer expected to recognize balanced chemical equations. However, understanding chemical formulas as a convention for representing chemicals can still be expected. Students can still be expected to recognize that a chemical equation represents a chemical reaction. An understanding of the micro-level perspective of chemical reactions and the conservation of mass comes from conceptual instruction of the phenomena. 

Students communicate and discuss their observations and record and organize data in their notebooks. Students will complete several laboratory investigations his year. They will demonstrate safe practices as outlined in the Texas Education Agency-approved safety standards and consider environmentally appropriate and ethical practices with resources during investigations.

*All of the introductions can be found on TEKS Resource IFD  “Unit 02: Investigating Chemical Formulas and Reactions”

https://www.teksresourcesystem.net/module/content/search/tcmpcbrowse/~/item/693370/viewdetail.ashx

Periodic Table Study Guide - Introduction & History: Organization of the Elements

Periodic Table Study Guide - Introduction & History: Organization of the Elements

Posted Date: 08/12/2021

By Anne Marie Helmenstine, Ph.D. Updated April 26, 2018 https://www.thoughtco.com/periodic-table-study-guide-introduction-history-608127 . . .

Full Article Source

Atoms and Atomic Theory: Study Guide Facts, Problems, and Quiz

Atoms and Atomic Theory: Study Guide Facts, Problems, and Quiz

Posted Date: 08/12/2021

By Anne Marie Helmenstine, Ph.D. Updated September 19, 2018 https://www.thoughtco.com/atoms-and-atomic-theory-study-guide-604134 . . .

Full Article Source

How Many Atoms There Are in the Human Body: Atoms in the Body

How Many Atoms There Are in the Human Body: Atoms in the Body

Posted Date: 08/12/2021

By Anne Marie Helmenstine, Ph.D. Updated April 05, 2019 https://www.thoughtco.com/how-many-atoms-are-in-human-body-603872 . . .

Full Article Source

10 Interesting Facts About Atoms: Useful and Interesting Atom Facts and Trivia

10 Interesting Facts About Atoms: Useful and Interesting Atom Facts and Trivia

Posted Date: 08/12/2021

By Anne Marie Helmenstine, Ph.D. Updated September 14, 2019 https://www.thoughtco.com/interesting-facts-about-atoms-603817 . . .

Full Article Source

The Most Important Molecules in Your Body

The Most Important Molecules in Your Body

Posted Date: 08/08/2022

By Anne Marie Helmenstine, Ph.D. Updated on July 03, 2019 . . .

Full Article Source

What Is a Molecule?

What Is a Molecule?

Posted Date: 08/08/2022

By Anne Marie Helmenstine, Ph.D. Updated on July 18, 2022 https://www.thoughtco.com/what-is-a-molecule-definition-examples-608506 . . .

Full Article Source

Is Energy Released When Chemical Bonds Are Broken or Formed?

Is Energy Released When Chemical Bonds Are Broken or Formed?

Posted Date: 08/08/2022

By Anne Marie Helmenstine, Ph.D. Updated on July 17, 2019 https://www.thoughtco.com/when-energy-is-released-in-chemical-bonding-603989 . . .

Full Article Source

How Flame Test Colors Are Produced

How Flame Test Colors Are Produced

Posted Date: 08/08/2022

By Anne Marie Helmenstine, Ph.D. Updated on February 21, 2020 https://www.thoughtco.com/how-flame-test-colors-are-produced-3963973 . . .

Full Article Source

Recognize 

USES OF CHEMICAL FORMULAS

Including, but not limited to:

  • Chemical formula – a representation of a molecule or compound in which the elements are represented by their symbols and subscripts represent the number of atoms of each element
  • Identify substances

Determine

THE NUMBER OF ATOMS OF EACH ELEMENT IN CHEMICAL FORMULAS CONTAINING SUBSCRIPTS

Including, but not limited to:

  • Differentiate between numbers of atoms of each element present in a formula and total number of atoms present in the formula overall
    • Count number of atoms of individual elements present within a formula
      • Parentheses are used to indicate more than one group of atoms within a chemical formula. The subscript outside the parentheses indicates the number of groups within the parentheses
        • Example:
          • (NH4)2CO3 - there are two groups of NH4
          • NH4 + NH4 = (NH4)2
          • N = 2 atoms
          • H = 8 atoms
    • Coefficients in front of a chemical formula represent how many molecules of a given formula are present. No coefficient represents one.
      • Example:
        • N2 + 3H2 → 2NH3
        • 1 N2 = N2
        • 3H2 = H2 + H2 + H2 (6 atoms of H)
        • 2NH3 = NH3 + NH3 (2 atoms of N and 6 atoms of H)
    • Coefficients with parentheses and subscripts in a chemical formula represent how many molecules of a given formula are present. No coefficient represents one.
      • Example:
        • 4Fe(OH)3 = Fe (OH)3 +  Fe (OH)3 + Fe (OH)3 + Fe (OH)3
        • 4 atoms of Fe
        • 12 atoms of O
        • 12 atoms of H
    • Count number of total atoms within a formula
      • Example:
        • (NH4)2CO3 - there are two groups of NH4
        • 2×(1 atom of N + 4 atoms of H) + 1 atom of C + 3 atoms of O
        • 2(1 + 4) + 1 + 3
        • 2(5) + 4
        • 10 + 4 = 14 total atoms in (NH4)2CO3

 

Key Content Vocabulary:

  • Chemical change – the formation of a new substance with different properties; cannot be undone by physical means
  • Chemical equation – a representation of a chemical reaction by symbols and numbers
  • Chemical formula – a representation of a molecule or compound in which the elements are represented by their symbols and subscripts represent the number of atoms of each element
  • Chemical reaction – a change caused by the interaction of two or more substances (reactants) resulting in the formation of new substances (products)
  • Coefficient – the number placed in front of a chemical formula in a chemical equation; represents the number of molecules of that substance
  • Compound – a substance made of two or more elements
  • Law of conservation of mass – matter is not created or destroyed; only rearranged
  • Molecule – combined atoms of the same element (e.g., O2)
  • Precipitate – the formation of solids from a solution
  • Product – substance(s) resulting from a chemical reaction
  • Reactant – substances that are combined and changed during a chemical reaction
  • Subscript – the number written to the right and slightly below an element in a chemical formula; represents the number of atoms of an element present in a chemical formula

 

Related Vocabulary:

  • Element
  • Physical change

 

*All of the introductions can be found on TEKS Resource IFD  “Unit 02: Investigating Chemical Formulas and Reactions”

https://www.teksresourcesystem.net/module/content/search/tcmpcbrowse/~/item/693370/viewdetail.ashx

Investigate

HOW EVIDENCE OF CHEMICAL REACTIONS INDICATES THAT NEW SUBSTANCES ARE FORMED AND HOW THAT RELATES TO THE LAW OF CONSERVATION OF MASS

Including, but not limited to:

  • Physical change – change which alters the physical properties of a substance without changing its identity
  • Chemical change – the formation of a new substance with different properties; cannot be undone by physical means
  • Observe and recognize signs of chemical change
    • Production of a gas
      • Odor
      • Bubbling
    • Change in temperature
    • Production of a precipitate
      • Precipitate – the formation of solids from a solution
    • Color change (permanent)
    • Production of light
    • Change in mass
  • Compare differences in properties between products and reactants of chemical equations
  • Law of conservation of mass – matter is not created or destroyed; only rearranged
    • Macro scale
      • Scientists can measure the total mass of the reactants prior to a chemical reaction and the total mass of products after a chemical reaction
      • When all matter is accounted for, scientists can confirm the law of conservation of mass by showing the total mass of the reactants equals the total mass of the products
        • If a chemical reaction is performed in an open system, the total mass of the products may be more or less than the total mass of the reactants
        • Possible examples:
          • Gas is formed during a chemical reaction and not massed
          • Solid particles (e.g., smoke) are released and not massed
          • Gaseous water cooling and condensing on the system containing the reaction adding to the mass
    • Micro scale
      • During chemical reactions, atoms of substances (reactants) are rearranged to form a new substance (products)
      • Recognize that all of the atoms involved in a chemical reaction can be accounted for