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9th grade · Science · Unit 7 of 10

Chemical Change

Reactions, mass and chemical names

What your child practices

Telling a chemical change from a physical change, reading simple word and symbol equations, why mass is conserved, what speeds a reaction up, exothermic and endothermic changes, and GHS safety.

Arizona Academic Standards learning standard: HS.P1U1.3 · chemical reactions and conservation of atoms

How we explain it

Children can open this short lesson before they practice.

  1. Signs of a chemical change: gas bubbles, a color change, a new solid, light or heat.
  2. In an equation, count atoms: the big number multiplies the whole formula; the small number counts one element.
  3. Mass is conserved. Read the hazard symbols on chemicals before using them.

Example: In 2H₂O, how many hydrogen atoms are there? Each H₂O has 2 hydrogen atoms. There are 2 molecules: 2 × 2 = 4. Answer: 4

Chemical Change in Arizona

In the Arizona Academic Standards, this is unit 7 of 10 in 9th grade Science, after The Periodic Table and before Evolution & Selection.

Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.

Source: Arizona Department of Education

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Which is a physical change?

    • wood burning
    • milk turning sour
    • ice melting
    • iron rusting

  2. Question 2

    A cold pack gets cold when the chemicals inside are mixed. This is an example of…

    • a physical change only
    • an endothermic reaction
    • combustion
    • an exothermic reaction

  3. Question 3

    In 2Mg + O₂ → 2MgO, how many magnesium atoms are on the left side (reactants)?

    answer box

  4. Question 4

    In a chemical reaction, what are the products?

    • the catalysts
    • the new substances formed
    • the starting substances
    • the equipment

  5. Question 5

    18 g of one substance reacts completely with 32 g of another in a closed container to form one product. What is the mass of the product?

    • 47 g
    • 14 g
    • 50 g
    • 55 g

  6. Question 6

    Which is a way to make a reaction go faster?

    • use bigger lumps instead of powder
    • raise the temperature
    • make the reactants more dilute
    • lower the temperature

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