8th grade · Science · Unit 1 of 13
Elements & Compounds
Formulas, properties and changes
What your child practices
Elements, compounds and mixtures, reading chemical formulas, physical and chemical properties and changes, density, and common household chemicals, including why some should never be mixed.
Texas Essential Knowledge and Skills (TEKS) teks student expectation: 8.6(A); 8.6(B) · classifying elements and compounds, and atoms in chemical formulas
How we explain it
Children can open this short lesson before they practice.
- Elements have one kind of atom; compounds are elements chemically joined, shown by formulas.
- Density = mass ÷ volume. Ice floats because it is less dense than liquid water.
- Some household chemicals, like bleach and ammonia, must never be mixed.
Example: A sample has a mass of 18 g and a volume of 12 cm³. What is its density? Density = mass ÷ volume. 18 ÷ 12 = 1.5. Answer: 1.5 g/cm³
Elements & Compounds in Texas
In the Texas Essential Knowledge and Skills (TEKS), this is unit 1 of 13 in 8th grade Science, before Chemical Change.
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: Texas Education Agency (TEA)
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
Baking soda is NaHCO₃. How many different elements does it contain?
- 4
- 2
- 6
- 3
4
Hint: Sodium, hydrogen, carbon and oxygen.
Question 2
How many atoms are in one unit of baking soda, NaHCO₃?
answer box
6
Hint: Add the subscripts, counting a missing subscript as 1, in NaHCO₃. The total is 6.
Question 3
Which is a chemical property?
- color
- melting point
- flammability (burns in air)
- density
flammability (burns in air)
Hint: A chemical property describes how a substance changes into a new substance.
Question 4
Water has a density of 1.0 g/cm³. A sample of cork has a density of 0.2 g/cm³. In water it will…
- sink
- float
float
Hint: A material floats if its density is less than the liquid's density, and sinks if it is greater.
Question 5
Physical or chemical property? Sort each.
Sort into: 📏 Physical property · 🧪 Chemical property
- 🟤Iron rusts in damp air
- ⚗️Baking soda reacts with vinegar
- 🥇Gold is malleable
- 🔌Copper conducts electricity
- ⚫Silver tarnishes
- ♨️Water boils at 100 °C
📏 Physical property: Gold is malleable, Copper conducts electricity, Water boils at 100 °C · 🧪 Chemical property: Iron rusts in damp air, Baking soda reacts with vinegar, Silver tarnishes
Hint: A physical property can be seen or measured without making a new substance. A chemical property describes how a substance reacts or changes into a new one.
Question 6
A student says, 'Burning a candle is only a physical change because the wax disappears.' What is wrong with this?
- Nothing is wrong
- Burning needs no oxygen
- Candles do not contain wax
- The wax reacts with oxygen to make new substances, such as carbon dioxide and water
The wax reacts with oxygen to make new substances, such as carbon dioxide and water
Hint: New substances form, so it is a chemical change.