Elements and compounds
Grade 8 Science Worksheet
Circle the best answer, or write your answer on the line.
A tray holds 3 units of Mg(NO₃)₂. How many oxygen atoms are on the tray in all, Ari?
Bo's sheet lists 4 units of Al₂(SO₄)₃. How many oxygen atoms is that altogether?
Which box holds a mixture of an element and a compound, Jun?
- Box B, since its balls are joined in pairs
- Box D: O₂ pairs and CO₂ groups together
- Box A, since it holds single balls of one kind
- Box C, since it has two kinds of atoms in a group
Which box holds a pure compound, Kira?
- Box D, since every ball in the box matches
- Box B, since it contains some water groups too
- Box C, since it has H and O in the same box
- Box A: every group is H and O joined
Which box holds one element made of molecules (joined atoms), Luz?
- Box B, since the joined atoms are all one kind
- Box A, since it is a pure element of single balls
- Box D, since it holds joined pairs of atoms
- Box C, since every group in it is identical
Read this first
Burning a metal in air (example data)
property before burning after burning appearance shiny gray metal white powder melting point (°C) 660 2072 mass (g) 2.7 5.1 A shiny metal burns in air and leaves a white powder. What do the data show, Milo?
- A physical change, since the metal just got hotter and brighter
- A new substance formed, since the melting point changed
- No new substance, since a melting point cannot ever change
- Mass was created, since 5.1 g is more than the 2.7 g
Melting ice (example data)
property ice liquid after melting state solid liquid freezing or melting point (°C) 0 0 density (g/cm³) 0.92 1.00 Ice melts in a cup. Which conclusion do the data support, Noah?
- Chemical: the density changed from 0.92 to 1.00
- Chemical: it changed from a solid to a liquid state
- Physical: it still freezes at 0 °C, so it is water
- Chemical: it needed heat to make the change
Salt and water (example data)
property salt before salt after the water boiled away appearance white crystals white crystals melting point (°C) 801 801 mass (g) 5.0 5.0 Pia dissolves salt in water, then boils the water away. Does the table show a chemical change?
- Yes: the mass should have changed after dissolving
- Cannot tell, since a melting point does not identify salt
- No: the crystals return with the same melting point
- Yes: the crystals disappeared when put in the water
Hydrogen, oxygen and water (example data)
substance state at room temperature with a flame hydrogen (H₂) gas burns oxygen (O₂) gas helps things burn water (H₂O) liquid puts out many fires What does the table show about water compared with hydrogen and oxygen, Ren?
- Water acts like a mix of the two gases it is made of
- Water has its own properties: a liquid that does not burn
- Water burns the same way, since hydrogen burns
- Water is just hydrogen and oxygen gas stirred together
Sodium, chlorine and sodium chloride (example data)
substance appearance with water sodium (Na) soft shiny metal reacts strongly chlorine (Cl₂) yellow-green gas dissolves a little sodium chloride (NaCl) white crystals dissolves without a strong reaction Which statement does the table support about sodium chloride, Tala?
- NaCl has properties unlike either element in it
- NaCl is a gas, since chlorine is in it
- NaCl reacts strongly with water, since sodium does
- NaCl is a shiny metal, since sodium is in it