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?
A bag holds 3 units of Ca₃(PO₄)₂. How many oxygen atoms does Esme count in the bag?
Which box holds a mixture of an element and a compound, Jun?
- Box C, since it has two kinds of atoms in a group
- Box A, since it holds single balls of one kind
- Box D: O₂ pairs and CO₂ groups together
- Box B, since its balls are joined in pairs
Which box holds a pure compound, Kira?
- 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
- Box D, since every ball in the box matches
Which box holds one element made of molecules (joined atoms), Luz?
- Box D, since it holds joined pairs of atoms
- Box C, since every group in it is identical
- Box B, since the joined atoms are all one kind
- Box A, since it is a pure element of single balls
Read this first
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 crystals disappeared when put in the water
- No: the crystals return with the same melting point
- Yes: the mass should have changed after dissolving
- Cannot tell, since a melting point does not identify salt
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 is just hydrogen and oxygen gas stirred together
- Water burns the same way, since hydrogen burns
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 is a gas, since chlorine is in it
- NaCl has properties unlike either element in it
- NaCl is a shiny metal, since sodium is in it
- NaCl reacts strongly with water, since sodium does
Carbon, oxygen and carbon dioxide (example data)
substance state at room temperature with a flame carbon (C) black solid burns oxygen (O₂) gas helps things burn carbon dioxide (CO₂) gas does not burn and is used in some fire extinguishers Which statement does the table support about carbon dioxide, Zee?
- CO₂ helps things burn, since oxygen does
- CO₂ burns just like carbon does in air
- CO₂ is a black solid, since carbon is in it
- CO₂ differs from both carbon and oxygen