Elements and compounds
Grade 8 Science Worksheet - Building Confidence
Circle the best answer, or write your answer on the line.
A box holds 2 molecules of C₂H₆O. How many atoms are in the box altogether, Ari?
Baking soda is NaHCO₃. A tray holds 3 units of it. How many atoms is that in all, Esme?
Jun says this box holds a compound because the atoms are joined. Is that right?
- No: it is a mixture, since the balls are separate from each other
- No: every ball is one kind, so it is an element
- Yes: joined atoms always make a compound, whatever kind they are
- Yes: each particle has two atoms in it, and two atoms make a compound
All the particles in this box match. Kira says it must be an element. Is that right?
- Yes: identical particles mean they hold a single kind of atom
- No: it is a mixture of H and O gases stirred together
- No: each particle holds H and O joined, so a compound
- Yes: water is very common, so it must be an element
What kind of matter is in this box, Luz?
- A compound of He and N, since both are in one box
- A compound, since it has two kinds of atoms in it
- A mixture of elements: He and N₂ are not joined
- A compound, since it has a joined pair of atoms
Baking soda is NaHCO₃. How many kinds of atoms does it have, and what is it, Noah?
- Six kinds, since it has six atoms in total
- Three kinds: Na, H and CO, so it is a compound
- Four kinds (Na, H, C, O): a compound
- Two kinds: Na and HCO₃, so it is a compound
Pia sees CO and Co on a chart. Are they the same substance?
- No: CO is carbon and oxygen; Co is cobalt
- Yes: both are cobalt, whichever letters are capitals
- No: both are compounds made of two elements
- Yes: capital letters do not change the substance
Read this first
Baking soda and vinegar (example data)
observation before mixing after mixing what is in the cup white powder and clear liquid bubbling liquid temperature (°C) 22 17 Baking soda meets vinegar in a cup. Which clue best shows a new substance formed, Ren?
- Gas bubbles formed that were not there before
- Nothing new formed, since the same atoms are in the cup
- The temperature changed, and any change means a new substance
- The powder was gone, so it must have melted
Ice melts, wood burns and sugar dissolves in tea. Which one is a chemical change, and why, Tala?
- The three together, since each change makes new substances
- Sugar dissolving, since the sugar seemed to disappear
- Wood burning, since ash and gas are new substances
- Ice melting, since it changed from a solid to a liquid
Heating sugar (example data)
property before heating after heating color white black dissolves in water yes no smell none burnt Zee heats sugar in a spoon, and the table shows what happened. Is this a chemical change?
- Yes: color, solubility and smell all changed
- Cannot tell without weighing the sample first
- No: it is the same sugar, just hotter and darker
- No: only the color changed, which is just looks