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?
All the particles in this box match. Kira says it must be an element. Is that right?
- Yes: water is very common, so it must be an element
- No: it is a mixture of H and O gases stirred together
- No: each particle holds H and O joined, so a compound
- Yes: identical particles mean they hold a single kind of atom
What kind of matter is in this box, Luz?
- A compound, since it has two kinds of atoms in it
- A compound of He and N, since both are in one box
- A mixture of elements: He and N₂ are not joined
- A compound, since it has a joined pair of atoms
Milo's sulfur sample has the formula S₈. Is it an element or a compound, and why?
- A compound, since eight atoms mean eight different kinds
- A compound, since the eight atoms are joined together
- An element, since all eight atoms are sulfur
- A mixture, since the eight atoms are mixed together
Baking soda is NaHCO₃. How many kinds of atoms does it have, and what is it, Noah?
- Three kinds: Na, H and CO, so it is a compound
- Four kinds (Na, H, C, O): a compound
- Six kinds, since it has six atoms in total
- 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
- Yes: capital letters do not change the substance
- No: both are compounds made of two elements
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
- The powder was gone, so it must have melted
- The temperature changed, and any change means a new substance
- Nothing new formed, since the same atoms are in the cup
Ice melts, wood burns and sugar dissolves in tea. Which one is a chemical change, and why, Tala?
- Ice melting, since it changed from a solid to a liquid
- Sugar dissolving, since the sugar seemed to disappear
- Wood burning, since ash and gas are new substances
- The three together, since each change makes new substances
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?
- No: it is the same sugar, just hotter and darker
- No: only the color changed, which is just looks
- Yes: color, solubility and smell all changed
- Cannot tell without weighing the sample first