Chemical and Physical Changes — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. To separate iron filings from sand, you can use a strong magnet to attract the metal pieces.
Magnets work because iron is magnetic while sand is not, so the iron filings stick to the magnet and sand stays behind.
2. Pouring muddy water through paper traps the dirt and lets clean water pass; this method is called filtering.
Filtering separates solids from liquids based on size: large dirt particles cannot pass through tiny holes in the filter paper, but water molecules can.
3. When salt water sits in the sun, the water turns to vapor and salt is left behind through evaporation.
Evaporation works for solutions because the liquid water leaves as vapor while the dissolved salt stays as solid crystals, called the residue.
4. A kitchen sieve can separate flour from rice through a process called sifting, which uses a mesh.
Sifting separates solids of different sizes: small flour particles fall through the mesh holes while larger rice grains stay on top.
5. The clear liquid that passes through a coffee filter is called the filtrate, while the grounds left behind are residue.
Filtrate is the scientific term for the liquid that passes through a filter, while residue is the solid material trapped on the filter paper.
6. The solid material left behind on filter paper after filtering muddy water is called the residue.
Residue is the solid material that does not pass through a filter. In muddy water, dirt and sand particles form the residue while clean water becomes the filtrate.
7. To separate a mixture of sugar and pebbles, you could dissolve the sugar in water, then filter to remove pebbles.
Sugar dissolves in water but pebbles do not. Filtering catches the solid pebbles, and you can later evaporate the water to recover the sugar.
8. Separating mixtures is a physical change because no new substances are formed during the process.
Separation methods are physical changes because the original substances keep their identities; we only sort them apart, never create new compounds.
9. Iron filings stuck to a magnet can be wiped off and used again because separation is reversible.
Separation is a physical change and reversible because the iron filings keep all their original properties and can be remixed with sand again.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Iron filings from sand
→ Magnet
Sifting
Mud from water
→ Filtering
Magnet
Salt from saltwater
→ Evaporation
Filtering
Flour from rice grains
→ Sifting
Evaporation
Magnets attract iron, filters trap solids in liquids, evaporation removes water from dissolved solids, and sifting sorts solids by size through mesh.