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Grade 7 Science — Physical and chemical changes

Decide whether a change has made a new substance or merely rearranged an old one, say what combustion needs, and name the slow changes that turn rock into soil.

Name: ________________________

Before you start

Look closely at a burning candle. Two different kinds of change are going on at once.

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  1. 1. The flame
  2. 2. The wick
  3. 3. The pool of melted wax
  4. 4. The solid wax of the candle

The wax melting into a pool, being drawn up the wick and turning to vapor are physical changes — it is still wax. Inside the flame, that vapor burns and becomes something else. That part is a chemical change.

  1. 1.Sand that a river carried settles on the riverbed, and over thousands of years it becomes rock. What does the chapter say about undoing changes like that?

    • a) They cannot be reversed
    • b) They can be reversed, but just in a laboratory
    • c) They can be reversed by grinding the rock back to sand
    • d) They reverse themselves each rainy season

  2. 2.Sort each change by whether a new substance was formed.

    Groups: Physical change · Chemical change

    • Cooking food
    • Crushing chalk into powder
    • Melting an ice cube
    • Folding paper into a shape
    • Adding baking soda to vinegar
    • Burning a magnesium ribbon
    • Rusting of an iron nail

  3. 3.A different question about the same changes: sort each one by whether you can get back what you started with.

    Groups: Can be reversed · Cannot be reversed

    • Melting an ice cube
    • Making popcorn from corn
    • Boiling water
    • Chopping vegetables

  4. 4.A candle burns. Which kinds of change are going on?

    • a) Chemical alone, because a flame is a chemical reaction
    • b) Physical alone, because the wax is still wax the whole way through
    • c) Neither, because the candle is just getting shorter
    • d) Both — the wax melting and evaporating are physical, and the vapor burning is chemical

  5. 5.Someone's clothes catch fire and a woollen blanket is wrapped around them. Why does that put the fire out?

    • a) It soaks up the fuel that is feeding the flames
    • b) It reflects the heat back onto the fire and smothers it with light
    • c) It cools the clothes below their ignition temperature at once
    • d) It cuts off the supply of air, so the burning has no oxygen

  6. 6.Water is boiled in an open pan and the steam is caught and cooled back into water. Which pair of words fits this change?

    • a) Physical, and it can be reversed
    • b) Chemical, and it can be reversed
    • c) Physical, and it cannot be reversed
    • d) Chemical, and it cannot be reversed

  7. 7.A black basalt rock exposed to damp air for a long time grows a red layer on its surface. What kind of change is that?

    • a) Neither — the rock was red underneath a black coating from the start
    • b) A physical change — the red is dust that has settled and stuck
    • c) A chemical change — iron in the rock has turned into iron oxide
    • d) A physical change — the surface has been worn smooth by rain

  8. 8.Follow the wax of a burning candle, starting from the step where the flame's heat melts the wax at the top of the candle.

    • The heat turns that wax into vapor
    • The liquid wax is drawn up the wick
    • The flame's heat melts the wax at the top of the candle
    • The wax vapor burns, and the flame is what you see

Answer key — Grade 7 Science — Physical and chemical changes

  1. 1. a) They cannot be reversed
  2. 2. Melting an ice cube → Physical change; Crushing chalk into powder → Physical change; Folding paper into a shape → Physical change; Rusting of an iron nail → Chemical change; Burning a magnesium ribbon → Chemical change; Adding baking soda to vinegar → Chemical change; Cooking food → Chemical change
  3. 3. Melting an ice cube → Can be reversed; Boiling water → Can be reversed; Chopping vegetables → Cannot be reversed; Making popcorn from corn → Cannot be reversed
  4. 4. d) Both — the wax melting and evaporating are physical, and the vapor burning is chemical
  5. 5. d) It cuts off the supply of air, so the burning has no oxygen
  6. 6. a) Physical, and it can be reversed
  7. 7. c) A chemical change — iron in the rock has turned into iron oxide
  8. 8. 1. The flame's heat melts the wax at the top of the candle 2. The liquid wax is drawn up the wick 3. The heat turns that wax into vapor 4. The wax vapor burns, and the flame is what you see