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Mix and flow of matter

Ten questions of mixed difficulty, covering Mix and flow of matter. Print it, or work through it on screen — the answer key starts on its own page.

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Mix and flow of matter

Science 8 · maddyhelps.com

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  1. Four blocks are dropped into water, which has a density of 1.0 g/cm³. W: 90 g, 100 cm³. X: 30 g, 20 cm³. Y: 45 g, 60 cm³. Z: 8 g, 10 cm³. Which will sink?

    1. a) X only
    2. b) W and Y
    3. c) X and Z
    4. d) W only

  2. Much of the bitumen in Alberta's oil sands is too deep to dig up and too thick to pump. In one method, steam is pumped down one well, and bitumen flows into a second well just below it. What does the steam do?

    1. a) It dissolves the bitumen into a watery solution
    2. b) It makes the bitumen denser, so it sinks through the sand
    3. c) It heats the bitumen, making it less viscous so it flows
    4. d) It pushes the bitumen up the same well by its pressure

  3. At 20 °C, at most 36 g of salt dissolves in 100 mL of water. A student stirs 40 g of salt into 100 mL of water at 20 °C for ten minutes. What is the result?

    1. a) 36 g dissolves, and the solution is still unsaturated
    2. b) 36 g dissolves, 4 g stays solid, and it is saturated
    3. c) None dissolves, because 40 g is over the limit
    4. d) All 40 g dissolves, because she stirred for long enough

  4. A student dissolves 12 g of salt in water to make 200 mL of solution. What is the concentration in grams per 100 mL?

    1. a) 0.06 g per 100 mL
    2. b) 12 g per 100 mL
    3. c) 6 g per 100 mL
    4. d) 24 g per 100 mL

  5. Muddy pond water looks cloudy when it is stirred. Left in a jar overnight, it clears as a layer of silt settles to the bottom. The muddy water was

    1. a) a solution
    2. b) a saturated solution
    3. c) a pure substance
    4. d) a suspension

  6. A student blocks the tips of two syringes, one full of air and one full of water, and pushes hard on each plunger. What happens?

    1. a) The water squeezes into less space; the air barely does
    2. b) Neither moves, since both tips are blocked
    3. c) Both squeeze into about half of their space
    4. d) The air squeezes into less space; the water barely does

  7. At most, 32 g of potassium nitrate dissolves in 100 mL of water at 20 °C, 64 g at 40 °C and 110 g at 60 °C. A saturated solution in 100 mL of water at 60 °C is cooled to 20 °C. About how much solid forms?

    1. a) 32 g
    2. b) 46 g
    3. c) 78 g
    4. d) 142 g

  8. A bottle in the school science lab has a WHMIS label showing liquid dripping from two test tubes onto a hand and a metal bar, eating into both. What is the hazard?

    1. a) It can catch fire if it is anywhere near a flame
    2. b) It is a gas stored under high pressure
    3. c) It can burn skin and eyes, and eat into metal
    4. d) It can make a fire burn hotter and faster

  9. A clear, colourless liquid looks exactly like water. A student heats 50 mL of it in a dish until all the liquid is gone, and a thin white crust is left behind. The liquid was

    1. a) a pure substance, since it looked like one
    2. b) a solution with a solid dissolved in it
    3. c) a suspension with solid particles in it
    4. d) pure water that turned into a white solid

  10. A raw egg sinks in a glass of tap water. A student stirs in salt, one spoonful at a time, until the egg floats. Why does it float now?

    1. a) The salt water has become denser than the egg
    2. b) The salt made the water thicker, so it holds the egg up
    3. c) The salt water has become less dense than the egg
    4. d) The salt soaked into the egg and made it lighter

Answer key · Mix and flow of matter

Science 8 · maddyhelps.com

  1. a) X only — Work out each density, d = m/V: W is 0.9, X is 1.5, Y is 0.75 and Z is 0.8 g/cm³, and only X is denser than water. W is the heaviest block, but its mass is spread through a large volume, so it floats: density decides what sinks, not mass.
  2. c) It heats the bitumen, making it less viscous so it flows — Heat lowers a thick liquid's viscosity, so the warmed bitumen flows down to the lower well, where it is brought to the surface; the method is called steam-assisted gravity drainage. Oil and water do not mix, so the steam cannot dissolve the bitumen; it only warms it.
  3. b) 36 g dissolves, 4 g stays solid, and it is saturated — 36 g per 100 mL is the salt's solubility at this temperature, the most that can dissolve, so 40 − 36 = 4 g stays at the bottom and the solution is saturated. Stirring makes salt dissolve faster, but it cannot make more of it dissolve than the limit.
  4. c) 6 g per 100 mL — 200 mL is two lots of 100 mL, so each 100 mL holds 12 ÷ 2 = 6 g of the salt, which is the solute. Answering 12 g forgets that the salt is spread through 200 mL, not 100 mL, and 24 g doubles where it should halve.
  5. d) a suspension — In a suspension, solid particles are spread through a liquid but are big enough to settle out when it stands, and a filter can catch them. The dissolved particles in a solution never settle, however long it stands, so water that clears overnight was never a solution.
  6. d) The air squeezes into less space; the water barely does — Gas particles are far apart, with empty space between them, so a push crowds them closer together; liquid particles are already touching, so water hardly compresses at all. That is why tires are filled with air, which cushions bumps, while hydraulic machines use liquids, which pass a push straight along.
  7. c) 78 g — At 60 °C the water holds 110 g, but at 20 °C it can hold only 32 g, so 110 − 32 = 78 g comes out of solution as crystals. 32 g is what stays dissolved, not what comes out, and 46 g is the answer for cooling only to 40 °C.
  8. c) It can burn skin and eyes, and eat into metal — That is the corrosion pictogram: the chemical can damage skin, eyes and metal on contact, so it is handled with gloves and goggles. A flame means flammable, a gas cylinder means a gas under pressure, and a flame over a circle means an oxidizer, which makes a fire burn more fiercely.
  9. b) a solution with a solid dissolved in it — A solution looks like a single substance because the solute's particles are spread evenly among the water particles, too small to see, and evaporating the water leaves the solute behind. Pure water leaves nothing when it evaporates, and a suspension would have looked cloudy.
  10. a) The salt water has become denser than the egg — An object floats in a fluid that is denser than it is, and dissolving salt adds mass to the water with little change in volume, so the water's density rises past the egg's. Thicker, meaning more viscous, is the tempting mix-up: viscosity decides how fast a liquid flows, not what floats in it.