Topic 11 Why the total mass always holds — what needs James (21)

Topic 11 · Why the total mass always holds · Type 1 · tests MAT5-060

EasyMCQType 1 — what the scale reads after a change inside a sealed container (a shake, a squeeze, a melt, a freeze, a thaw)

A sealed glass jar of marbles reads 540 grams, and Lucas shakes it until the marbles are all jumbled up. What does the scale read now?

1. a) less than before
2. b) the same as before ← key
3. c) more than before
4. d) you cannot tell from this

Why it is this type: The total mass stays the same, whatever change the stuff goes through. The jar stayed sealed, so no stuff was added or taken away. So the scale reads the same as before: 540 grams.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 2 · tests MAT5-060

EasyMCQType 2 — the word for a mass that stayed the same through a change

A sealed pencil case reads 200 grams before and after a pencil inside it breaks in two. Which word says what happened to the mass?

1. a) conserved ← key
2. b) dissolved
3. c) evaporated
4. d) melted

Why it is this type: When the mass stays the same through a change, we say it is conserved. The case reads 200 grams before and after. So the mass was conserved.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 3 · tests MAT5-063

EasyMCQType 3 — what the student does first / next in the routine

Lena wants to check that the mass is conserved when popping candy fizzes in a sealed jar of water. What does she do first?

1. a) measure the mass of everything that will be in the test ← key
2. b) make the change
3. c) measure the mass of everything again
4. d) compare the two readings

Why it is this type: Measure the mass of everything before the change. So Lena measures the mass of everything that will be in the test first.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 4 · tests MAT5-062

HardMCQType 4 — which sentence says why the reading held, or tells you the mass is conserved, or is true of the jar in the morning

A sealed glass jar of warm water reads 400 grams. Overnight some water evaporates inside, and drops form under the lid. Which sentence about the jar in the morning is true?

1. a) The jar reads less than 400 grams, because some water turned into water vapor.
2. b) The jar reads 400 grams, because the water vapor and the drops are all still inside. ← key
3. c) The jar reads more than 400 grams, because drops were added under the lid.
4. d) The jar reads less than 400 grams, because warm water loses mass as it cools.

Why it is this type: Inside a sealed container, the mass stays the same when ice melts, water freezes or water evaporates. The water that evaporated is water vapor inside the jar, and the drops are that water turned back into liquid. So the water is all still inside, and the jar reads 400 grams. Hard because: the hard-to-spot member, read against three readings that each follow a belief. (every option here is a reading WITH its reason in one frame — the four share one shape, so no option is a bundled verdict against bare ones; recorded as the one type whose options carry a because, all four alike)

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 5 · tests MAT5-063

EasyMCQType 5 — what two matching readings tell you; what to measure after; what to ask when the readings differ

A sealed jar of sand and water reads 730 grams before shaking and 730 grams after. What do the two readings tell you?

1. a) The sand lost some mass.
2. b) The water gained some mass.
3. c) The scale needs fixing.
4. d) The mass was conserved. ← key

Why it is this type: If the two readings are the same, the mass was conserved. Both readings are 730 grams. So the two readings tell you the mass was conserved.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 6 · tests SR5-007

MediumMCQType 6 — how much the result counts now; which thing makes it count for more; which result counts for the most

[pp_table_swing: group one 30; group two 30; group three 31] Three groups count how many times a swing goes back and forth in one minute, and here are their counts. Which result counts for the most?

1. a) the third group's result, 31 times
2. b) no result, because the groups did not all agree
3. c) the first group's result on its own
4. d) the result that two groups got, 30 times ← key

Why it is this type: When a different group does the same test and gets the same result, the result counts for more. Two groups got 30 times, and one group got 31. So the result two groups got counts for the most.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 7 · tests MAT5-063

MediumMCQType 7 — the correct reason: why the second reading is more (the lid slip); why two groups' result counts for more; why one result could still be wrong; what happened when the lid was forgotten (hard)

A boy measures a jar of water without its lid at 400 grams, then drops in a stone, screws the lid on, shakes the jar and measures it with the lid at 420 grams. What is the correct reason the second reading is more?

1. a) The lid and the stone were not on the pan for the first reading. ← key
2. b) Shaking the jar hard made new stuff appear inside it.
3. c) A stone gets heavier when it is shaken in water.
4. d) A scale always reads more the second time it is used.

Why it is this type: The same set of things goes on the pan before and after. The lid and the stone were not on the pan for the first reading. So the second reading is more by their masses.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 8 · tests MAT5-061

EasyMCQType 8 — which two masses give the solution's mass; which sentence is true of a dissolved powder; the total as a number

A pail of water reads 900 grams and a cup of pickling salt reads 60 grams, and the salt is stirred in until it dissolves. Which two masses give the mass of the salty water?

1. a) the water's 900 grams and the salt's 60 grams ← key
2. b) the water's 900 grams on its own
3. c) the salt's 60 grams on its own
4. d) the pail's mass and the cup's mass only

Why it is this type: When a solid dissolves in a liquid, the solution's mass is the solid's mass added to the liquid's mass. The salt is the solid, and the water is the liquid. So the two masses are 900 grams and 60 grams.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 9 · tests MAT5-064

EasyMCQType 9 — where the missing water went

A saucer of water sits on a warm heater with nothing over it, and by evening the saucer reads 40 grams less. Where did the missing water go?

1. a) into the air, as a gas ← key
2. b) into the pan of the scale
3. c) into the table under the scale
4. d) nowhere, because it stopped existing

Why it is this type: When the reading goes down, the stuff is not gone. It left the container as a gas into the air. The saucer was open, and the water evaporated. So the missing water went into the air, as water vapor.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 10 · tests MAT5-061

HardtypedType 10 — the solution's total (medium); the reading after a sealed change (medium); the difference that went into the air (medium, drawn); the solid's mass from the total and the liquid (hard); two changes chained in a sealed can (hard)

A bucket of water reads 2,000 grams. A boy stirs in some salt for a science-fair test, and it dissolves. Now the bucket of salty water and the empty salt box together read 2,130 grams. How many grams of salt did he stir in? Type the number.

Accepted answers (typed): 130 / 130 grams / 130 g / 130 gram / one hundred thirty / one hundred thirty grams / one hundred and thirty / one hundred and thirty grams.

Why it is this type: When a solid dissolves in a liquid, the solution's mass is the solid's mass added to the liquid's mass. So the solid's mass is the solution's mass take away the liquid's mass. 2,130 − 2,000 = 130, so he stirred in 130 grams of salt. Hard because: the rule is run backwards.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 11 · tests MAT5-064

MediumMCQType 11 — which sentence says why (the pot reads more; the foggy jar reads the same; the dog bowl reads less)

A dog's open water bowl reads less every evening than it did in the morning, even on a day the dog is away. Which sentence says why?

1. a) The bowl's mass shrinks a little every day.
2. b) The water sinks into the floor under the bowl.
3. c) Some of the water evaporates into the air during the day. ← key
4. d) Water left in a bowl slowly stops existing.

Why it is this type: When the reading goes down, the stuff is not gone. It left the container as a gas into the air. The bowl is open, so some water evaporates into the air. So the bowl reads less by evening.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 12 · tests MAT5-064

HardMCQType 12 — what left the container

An open loaf of bread is left out for two days, goes stale and hard, and reads 30 grams less than before. What left the bread?

1. a) flour, as a powder onto the counter
2. b) water, as water vapor into the air ← key
3. c) the bread's color, into the air
4. d) nothing, because bread cannot lose mass

Why it is this type: When the reading goes down, the stuff is not gone. It left the container as a gas into the air. Fresh bread holds water, and the loaf was open to the air. So water left the bread as water vapor into the air, and the loaf reads less. Hard because: the water is hidden inside a solid.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 13 · tests MAT5-064

HardMCQType 13 — X is wrong. What is the correct reason?

Lena is wrong to say that the 50 grams of water missing from an open fish tank after a week stopped existing. What is the correct reason?

1. a) The fish in the tank drank the 50 grams of water during the week.
2. b) The glass walls of the tank soaked up the 50 grams of water.
3. c) The water left the tank as water vapor and is in the air of the room. ← key
4. d) The water sank down through the tank's base into the stand.

Why it is this type: When the reading goes down, the stuff is not gone. It left the container as a gas into the air. The tank was open, so water evaporated into the air of the room. So the water did not stop existing, and Lena is not right. Hard because: the belief (MAT5 SD-4: mass disappears when water boils or dries away) is judged by its correct reason.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 14 · tests MAT5-062

Hardmulti-selectType 14 — select the TWO true sentences

Select the TWO sentences that are true of a sealed bag of ice after the ice has melted.

1. a) The bag holds water, the same stuff as the ice. ← key
2. b) The bag's mass is the same as before the ice melted. ← key
3. c) The bag's mass is less, because melted ice takes up less room.
4. d) The bag holds a new liquid that is not water.

Why it is this type: Ice, water and water vapor are the same stuff: water. Inside a sealed container, the mass stays the same when ice melts, water freezes or water evaporates. So the bag holds water, and its mass is the same as before. Hard because: the answer is a set.

Recommendation: Approve as the exemplar

Topic 11 · Why the total mass always holds · Type 15 · tests MAT5-064

HardMCQType 15 — the minimal pair (lid on / lid off) read with both rules; what to count to show the total holds

[pp_pots_800: a pot with a lid on, 800 g; a pot with no lid, 800 g; both before warming] Here are two matching pots of water, each reading 800 grams before warming. Both are warmed gently for an hour, so that some water in each pot evaporates. Which sentence is true afterward?

1. a) The pot with no lid reads less, and the pot with the lid reads 800 grams. ← key
2. b) Both pots read less than 800 grams.
3. c) Both pots read 800 grams.
4. d) The pot with the lid reads less, and the pot with no lid reads 800 grams.

Why it is this type: In the pot with the lid, the water vapor stays inside. In the pot with no lid, the water vapor leaves into the air. So the open pot reads less, and the lidded pot reads 800 grams. Hard because: two rules chained on a minimal pair.

Recommendation: Approve as the exemplar