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Shaken, Chilled or Scorched? Three Kitchen Changes

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Shaken, Chilled or Scorched? Three Kitchen Changes

Use density, melting point and solubility to decide which kitchen change made a new substance.

The dressing and the ice cream

Read the passage and Table 1, then answer the questions.

Three Changes on a Saturday

1On a Saturday afternoon, seventh grader Ilaria Conti and her younger brother Rocco cooked with their aunt, Dr. Nkem Obi, a food scientist. They made a salad dressing, a batch of ice cream in a bag and a sugar topping that Dr. Obi heated on the stove. Ilaria wanted to know which of the three changes made a new substance. She measured masses and volumes at home, and Dr. Obi took small samples to her lab to measure melting points.

2For the dressing, Ilaria poured 40.0 cm³ of olive oil and 20.0 cm³ of vinegar into a jar, closed the lid and shook it hard. The liquid turned cloudy and pale yellow. She weighed the jar's contents: 57.0 g. After standing for 10 minutes, the dressing separated into two clear layers, with the oil on top. Ilaria used a dropper to lift each layer into its own measuring cup.

3For the ice cream, Rocco sealed cream, sugar and vanilla in a small bag and packed it inside a larger bag of crushed ice. He poured 60 g of table salt over the ice, sealed the big bag and shook it for 10 minutes. A thermometer in the ice dropped from 0 °C to −11 °C, and the cream froze. Afterward, Dr. Obi boiled the salty water from the big bag in a pot until only white crystals were left.

4For the topping, Dr. Obi heated 20.0 g of white table sugar in a dry pan while the children watched from a safe distance. The crystals melted into a clear liquid, which turned golden and then brown. Steam rose from the pan, and the kitchen filled with a sharp, burnt smell. She kept heating until only a black, crumbly solid was left in the pan.

5Dr. Obi reminded them that each pure substance has its own set of properties, such as density, melting point and whether it dissolves in water.

Table 1. Properties Before and After
SampleLooks likeDensity (g/cm³)Melting point (°C)Dissolves in water?
Olive oil, beforeYellow liquid0.92Liquid at 20 °CNo
Vinegar, beforeClear liquid1.01Liquid at 20 °CYes, mixes in
Top layer, afterYellow liquid0.92Liquid at 20 °CNo
Bottom layer, afterClear liquid?Liquid at 20 °CYes, mixes in
Table salt, beforeWhite crystals2.17801Yes
Crystals from ice waterWhite crystals2.17801Yes
Table sugar, beforeWhite crystals1.59186Yes
Black solid from panBlack, crumblyNot measuredNone: still solid at 500 °CNo
The black solid is full of tiny air pockets, so its density could not be measured well.
1.
Density = mass ÷ volume. The bottom layer had a volume of 20.0 cm³ and a mass of 20.2 g.

The density of the bottom layer is g/cm³.

2.
Use paragraph 2. Find the density of the cloudy dressing right after shaking, before it separated.

The density of the cloudy dressing was g/cm³.

3.
The cloudy dressing's density did not match the oil or the vinegar. Does that show that a new substance formed?
Question 3 options
4.
Rocco thinks the salt and ice reacted, because the temperature dropped to −11 °C. Which evidence best shows that the salt was still there after the ice melted?
Question 4 options
5.
Rocco worries that the crystals from the ice water might be sugar from the ice cream. Which property in Table 1 would NOT help tell salt and sugar apart?
Question 5 options

The sugar in the pan

Use Table 1 and paragraph 4.

6.
Sort each observation from the afternoon.
7.
The next day, Ilaria finds a dark, crumbly lump on the stove top. Which single test would best show whether it is unburned sugar or the black solid?
Question 7 options
8.
Ilaria wonders what would happen if Dr. Obi heated sugar only until it melted into a clear liquid, then let it cool into a clear, glassy solid. Which data would best show whether the glassy solid is a new substance?
Question 8 options
9.
Which conclusion about the three changes does Table 1 support best?
Question 9 options
10.
Rocco says, "All three changes made new substances. The dressing turned cloudy, the ice got colder and the sugar turned black." Do you agree? Write a claim, give property evidence from Table 1 for at least two of the changes, and explain your reasoning.