Worksheet

Shaken, Chilled or Scorched? Three Kitchen Changes

Grade 7 physical science: compare properties before and after mixing a dressing, chilling ice cream with salt and scorching sugar to spot a chemical reaction.

Grade 7ScienceInteractive · Printable

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What’s included

Single worksheet
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1 NGSS standard
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10 questions
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Details

Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students calculate densities and compare properties of substances before and after a change.
Students use density, melting point and solubility data to decide whether a new substance formed.
Students explain why a change in temperature, color or a mixture's density does not by itself show a reaction.

How to use

Read the three changes aloud and ask students to predict which made a new substance before they look at Table 1. Discuss why a mixture can have a density between those of its parts.

Curriculum

NGSS standard covered

MS-PS1-2

Use data on properties such as density, melting point and solubility to decide whether a chemical reaction happened.

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Name Date

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
Sample	Looks like	Density (g/cm³)	Melting point (°C)	Dissolves in water?
Olive oil, before	Yellow liquid	0.92	Liquid at 20 °C	No
Vinegar, before	Clear liquid	1.01	Liquid at 20 °C	Yes, mixes in
Top layer, after	Yellow liquid	0.92	Liquid at 20 °C	No
Bottom layer, after	Clear liquid	?	Liquid at 20 °C	Yes, mixes in
Table salt, before	White crystals	2.17	801	Yes
Crystals from ice water	White crystals	2.17	801	Yes
Table sugar, before	White crystals	1.59	186	Yes
Black solid from pan	Black, crumbly	Not measured	None: still solid at 500 °C	No

The black solid is full of tiny air pockets, so its density could not be measured well.

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.

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.

The cloudy dressing's density did not match the oil or the vinegar. Does that show that a new substance formed?

A. Yes, because the liquid turned cloudy, and a change in look shows a reaction.
B. No, because they were only mixed: after separating, each layer had its starting density.
C. Yes, because the shaken liquid had a density that neither starting liquid had.
D. No, because density can be measured for solids but not for liquids like these.

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?

A. The crystals left after boiling matched table salt's density, melting point and solubility.
B. The ice in the big bag melted faster once the salt was poured over it.
C. The thermometer in the bag read −11 °C, colder than the 0 °C ice at the start.
D. The cream in the small bag froze solid after 10 minutes of hard shaking.

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?

A. Whether it dissolves in water
B. Density
C. Melting point

The sugar in the pan

Use Table 1 and paragraph 4.

Sort each observation from the afternoon.

The black solid does not dissolve in water, but sugar does.The sugar crystals melted into a clear liquid.The black solid stayed solid at 500 °C, far above sugar's melting point.The pan and the sugar became very hot on the stove.The dressing turned cloudy when it was shaken.Steam rose from the pan, though only dry sugar was in it.The ice in the big bag got colder than 0 °C.

Evidence that a new substance formed	Not evidence of a new substance

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?

A. Hold the lump up to a light to see its color.
B. Drop the lump in water to see if it dissolves.
C. Weigh the lump on the kitchen scale.
D. Check whether the lump is still warm.

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?

A. How hot the pan was when the sugar first began to melt
B. How long it takes to cool from the pan to room temperature
C. Its melting point and solubility, compared with sugar's
D. Its mass, compared with the 20.0 g of sugar Dr. Obi used

Which conclusion about the three changes does Table 1 support best?

A. The ice cream and the topping made new substances, since both changed temperature.
B. All three made new substances, since each looked different at the end.
C. Only the topping made a sample with properties unlike any starting material.
D. The dressing and the topping made new substances, since both changed color.

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.