Monday
Monday: Where Did the Salt Go?
Rocket stirs a spoonful of salt into a cup of water at the Bench. The grains swirl, shrink and vanish.
"It is gone," he says. "Did we destroy it?"
Raven holds the cup up to the window. "What do you notice? The water is clear, but the cup is heavier than before."
Nova hovers over the cup, her light passing straight through the water. "Think about Saturday's float challenge," she says.
"The salt water was denser," Rocket says. "So the salt is still in there. We just cannot see it."
Raven pours a little lemon juice and sugar into a second cup of water. "And this? Lemonade is a mixture too."
"Then a mixture is stuff together that is still itself," Rocket says. Nova hums. "Now you have the big idea."
What a mixture is
A mixture is a combination of two or more substances in any proportion. Lemonade can have more or less lemon juice and still be lemonade.
The substances in a mixture do not combine chemically to form a new substance. They intermingle and keep their original properties.
When salt dissolves in water, only the form of the salt changes. It keeps its composition and its properties.
Salt water is not a pure substance, because its composition can vary. You may dissolve a small or a large amount of salt.
Mixtures are everywhere. Air is a mixture of gases, mainly nitrogen and oxygen. A rock can be a mixture of smaller rocks and minerals.
- Read the question.
- Tap your answer.
Same throughout, or not?
A homogeneous mixture has the same composition throughout. Salt water is homogeneous because the dissolved salt is spread evenly through the whole cup.
Ocean water is homogeneous too. Samples from different places hold the same proportion of salt, about 3.5 percent.
A heterogeneous mixture varies in its composition. Vegetable soup is heterogeneous. So is soil, which changes from one scoop to the next.
When oil and water are combined, they do not mix evenly. They form two separate layers. Each layer is called a phase.
| Kind of mixture | Composition | Examples |
|---|---|---|
| Homogeneous | The same throughout | salt water, ocean water, lemonade, air |
| Heterogeneous | Varies from place to place | vegetable soup, soil, oil and water, a rock |
| Statement | True or false? |
|---|---|
| A mixture is a combination of two or more substances in any proportion. | ? |
| The substances in a mixture combine chemically into a new substance. | ? |
| Salt water is a homogeneous mixture. | ? |
| Oil and water form a homogeneous mixture. | ? |
| Ocean water is about 3.5 percent salt. | ? |
Pour a spoonful of cooking oil into a second cup of water. Do not stir. Wait one minute.
Write in your Bench Log which cup is homogeneous and which is heterogeneous, and how you can tell.
Only tap water is used, and nothing is tasted. Coffee filters or paper towels work for the filtering step.
The chemistry definitions this week come from a chemistry text hosted by the University of California, Davis, and from the U.S. Geological Survey.
Good thinking. Tomorrow you will zoom in on solutions and find out what dissolving really does to the salt.