Chemistry Activity: Hide and Seek

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Objective:

The purpose is to develop students’ understanding of substances that dissolve in one solution and not in another solution.

Content Connections:

Properties of objects and materials, soluble and insoluble

Background Information:

Substances that are ionic compounds like table salt and salt substitute, will easily dissolve in water. Oil does not easily dissolve in water, but will dissolve more readily in an oily compound like a stain treatment stick or spray.

Materials
Watercolor paper, watercolors, paint brushes, wax crayon, wax candles, pieces of cloth

Engage:

Before class write a message on a white piece of watercolor paper with a white crayon. Hold up the paper and ask the children, “I have a message from my best friend. Can anyone read this message?” Allow the children to briefly tell you what the message says. Wash the watercolor paper with watercolors.

The message appears. Ask the children to read the message now.“Why do you think the message appeared?”

Record the children’s initial ideas.

Ask the children questions like the following: Did you ever get in trouble for coloring on the wall with a crayon? How did you parents remove the crayon? Allow the children to briefly share their experiences.

Explore:

Pass out white crayons. Allow the children to write secret messages on white watercolor paper. Then allow the children to paint over the message with watercolors so the message appears. Ask the children, “Do you think this is how I
wrote the message to my best friend?” Record the children’s initial ideas.

Explain:

Share class data. Ask the children what evidence makes them think the watercolors allowed the message to be read. Allow the children to feel the crayon. How does a crayon feel? Write down observations of the crayons. Pass out white, wax candles. Allow the children to feel the candle. Do crayons and candles feel the same or different? Allow the children to use the candles to write messages on another sheet of white watercolor paper. Wash the message with watercolors. What happens?

Drop small pieces of wax in water. Does the wax dissolve? Drop small pieces of table salt in the water. Does the salt dissolve?

Use the children’ ideas to lead into the discussion. Crayons are basically made of wax with a little color. Wax is a compound with a long carbon chain and has a greasy feel. Water and wax or oil do not mix because water has a slight charge (polar molecule). The salt molecule has positive and negative charges like water and so will dissolve in water.

Allow students to feel soap. How does it feel? Soap and detergents have an oil-like part of the molecule and will dissolve grease and a positive charge on the other end of the molecule like water. Allow the children to feel a laundry stain remover.

Stain removers typically have a more oily feel than soap or detergent and are better at removing grease or oil. Water will dissolve dirt because dirt is not greasy.

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Elaborate:

Rub various stains on scraps of cloth. Place the cloths in water. Which stains will dissolve in water and which stains require stain stick?

Sample stains can be made from: a paste of salt (like perspiration, which contains a lot of salt), toothpaste, baking soda, shortening, oil, chocolate and sunflower butter (instead of peanut butter in case there is a child with a peanut allergy).

The salt, baking soda, and toothpaste should dissolve in the water. The shortening, oil, chocolate, and sunflower butter will not dissolve. Try treating the stain first with a stain stick and see if the stain is easier to remove.

Evaluate:

Assess participation and check for understanding of oily substances will dissolve oily stains and salt stains and dirt are easily dissolved in water because these stains are not oily.

Summer Time Fun:

Watch the movie National Treasure. Talk about different ways secret messages have been written. Parents and children can discuss how heating The Declaration of Independence or adding lemon juice to the document would cause damage to the manuscript. Discuss how writing on the wall with a crayon is very difficult to remove because it takes oil to dissolve crayon .

Once the crayon is dissolved by the oil, an oily spot is left on the wall. So, most of the time, parents have to re-paint the wall, which takes time and money, so there is less time for fun and less money for new toys.