Lesson 7: Classifying Matter

Activities

Activity 1: What Is a Compound?

In this activity, you will explore the idea of a compound. A compound is a substance formed by the chemical combination of elements in fixed proportions. Compounds can be separated into simpler substances by chemical reactions. In this lesson, you will use some compounds common in your home — sugar and salt — and examine them more closely.

Remember that fixed proportions are unchanging quantities of an element that are part of a compound. For a compound to always be the same, the amount of each element must remain the same or else the compound changes into a different compound. Some examples of common household compounds are included in the "Compounds" table:

Compounds

Look carefully at the "Sample Compounds" table and use the information presented to complete the table on the "What Is a Compound?" activity page. Not all cells will be filled with an element. If a cell is empty, put "n/a" or a dash (—) through the cell. When you are done, answer the questions at the bottom of the page. In the chemical formulas, if an element does not have a subscripted number, the quantity is understood to be one. If it does have a subscripted number, the quantity is that number.

Sample Compounds
Compound Name	Chemical Name	Chemical Formula
Baking soda	sodium bicarbonate	NaHCO₃
Limestone	calcium carbonate	CaCO₃
Lime	calcium oxide	CaO

Student Activity Page

What Is a Compound?

In this activity, your child looked at the chemical formulas for baking soda, limestone and lime. In each compound, there are specific elements combined with other elements. For baking soda, they are sodium (Na), hydrogen (H), carbon (C), and oxygen (O). For limestone they are calcium (Ca), carbon (C), and oxygen (O); and for lime they are carbon (C) and oxygen (O). Each letter has a subscript that represents the number of these elements that are combined with specific amounts of each element present. Capital letters represent the first or only letter of the chemical symbol of an element.

Use the following answer key to check your child's answer:

Answer Key

Activity 2: Sweet and Salty

Materials: measuring cups, oven mitt or potholder, safety goggles (kit), salt, small cups (kit), sugar, tablespoon, tea light candle (kit), test tube clamp (kit), test tube holder (kit), test tubes (kit)

In Activity 1, you learned that compounds are made up of different elements in certain proportions. Keep in mind that changing the proportion or changing the elements means that a new compound is formed. In this activity you will learn that changing the conditions for a compound does not change the compound unless a chemical reaction occurs. In the final portion of condition three, you will see or taste, this.

Condition 1: Solution

Follow these steps:

Put a tablespoon of sugar and a tablespoon of water in a cup.
In a different cup, put in a tablespoon of salt and add a tablespoon of water.
Stir each solution until each the salt and sugar dissolve.
Taste the solution in each cup and note your observations on the "Sweet and Salty" activity sheet.
Keep the cups and their contents; you will use them in Conditions 2 and 3.

NOTE: You will need to use safety goggles for Condition 2 and Condition 3. You should also use an oven mitt or silicone potholder to hold the test tube clamp.

Condition 2: Applied Heat to Solution

Follow these steps:

Carefully light a tea light candle (or ask a parent to light it).
Put a small amount of the sugar water mixture into a test tube. Use the test tube clamp to hold it over the flame. NOTE: You should hold the test tube clamp using an oven mitt or silicone potholder; metal conducts heat well, and the handle could get very warm.
Keep applying heat until the water bubbles.
Place the test tube in the test tube rack and let it cool; don't spill any of the liquid.
Repeat Steps 2-4 for the salt water.
Once both liquids have cooled completely, taste each one and record your observation on the activity sheet.

Condition 3: Applied Heat to Solid

Follow these steps:

Put a small amount of sugar in a test tube and place it over the flame. Do NOT add water. (As with Condition 2, be sure to hold the test tube clamp with an oven mitt or pot holder.)
Keep applying heat until the sugar melts.
Place the test tube in the test tube rack and let it cool; don't spill any of the melted sugar.
Put a small amount of salt on another test tube and place it over the flame. Do NOT add water.
Keep applying heat; the salt may not bubble, and that is okay.
Place the test tube on the rack and let it cool; don't spill any of the salt.
Once both test tubes have cooled down until warm or cool to the touch, do a taste test and record your observations on the activity page.

Consider the following chemical formula for what you observed with the sugar:

Table Sugar Burning

This is a chemical formula that represents table sugar (sucrose) when it combusts or burns. Although your sugar did not combust (sugar burns at 325 degrees Fahrenheit), you should see some discoloration. That discoloration is the result of a chemical change. After you applied heat, did you have the same compound as before or did you have different ones? Answer that question at the bottom of the activity page.

Student Activity Page

Sweet and Salty

Use the following answers to review your child's activity page:

Answer Key:
Compound	Taste: Condition One	Taste: Condition Two	Taste: Condition Three
Sugar	sweet	sweet	burned taste; has lost some sweetness, some dark color
Salt	salty	salty	salty

After applying heat, do you have the same compound as before or do you have different ones? Salt will not change because its melting point is approximately 700 degrees Celsius and the temperature of the flame is not that high. Sugar should melt and has the potential to burn, creating a chemical reaction that changes sugar into different compounds. Sugar is not the same, but salt is.

NOTE: The sugar may not break down in Condition 3 because the temperature was not hot enough. The flame on the wick would burn sugar, but that requires extra precautions to test.