Teaching Chemical Changes in the Elementary Classroom

Sodium Bicarbonate, Calcium Chloride and Phenol Red

Let’s go over the procedure first and then we will discuss what is happening.

1. In a quart baggie, place sodium bicarbonate(1 tsp) in one corner and calcium chloride(1 tsp) in the other.
2. Lay the bag on its side and place a small cup (medicine cup size – 1 oz) of phenol red in center of the bag. Be careful not allow the any on the chemicals to mix yet. Seal the bag
3. Gently pour the phenol red where it spills into each corner. Do not mix the two corners yet.
4. Have the students feel each corner and make observations. Continue the observations for a few minutes.
5. Pick the bag up and gently move the bage side to side, mixing the chemicals. What happens?

Explanation

The side of the bag with calcium chloride becomes warm. The calcium chloride dissolves forming calcium and chloride ions. The release of heat (exothermic) is a result of the calcium chloride dissolving and not a chemical reaction.

When the sodium bicarbonate dissolves to form sodium, hydrogen and carbonate. It becomes cool (endothermic). The baking soda absorbs heat in order to dissolve. This is not a chemical change.

When the two sides are mixed, calcium carbonate is formed which is insoluble. Also formed are water and carbon dioxide. The carbon dioxide (gas) causes the bag to inflate. When the carbon dioxide dissolves in the liquid, carbonic acid is formed. This change in pH causes the phenol red to turn yellow. A chemical change has now occurred.

Remember to have the students use all lab safety measures. If the bag becomes over inflated, release some of the gas.

Glacial Heat – Teaching Supersaturated and Supercooled Solutions

Glacial Heat

Glacial Heat

What’s Happening?

Inside the Glacial Heat exists a supersaturated and super-cooled (below its freezing point) solution of sodium acetate and water.This supersaturated solution was created by mixing the salt (sodium acetate) in hot water. Hot liquid will dissolve more salt than a cold one. When this solution is cooled slowly, the salt stays in solution.

CH3COONa.3H2O + Heat -> CH3COO-(aq) + Na+(aq)+3H2O
(Solid) (Liquid)
A small, stainless steel, metal chip provides the “spark”. When the chip is squeezed, a small, single, solid salt molecule is created. This is the seed ,on which, the other salt crystals begin to form. The normal freezing point for sodium acetate is 130 degrees F (54 degrees C). The reaction occurs quickly, with heat being released and the liquid becomes solid (freezes). The heat being released is equal to the freezing point of the solution (54 degrees C). The sodium acetate (a salt) dissolving and freezing in the water is an example of a physical change.

 

How do I Teach With Glacial Heat?

Discuss physical and chemical properties:

Physical properties are observable (color, size, luster and smell) and also include characteristics, such as, freezing point, melting point, malleability, conductivity, volume, mass, weight and length.

Chemical properties are only observable during a chemical reaction and can include flammability or the ability to rust. In each of these examples, a new compound has been formed.

Discuss physical and chemical changes:

Physical changes include ice melting, molding clay, water evaporating, a coke freezing and sugar dissolving in water. In these examples, no chemical changes have occurred and the changes can be reversed.

Chemical changes include metal rusting, lighting a match, milk souring and the stomach digesting food. These changes are not easily reversed. The presence of light, color change, odor, gas production, heat or sound can indicate that a chemical change has taken place.

The Glacial Heat can be boiled (melted) for 7-10 minutes and reused over and over again.