Tuesday

Raven pours a pile of dry beans onto the Bench. "Atoms," she says. "Red beans are oxygen, white beans are hydrogen."

Rocket pairs the white beans up. "Hydrogen comes in twos. And oxygen comes in twos." He makes two red pairs and two white pairs.

"Now react them," Raven says. "Make water. What do you notice?"

Rocket pulls the pairs apart and rebuilds: one red bean with two white beans, twice. Two water molecules.

Nova counts the beans with a sweep of her light. "How many beans did you start with?" she asks. "And how many now?"

"Six and six," Rocket says. "I only moved them."

"That is a model," Raven says. "The beans did not change. Neither do atoms."

A chemical reaction is a process in which some substances change into different substances. Reactants go in. Products come out.

The reactants and products contain the same atoms, but the atoms are rearranged during the reaction.

This happens because chemical bonds break in the reactants and new chemical bonds form in the products.

When water forms from hydrogen and oxygen gas, bonds break in the hydrogen and oxygen molecules. Then new bonds form in water molecules.

In both the reactants and the products there are four hydrogen atoms and two oxygen atoms. They are simply combined differently in water.

The total number of each type of atom is conserved, and so the mass does not change. That is the sentence at the heart of this week's standard.

A balanced description of a reaction has equal numbers of atoms of each element before and after.

Chemists check their work by counting. If the atom counts match, mass is conserved.

You will not need to balance symbolic equations. You will count beans and drawn circles, which is a model.

Clear modeling. Tomorrow is Bench Lab: the sealed bag, the scale and the number that should not move.