A strategy for teaching chemical change.

Educators Dutch de Vos and Verdonk proposed a strategy for introducing the topic of chemical reactions on "A New Road to Reactions". This technique requires five steps that teachers avoid a traditional approach based on detailed understanding terminology, however, present chemical processes that force students to think explanations about what they see.
This sequence of steps describes a valuable way of providing images to help students form an accepted view of the chemical changes. In the end it helps them distinguish between physical change and chemical change, and after this, to understand that chemical changes occur on a microscopic scale between atoms. This approach suggests that the sequence commonly used to teach basic chemical ideas seems to create confusion in many high school students. 

Help them recognize that they form a new substance.
Students milled separately (in mortars and their respectivepistils) potassium iodide and lead nitrate. Then pour over each other. By mixing the powders, immediately produce a bright yellow solid (lead iodide) mixed with a white solid (potassium nitrate). The teacher feigns anger and asks, "Who put this yellow solid in mortar?". This causes confusion, "I do not know, just appeared," "I do not know where it came from," "not me". The teacher's response is, "Well, could not have simply appeared, must have come from somewhere! Where did it come? ". Perhaps the students say that the egg white powders are like tiny yellow powder that was inside of them, so that, when mixed broke the "eggs" and made the yellow material appeared.
 Andersson (1990) suggests that this reasoning arises because: "It seems that most children 14 years old, still adhere firmly to the idea that tacit and unconscious of each individual substance is preserved, we pass what happens" (p. 4).
The recognition of yellow material as a new substance is the important point, therefore, made to see if a white substance was made of "tiny eggs", the material should have appeared yellow since the ground, ie before mixing with the other white powder. Students prefer to think intuitively that the two original substances and the material contained yellow but something prevented them from seeing it from the beginning.Through a continuous questioning, students admit that the substance is new and "just appeared". The experiment creates a cognitive conflict, as both the result of the teacher signs put into question the thinking of students.De Vos and Verdonk show that:
"The teacher's role is to make harder, not easier for the student to abandon his earlier idea. The new point of view on the substance must be a student's personal victory and something to be proud "(p. 239).
Extend this reasoning to other reactions
Students carry out the same reaction, but add small amounts of solids to water in a petri dish. Are made small amounts of lead nitrate and potassium iodide on opposite sides of the box. After a few moments, a line of yellow crystalline lead iodide appears in the center of the box. Students can explain this by the idea that "molecules" substance "attract" each other. However, this idea vanishes when repeating the experiment and add a reagent few minutes before the other, and there is then the instantaneous formation of the precipitate. Other combinations of substances, including salt or sugar and salt and silver nitrate to help students understand that there is always a precipitate, even when the "molecules" of substances collide. At this stage, they can be encouraged to think that the particles are very small, otherwise the water would move in some way "
(From Vanessa Kind (2004) Beyond appearances. Students' previous ideas on basic concepts of chemistry, Santillana, Mexico.)