There are two activities in this class: "Looking for traces of starch" and "Starch disappeared". The overall design of the activity is to observe and compare starch and talcum powder, so that students can design their own distinguishing methods (appearance, hand twisting, touching and using iodine), and to compare the reaction of dropping iodine on the mixture of starch and talcum powder with different proportions. (The amount of starch gradually decreases) What happens when students' guesses are reduced to a certain extent? The students found from the experiment that these methods can not only test whether the substance contains starch, but also judge the starch content according to the depth of blue, and then the game of "rice noodle mouse" played a stimulating role. Third, use iodine to identify whether there is starch in food. (Guess before you do it) "Starch has disappeared" is the last content of this unit. The specific experiment is divided into three bottles of starch paste, one with iodine and the other with saliva. After shaking well, drop iodine to observe and discuss why there are different phenomena. Teachers should use thinking to help guide teaching materials. The problem of "starch disappearing" should pay attention to the following points: a, time may not be enough. The game of "rice noodle mouse" can be played, which has little significance and connection with other activities and can be saved. B, pay attention to the collection of saliva.

The lesson "Traces of Starch" takes starch as the observation object, and through the exploration of starch, let children know that there are many scientific reasons in the things around us.

(1) "Looking for traces of starch" The first experiment was to observe and compare the reactions of different proportions of starch and talcum powder after dropping iodine. In the experiment, children will find that not only can iodine lake reaction be used to test whether there is starch in talcum powder, but also the content of starch can be judged according to the depth of blue.

The second experiment of "looking for traces of starch" is to use iodine to identify whether there will be starch in some foods. Let children make predictions first, and then carry out experimental verification to cultivate students' scientific inquiry ability.

(2) In the activity of "starch disappeared", let students add saliva to the starch paste, shake it well and add iodine dropwise to make the liquid in the cup no longer turn blue, and let students try to explore the reasons for the change.

● Teaching objectives

▲ Scientific knowledge objectives:

(1) Know the difference between starch and talcum powder.

(2) Knowing that starch will turn blue when it meets iodine, the content of starch can be judged according to the depth of blue, and the iodine lake reaction can be used to detect whether there is starch in food.

▲ Scientific inquiry objectives:

(1) will be observed with a magnifying glass to further understand the difference between starch and talcum powder through comparison.

(2) Learn to use prediction when detecting whether the food contains iodine-containing starch; When observing the changes of starch paste after adding iodine, try to explore the reasons for the changes.

▲ Emotional attitude goal: guide students to learn to respect scientific facts in activities, cultivate good scientific study habits, experience the fun of scientific inquiry, and gradually develop the habit of daring to express their opinions and listen to others' opinions in discussions.

● Teaching emphasis: organizing students to explore the process of starch is a key point of this course;

● Teaching difficulty: Starch disappears after contact with saliva.

● List of active materials:

Each group: (starch and talcum powder are marked with 1 and 2), iodine, dropper, glass, 5 pieces of white paper and 2 toothpicks. Calligraphy Axis (Title, Evaluation)

Students bring their own: vegetables, fruits, rice, steamed bread, meat food, etc. A note with a title.

● Teaching process

First, the introduction of magic leads to the research topic of this lesson.

1. Students, do you like to watch magic shows? The teacher will perform a magic trick for you today, ok? Magic: Show a calligraphy work (this is a blank piece of paper). Ask a student to read aloud to the class. Everyone thinks there are no words on this paper, right? The teacher can make it change words. Can you believe it? At this time, I asked the students to smear iodine on the paper to show the words on the paper. What is written on the paper? Who can tell what color these words are?

2. So, can you ask some questions about this little magic trick? Can you explain this phenomenon with what you have learned? (Courseware: blackboard writing: starch+iodine → blue)

3. Summary: It turns out that we can find traces of starch with iodine. I'm so happy, and I'm even happier that the students can use what they have learned to solve the phenomenon they saw today.

Second, inquiry activities

(1) Observe and compare starch and talcum powder.

Situation: The teacher bought a bag of starch and talcum powder. I want to ask my classmates to help me distinguish. Will you help me? 1. Before the observation, the group will discuss the methods they have learned can be used.

2. Discuss the methods of group communication. (Label per bag 1, 2) Nose, hands, eyes, iodine.

The students must return home with a full load. Communicate with the whole class in groups and introduce the methods and findings of your group.

2. Observe and compare the reaction of the mixture of starch and talcum powder with different proportions after dropping iodine. If starch and talcum powder are mixed, the amount of starch will gradually decrease, and what will happen to the color? How to do this experiment? Yaozhu

What do you mean? The amount of talcum powder and the amount of iodine. ) (Board: talcum powder+iodine → no discoloration)

(2) Use iodine to identify whether some foods contain starch.

Situation: We know that the iodine lake reaction will turn blue, and we also know that different starch contents have different degrees of discoloration. The teacher has an idea. Guess what the teacher thinks. (Student guesses) How about we apply this knowledge to our lives?

(1) Ask the students to show their own food, guess whether these foods contain starch, put the food that is considered to contain starch in one pile, and put the food that does not contain starch in another pile. What makes you guess? ) (Table 2)

(2) Is our guess correct? Verified by dropping iodine. Students' experimental operation. (Table 2)

(3) student statements.

Inquiry activity 2. Do the experiment of starch disappearing.

Situation: Next, let's do a more interesting magic trick. Put 1, 2, 3 and 4 transparent cups, add some starch paste and stir. Hurry up and add saliva to No.2, so that students can't find it.

(1) 1 Cup iodized, guess, look carefully, what's the change? What does turning blue tell us? Is this magic good? But we found starch here.

(2) Add iodine to No.2 cup, guess and shake it a few times, what do you find? What does Henglan tell us? Where did these starches go?

Situation: The teacher wants to take his apprentice. Guess why. After full development, the students tried to explore the reasons for the change. A milkshake. Students try the third cup to exclude. B became something else. Into what? What method does the teacher use? Students try.

... the teacher led the students to discuss: "Why do two bottles have different phenomena?"

Solution: Food containing starch is chewed with teeth after being eaten by people. Is starch still there?

Third, the expansion link:

In this class, we went to look for traces of starch, found it and let it run away. Are the students interested? After your research, you have gained a lot. What have you learned? But time is limited. What other questions are you interested in about starch? Students can use the research methods they have learned today to study new problems after class. Because starch and food are closely related to our lives, the teacher suggested that you set up a tabloid about food and people to fully display your talents.

Evaluation: Students spray with iodine solution.

Fourth, homework. "rice noodle tiger" game; Write a fairy tale.

Low blackboard writing:

Traces of starch

Starch? + ? Iodine → discoloration

Talcum powder? + ? Iodine → no discoloration

Starch paste? + ? Iodine → discoloration

Starch paste+saliva+iodine → no discoloration