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    How do we come to know that a chemical reaction has taken place?

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    How do we come to know that a chemical reaction has taken place?

    The presence of any of the following changes helps us to determine that a chemical reaction has taken place.

    Medium Open in App Solution Verified by Toppr

    (i) Formation of new substance(s)

    (ii) Change in state

    (iii) Change in colour

    (iv) Change in temperature

    (v) Formation of a precipitate

    (vi) Evolution of a gas

    For example, if on mixing two substances a gas is evolved, then we can say that a chemical reaction has taken place. 

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    स्रोत : www.toppr.com

    Investigate Chemical Changes

    This activity is a classroom lab activity that demonstrates chemical change in a dramatic way using a few inexpensive, easily obtainable materials.

    Investigate Chemical Changes - What are some signs of chemical change?

    Jeanine Salisbury, Robbinsdale Middle School, Robbinsdale, MN based on an original activity from McDougal Littell Science, Matter and Energy p.47 Author Profile


    This is an open inquiry lab that can be done in approximately 15-20 minutes with a few household materials. It dramatically, yet simply, illustrates the condition of color change and the formation of gas (bubbles) as a result of a chemical change.

    Learning Goals

    Skills:1. Following step by step written instructions2. Following a sequence3. Measuring4. Working collaboratively5.Recording observationsThinking Skills:1. Knowledge and comprehension.2. Synthesis and evaluation.3. Metacognition.

    Key Concepts:

    There are five signs of a chemical change:

    1. Color Change

    2. Production of an odor

    3. Change of Temperature

    4. Evolution of a gas (formation of bubbles)

    5. Precipitate (formation of a solid)

    Vocabulary:1.Chemical properties2. Chemical change3.Precipitate

    Context for Use

    This activity would be appropriate for just a few students up to as many as 30-35. The only limitation would be desk space and graduated cylinders. Everything else can be obtained from a grocery store. The lab is inquiry and it can be used to introduce the concepts of chemical change. It can also be used to demonstrate the chemical changes of color and bubbles. It is a lab that takes approximatelu 15-20 minutes of class time to do. It will also require 15-20 minutes for recording observations and answering questions, as well as additional time for set up and clean up. The time required for these, would depend on the number of children doing the lab. It can be done individually or in groups of two. If necessary, it can be done with larger groups, but it is much more fun with 1 or two students. The only special equipment that will be needed are graduated cylinders. It is helpful if students have studied physical changes, but not necessary. This lab can be used as an anticipatory set, or as an example(s) of chemical changes in materials. The children can do the lab and then learn why the changes took place, or they can use it to demonstrate learned concepts. This activity can be used in most any setting.

    Subject: Chemistry:General ChemistryResource Type: Activities:Lab ActivityGrade Level: Middle (6-8)

    Description and Teaching Materials

    Materials required for 1 experiment:

    1. 1 graduated cylinder2. Water3. 2 clear plastic cups4. 2 eyedroppers5. Iodine or Lugol's solution6. Cornstarch7. 2 spoons (plastic)8. 1 vitamin C tabletProcedure:1.Measure 80mL of water and pour it into one of the cups.2. Add three full droppers of iodine solution. Record your observations.3. Add one spoonful of cornstarch to the iodine solution and stir. Record your observations.4.Measure 50 mL of water and pour it into the second cup.5. Using a clean eyedropper, add 4 full droppers of the iodine/cornstarch solution to the second cup.6. Drop a vitamin C tablet into the second cup and stir the liquid with a clean spoon until the tablet is dissolved. Record your observations.What do you think?1. What changes did you observe in the first cup? In the second cup?2. Do you think that chemical changes occurred? Why or why not.3. What are some characteristics of chemical changes? Can you list all five?

    Challenge: Describe some chemical changes that you have seen take place in your home or school.


    McDougal Littell Science, Matter and Energy, Copyright 2005, McDougal Littell, a division of Houghton Mifflin Company, pg. 47

    Teaching Notes and Tips

    Tips: You can use iodine from any drug store. Just be careful as it stains and probably will not come out of clothing. Old adult male shirts work well as lab coats and can be obtained at thrift stores very reasonably. If you use Lugol's solution, add enough to make the solution dark yellow/orange. Plain iodine will be dark red.

    If Vitamin C tablets are used, crush them first and they will dissolve faster. If you do not have Vitamin C tablets, orange or lemon juice works as well.

    Possible Answers to What do you Think?

    The color changed from yellow/orange, red to blue-black in the first cup. In the second cup, the color changed and bubbles formed. Yes; new substances formed, as evidenced by the color changes and bubbles. Some signs of a chemical change are a change in color and the formation of bubbles. The five conditions of chemical change: color change, formation of a precipitate, formation of a gas, odor change, temperature change.

    Challenge: food cooking, rust formation, tarnish forming on doorknobs, fuel burning for heat, tarnished penny.


    The assessment can be either formal or informal. The students can write the answers to the questions in their science journals, or on a piece of paper to turn in. They can also talk about their observations in small groups or large class discussions.

    स्रोत : serc.carleton.edu

    7.1 How do we know a chemical reaction has taken place?

    Siyavula's open Natural Sciences Grade 8 textbook, chapter 7 on Chemical reactions covering 7.1 How do we know a chemical reaction has taken place?

    7.1 How do we know a chemical reaction has taken place?


    Chapter 6: Particle model of matter


    Chapter 8: Matter and Materials Glossary


    What is a chemical reaction?

    What happens to atoms and the bonds between them during a chemical reaction?

    How can we identify the reactants and products of a reaction?

    What examples of chemical reactions are there in indigenous practices?

    In the last chapter we looked at the particle model of matter and specifically at changes of state. Do you remember heating and cooling candle wax to observe it melt and then solidify. The wax first changed from a solid into a liquid and then back to a solid again. These are physical changes. The chemical properties of the substance does not change.

    We are now going to look at what happens when we get chemical changes in substances. These take place during chemical reactions.

    How do we know a chemical reaction has taken place?

    chemical reaction reaction flask or reaction vessel

    During a chemical reaction, one or more substances are changed into new substances. Do you know of any chemical reactions? Can you mention one or two examples?

    How will we know when a chemical reaction is taking place? What are the signs?

    We can tell if a chemical reaction has taken place when one or more of the following things happen:

    There has been a colour change inside the reaction flask.

    A gas has formed. Usually we know a gas has formed when we can see bubbles. This should not be confused with boiling, which only happens when a liquid is heated to its boiling point.

    A solid has formed. Usually we know that some solid material has formed when we can see a sludgy or cloudy deposit, or crystals forming.

    All the signs listed above are visual, or recorded by sight. That means we can see them. Our other senses can also help us to say whether or not there was a chemical reaction:

    Sometimes chemical changes can be smelled, for instance when a new material, that has a strong smell, is formed.

    Other chemical changes can be felt, e.g when the reaction produces heat.

    Some chemical changes can be heard, e.g. when an explosion takes place.

    Video on physical and chemical changes.



    Below is a table with some different chemical and physical changes listed.

    You need to decide whether the change is physical or chemical and write the answer in the last column.

    ChangeIs it a physical or chemical change?

    Cutting up potatoes into cubes

    Boiling water in a pot on the stove

    Frying eggs in a pan

    Whipping egg whites

    Dissolving sugar in water

    Burning gas in a gas cooker

    Your ice cream melts in the sun

    Milk turning sour

    An iron gate outside rusts

    Here are the answers. Learners only need to state physical or chemical - some explanations have been provided as background for the teacher and if you wish to explain the changes further to your learners.

    ChangeIs it a physical or chemical change?

    Cutting up potatoes into cubes


    Boiling water in a pot on the stove


    Frying eggs in a pan

    Chemical (the egg proteins undergo a chemical change and crosslink to form a network)

    Whipping egg whites

    Physical (air is forced into the liquid but no new substance is made)

    Dissolving sugar in water

    Physical (the sugar grains are dispersed within the water, but the individual sugar molecules are unchanged)

    Burning gas in a gas cooker

    Chemical (water vapour and carbon dioxide form)

    Your ice cream melts in the sun

    Physical Milk turning sour

    Chemical (lactic acid is produced)

    An iron gate outside rusts

    Chemical (iron oxide forms - this will be discussed in more detail in Gr. 9)

    We will now put our checklist into practice by looking at a reaction safe enough to try at home. Have you ever wondered what a raw egg would look like without its shell? We are going to use a chemical reaction to strip away the shell of an egg, without breaking the egg!


    How can we make an egg look like this? http://www.flickr.com/photos/gemsling/2687069763/

    Video on the naked egg experiment


    eggs a glass white vinegar


    Carefully place the egg in the glass. Be careful not to crack the shell.

    Cover the egg with vinegar. Wait a few minutes. Can you see anything happening on the surface of the eggshell?

    Write your observations below.

    What is this observation a sign of?

    The eggshell gradually becomes covered in bubbles.

    The bubbles are a sign of a chemical reaction taking place.

    Leave the egg in the vinegar for 4 - 5 days. You should complete the rest of the activity after this.

    Note: It may be necessary to top up the vinegar if the reaction starts to slow down. Remember to return to the activity at the end of the week, when the eggshell has dissolved completely.

    स्रोत : www.siyavula.com

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