Which procedures and observations show a chemical change

This activity can be used in most any setting. Materials required for 1 experiment: 1. Water 3. Iodine or Lugol's solution 6. Cornstarch 7. Measure 80mL of water and pour it into one of the cups. Add three full droppers of iodine solution. Record your observations. Add one spoonful of cornstarch to the iodine solution and stir. Measure 50 mL of water and pour it into the second cup. Drop a vitamin C tablet into the second cup and stir the liquid with a clean spoon until the tablet is dissolved.

What do you think? What changes did you observe in the first cup? In the second cup? Do you think that chemical changes occurred? Why or why not. 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. 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. Plain iodine will be dark red. Frequently students are led to believe that a change is either physical or chemical. In fact this should be considered more of a continuum. For example salt dissolving in water is usually considered to be a physical change, however the chemical species in salt solution hydrated sodium and chlorine ions are different from the species in solid salt.

Dissolving of instant coffee in water seems to be a physical change but in most cases dissolving is accompanied by an energy change and is probably better considered to be a chemical process even though it is possible to recover the original components by physical means. Many examples of materials dissolving for example, an Alka Seltzer in water, metal in acid and the effect of acid rain on marble and concrete involve both chemical and physical processes.

Many junior school science texts state that chemical changes are irreversible while physical changes can be reversed. Cutting paper into tiny pieces or crushing a rock are obvious physical changes but to restore the original piece of paper or rock is difficult. Rechargeable batteries use one chemical reaction when discharging and recharging involves driving that reaction backwards, turning the products back into the original reactants.

The electrical generator alternator on a car recharges the car battery constantly while the car engine is running. Explore the relationships between ideas about physical and chemical change in the Concept Development Maps - States of Matter, Chemical Reactions.

When teaching about physical and chemical changes it is important to allow students to see the classification as a continuum. They should be able to observe a number of changes and formulate their views on the kind of change and problems with the classification process. Students should come to see that chemical reactions produce new chemicals distinct from the starting materials but that chemical processes can be reversed.

Examining examples of reversible chemical reactions and considering why it is difficult to reverse many chemical changes can be very useful. Promote reflection on and clarification of existing ideas The following activities are intended to get students identifying and then refining their ideas about physical and chemical change.

It is important for students to observe a number of changes and to record their opinions on what is happening. They could record this in a booklet where they write about and draw results and observations.

They should be encouraged to formulate and record hypotheses about what is happening with the knowledge that their opinions will not be assessed at this stage.

For an example of this technique see: Using logbooks in year 10 electricity. This can help bring out their existing ideas and help them challenge and extend their existing beliefs. Practise using and build the perceived usefulness of a scientific model or idea It is important that examples of changes are not confined to only the materials and chemicals students are exposed to in the classroom.

As a homework activity students could be asked to collect examples of changes they see around them and classify these on their scale of physical and chemical changes. Some examples they may collect are combustion of fuels, cooking and processes such as digestion, respiration and photosynthesis. Communicating their ideas to others can help students clarify and consolidate new and existing ideas about changes. Practise using and build the perceived usefulness of a scientific model or idea Science is an area where deeper meaning for a number of key ideas is built gradually by using them in a range of situations and stressing how the same idea helps make sense of many situations.

Both elements and compounds a key idea at the macro level and atoms and molecules which involve the same thinking at the micro level are examples of this, but their usefulness can be developed by showing among many other things how they can help make sense of physical versus chemical changes. These ideas may be introduced here, or referred back to if they have been introduced earlier. Writing chemical equations in word and symbolic form can be introduced as a useful way of describing some of the changes students have seen and also to show the advantages of chemical symbols in keeping track of the elements or atoms in ways that words do not.

If the exact chemical formulae cannot be written as is the case with most biochemicals a drastic simplification can still be useful. For example wood is mostly cellulose, a polymer of glucose, and a representation such as C 6 H 10 O 5 n can be used to track changes in processes like combustion. Methods of filtration vary depending on the location of the targeted material, i.

Learning Objectives Label a change as chemical or physical. List evidence that can indicate a chemical change occurred. Physical Change Physical changes are changes in which no bonds are broken or formed. Some types of physical changes include: Changes of state changes from a solid to a liquid or a gas and vice versa Separation of a mixture Physical deformation cutting, denting, stretching Making solutions special kinds of mixtures As an ice cube melts, its shape changes as it acquires the ability to flow.

Observations that help to indicate chemical change include: Temperature changes either the temperature increases or decreases Light is given off Unexpected color changes a substance with a different color is made, rather than just mixing the original colors together Bubbles are formed but the substance is not boiling - you made a substance that is a gas at the temperature of the beginning materials, instead of a liquid Different smell or taste do not taste your chemistry experiments, though!

Boiling water A nail rusting A green solution and colorless solution are mixed. The resulting mixture is a solution with a pale green color. Two colorless solutions are mixed. The resulting mixture has a yellow precipitate. Solution Physical: boiling and melting are physical changes. When water boils no bonds are broken or formed. Color changes indicate chemical change. The green mixture is still green and the colorless solution is still colorless.

They have just been spread together. No color change occurred or other evidence of chemical change. Chemical: the formation of a precipitate and the color change from colorless to yellow indicate a chemical change. A mirror is broken. An iron nail corroded in moist air Copper metal is melted.

A catalytic converter changes nitrogen dioxide to nitrogen gas and oxygen gas. Answer a: physical change Answer b: chemical change Answer c: physical change Answer d: chemical change. Separating Mixtures Through Physical Changes Homogeneous mixtures solutions can be separated into their component substances by physical processes that rely on differences in some physical property, such as differences in their boiling points.

The solution of salt in water is heated in the distilling flask until it boils. The resulting vapor is enriched in the more volatile component water , which condenses to a liquid in the cold condenser and is then collected in the receiving flask.

Summary Chemists make a distinction between two different types of changes that they study - physical changes and chemical changes. Physical changes are changes that do not alter the identity of a substance. Chemical changes are changes that occur when one substance is turned into another substance. Observations that indicate a chemical change occurred include color change, temperature change, light given off, formation of bubbles, formation of a precipitate, etc.