Close-up Look at Change

When discussing change with students, it's important to help them understand the makeup of familiar items at the particulate level. For example, some mixtures are evenly distributed and others aren't. Some pure substances are made only from a single element, but others are made of compounds. The Olympics awards gold and silver medals (which are metals themselves), but the bronze medal is a mixture of metals even though we think of it in the same category.

Students see physical and chemical throughout the day, but the science terminology describing change can be tricky. This unit tries to relate familiar changes, such as making lemonade from drink mix, condensation on the outside of a glass, or the rusting of a nail to what is happening at the particulate level. There are factors that help students identify change as physical or chemical.

 

Activities

Scene: Concentrating on Pink Lemonade

In this Science Scene, three friends are making pink lemonade and considering what happens to the drink as they add more powder.

Duration: Approximately 50 minutes

Lesson: Categories of Chemicals and Mixtures

This activity introduces students to the classification of matter into pure substances and mixtures by using familiar examples.

Duration: Approximately 40 minutes

Interactive: Condensation and Evaporation

Using the familiar examples of water droplets on the outside of an iced beverage and the phases of water (ice, liquid water, and water vapor), students students explore the concepts of condensation and evaporation.

Duration: Approximately 45 minutes

Conversation: Describing Physical and Chemical Changes

This activity focuses on how to discern and describe the difference between physical and chemical change.

Duration: Approximately 50 minutes

Interactive Reading: The Three Little Chemists and The Big Bad Wolf

This short tale about the three little chemists and the big bad wolf reinforces the main ideas in this unit, including physical change (as opposed to chemical change); mixtures; and density.

Duration: Approximately 50 minutes

Teacher Tune-ups

• What is the difference between physical and chemical change?
• What is the difference between chemicals and mixtures?

Student View of Visuals and Activities

Some teachers prefer to have students view the slides and other visual assets in this unit directly instead of projecting them in class. Below is a web page to share with students with links to some of same items that are within in the teacher lesson plans, but without the explanatory text for the teacher.

 

• Student Links to Unit M3 Slides and Other Visuals

Original SciGen Unit

This unit has been adapted from pieces of the three units of the "Introductory Chemistry Concepts" group in the Word Generation program led by Catherine Snow (Harvard University) through a SERP collaboration with the Boston Public Schools and other districts in Massachusetts and Maryland.

PDFs of that earlier unit's teacher and student editions are available at the Science Generation Download Center.

homogeneous

adjective – (of mixtures) even, smooth, the same in every part

Salt water contains evenly distributed sodium and chlorine atoms, making it a homogeneous mixture. Can you think of another?

heterogeneous

adjective – (of mixtures) uneven, lumpy, different in different parts

What foods do you eat that are heterogeneous mixtures?

mixture

noun—a combination of two or more substances

To get solid salt out of a saltwater mixture, evaporate the water molecules away by boiling it.

pure

adjective— composed completely of a single substance and not mixed with any other material

H2O is a very poor conductor—but water is almost never pure in the real world.

concentration

noun – the rate of dissolved substance to solvent in a solution

As the water in salt water evaporates, what happens to the concentration of salt?

solution

noun – a homogeneous mixture

Many people who wear contacts clean them with saline solution. Name a solution that you use in your daily life.

dilute

verb – to lower the strength of a solution by adding water (or other solvent)

Can you think of a substance that you would dilute by adding something other than water?

dissolve

verb – to make one substance (a solute) disperse into another (a solvent), forming a solution

Very gradually, rain erodes and dissolves rocks, so that the runoff that flows into streams and rivers carries some sodium, chlorine, and other ions.

chemical

noun – noun – a pure substance (either an element or a compound)

Water is a chemical, but mud is a mixture of many chemicals.

adjective – having to do with chemicals and the transformations between chemicals

Flammability is a chemical property, because you can’t judge the flammability of a substance without burning it; and burning is a chemical change because the starting substances (like wood and oxygen) are transformed into new substances (like carbon dioxide, water vapor, and ash).

physical

adjective— (used in contrast to “chemical”) having to do with properties of matter that don’t involve one substance changing into another

Density is a physical property, because you can measure the density of a substance without the substance undergoing a chemical change; and evaporation is a physical change because a substance retains its same chemical identity when it changes from a liquid to a gas.

compound

noun – a substance made of definite proportions of two or more elements

Water is a chemical compound. Can you name the elements that combine to form water molecules?

density

noun – the ratio of mass to volume

How do differences in density help balloons float in the air?

phase

noun – a state of matter, usually solid, liquid, or gas

What phase change takes place as snow melts?