Close-up Look at Change

Scene: Concentrating on Pink Lemonade

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Lesson: Categories of Chemicals and Mixtures

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Interactive: Condensation and Evaporation

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Conversation: Describing Physical and Chemical Changes

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

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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?