MIXTURES

A mixture is a jumble of different things. Soil is a mixture of sand, clay, stones, roots, and plant and animal remains. The air is a mixture of different gases. Seawater contains a mixture of different chemical compounds in SOLUTION.

WHAT IS THE DIFFERENCE BETWEEN A MIXTURE AND A COMPOUND?

The components of a mixture are physically mixed together, but they have not reacted chemically. When materials react chemically, chemical bonds break and reform, producing compounds with new properties.

DO SOLIDS, LIQUIDS, AND GASES MIX?

All the states of matter can mix, with themselves and each other. Solid powders mix easily. Most rocks are a mixture of different minerals. Some liquids mix easily, and some don’t. Water and alcohol mix together, but water and oil do not mix. Gases mix rapidly by DIFFUSION—their molecules can move between each other because gas molecules are widely spaced.

WHY IS IT EASIER TO MIX THAN TO SEPARATE?

It couldn’t be easier to mix a bucket of red balls with a bucket of white balls—just dump them together. Mixing makes the balls more jumbled. Scientists say that they have more entropy (disorder). Separating the balls is much harder. To make them ordered again—red balls in one bucket, white in another—you have to pick the balls out one at a time.

SOLUTION

A solution is a mixture in the liquid state. Molecules of one substance are dispersed (scattered) throughout molecules of another—the substance is dissolved. The amount of a substance that will dissolve in another is called its solubility.

WHY DON’T OIL AND WATER MIX?

Oil is insoluble (does not dissolve) in water because oil and water molecules repel each other. Cooking oil and water can be physically mixed together by shaking them vigorously in a bottle, but when the mixture is left to stand, the oil and water molecules gradually separate again. Oil is less dense (heavy) than water, so the oil floats on top of the water.

DIFFUSION

When two liquids or gases are in the same container, the random motion of their molecules makes them mingle together until the mixture is the same throughout. This is called diffusion.

WHAT IS A RANDOM WALK?

The movement of a single molecule in a liquid or a gas is a zigzag random walk. The molecule is continually moving and changing direction as it collides with other molecules. A group of molecules that were concentrated in one spot gradually spread apart. This explains how an odor—for example, perfume—spreads through a room.

FIND OUT MORE

Chemistry
Heat
Matter
Soil

Copyright © 2007 Dorling Kindersley