image Matter and Chemical Changes

Georgia Perimeter College

Objectives

  1. Classify matter into pure substances and mixtures.
  2. Classify pure substances into elements and compounds.
  3. Classify mixtures into heterogeneous and homogeneous mixtures.
  4. Distinguish between chemical changes and physical changes in matter.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S8P1. Students will examine the scientific view of the nature of matter.
    a. Distinguish between atoms and molecules.
    b. Describe the difference between pure substances (elements and compounds) and mixtures.
    d. Distinguish between physical and chemical properties of matter as physical (i.e., density, melting point, boiling point) or chemical (i.e., reactivity, combustibility).
    e. Distinguish between changes in matter as physical (i.e., physical change) or chemical (development of a gas, formation of precipitate, and change in color)
    g. Identify and demonstrate the Law of Conservation of Matter.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • All matter is made up of atoms, which are far too small to see directly through a microscope. The atoms of any element are alike but are different from atoms of other elements. Atoms may stick together in well-defined molecules or may be packed together in large arrays. Different arrangements of atoms into groups compose all substances.
  • The temperature and acidity of a solution influence reaction rates. Many substances dissolve in water, which may greatly facilitate reactions between them.
  • Scientific ideas about elements were borrowed from some Greek philosophers of 2,000 years earlier, who believed that everything was made from four basic substances: air, earth, fire, and water. It was the combinations of these "elements" in different proportions that gave other substances their observable properties. The Greeks were wrong about those four, but now over 100 different elements have been identified, some rare and some plentiful, out of which everything is made. Because most elements tend to combine with others, few elements are found in their pure form.
  • There are groups of elements that have similar properties, including highly reactive metals, less-reactive metals, highly reactive nonmetals (such as chlorine, fluorine, and oxygen), and some almost completely nonreactive gases (such as helium and neon). An especially important kind of reaction between substances involves combination of oxygen with something else-as in burning or rusting. Some elements don't fit into any of the categories; among them are carbon and hydrogen, essential elements of living matter.
  • No matter how substances within a closed system interact with one another, or how they combine or break apart, the total weight of the system remains the same. The idea of atoms explains the conservation of matter: If the number of atoms stays the same no matter how they are rearranged, then their total mass stays the same.

This section addresses, in whole or in part, the following National Science Education Standards:
  • A substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the sample. A mixture of substances often can be separated into the original substances using one or more of the characteristic properties.
  • Substances react chemically in characteristic ways with other substances to form new substances (compounds) with different characteristic properties. In chemical reactions, the total mass is conserved. Substances often are placed in categories or groups if they react in similar ways; metals is an example of such a group.
  • Chemical elements do not break down during normal laboratory reactions involving such treatments as heating, exposure to electric current, or reaction with acids. There are more than 100 known elements that combine in a multitude of ways to produce compounds, which account for the living and nonliving substances that we encounter.
  • Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together.
  • The atom's nucleus is composed of protons and neutrons, which are much more massive than electrons. When an element has atoms that differ in the number of neutrons, these atoms are called different isotopes of the element.

Matter

Matter is a term used to describe anything that has mass and takes up space.

States of Matter

There are three states of matter (four, if you count plasma).

  1. Solids - fixed shape and volume
  2. Liquids - fairly fixed volume, but shape varies with container
  3. Gases - both volume and shape are variable
  4. Plasma - ionized gas - both volume and shape are variable

Matter can be divided into two categories:

  1. Substances (or pure substances)
  2. Mixtures

Substances

Substances cannot be separated into different types of matter by any physical means. Atoms in a pure substance can be separated only by chemical changes.

  1. Elements are pure substances that are made of only one type of matter.
    The smallest possible particle of an element is called an atom.
    Examples: Silver, copper, oxygen, carbon (as diamond or graphite), sulfur, etc.
  2. Compounds are composed of more than one type of matter. The different substances cannot be separated without changing the substance.
    The smallest possible particle of a compound that retains the properties of a compound is called a molecule.
    Examples:
    1. Table salt (sodium chloride). Salt is composed of two elements, sodium and chlorine. If you could separate salt into into its two elements, it would no longer be salt.
    2. Pure water (H2O).
    3. Pure sucrose or sugar (C6H12O6).
    4. Quartz (SiO2).

    Law of Definite Proportions or Law of Constant Composition: a pure compound always contains the same proportions of elements by mass, regardless of the source of the compound. In other words, if you tested a sample of pure water (H2O), every sample would contain H and O atoms in a ratio of 2:1.

Mixtures

A mixture is a material made up of two or more substances. Mixtures are composed of more than one kind of matter, or more than one pure substance. Composition can vary between different samples. Pure substances in the mixture can be physically separated from each other by physical changes.
There are two types of mixtures:

  1. Homogeneous mixtures - A homogeneous mixture has two or more substances in it, but you cannot see them. They appear to be the same throughout. They may not look like mixtures, but if tested, they can be determined to be composed of more than one type of substance.
     
    Examples: Cake batter (which is a mixture of butter, eggs, sugar, and flour), soft drinks, salt water, sugar water, tap water, and brass (which is a mixture of copper and zinc).
     
    Salt water (or sugar water) would be a homogeneous mixture since you cannot actually see the salt in the water. A homogeneous mixture is sometimes called a solution.

  2.  

  3. Heterogeneous mixtures - A heterogeneous mixture has two or more substances in it, and you can see what is inside of it. Different samples are not necessarily the same. These are easy to spot because they look like mixtures. They are clearly composed of more than one type of matter, and contain regions with different properties.
    Mixtures can be separated into different types of matter by some physical means, such as sorting, filtering, heating, cooling, freezing, melting, evaporation, settling, etc.
     
    Examples: Trail mix (which is a mixture of raisins, peanuts, and chocolate candy m & m's), crunchy peanut butter, chocolate chip cookies, vegetable soup, spaghetti sauce, concrete, granite, raisin bran.
     
    Heterogeneous mixtures that are liquids can be subdivided into two types: colloids and suspensions.
     
    A colloid consists of solid particles in a liquid. These particles are usually very small, often less than 0.01 mm in diameter. A colloid is a heterogeneous mixture in which the contents do not settle over a long time. They tend to stay combined together. Paint, orange juice, ketchup, and most salad dressings are colloids.
     
    A suspension is a heterogeneous mixture in which the contents settle over a short period of time. Some examples of this may be muddy water or Italian salad dressing.


Image courtesy of NASA.
 
 
Discussion question:

Compare the diagram above (matter diagram from NASA) with the diagram in your text on p. 435. 

Why is a suspension listed under homogeneous mixtures in your textbook, but under heterogeneous mixtures here? Give some examples.


Physical and Chemical Properties of Matter

  1. Physical properties: Properties that describe the look or feel of a substance.
     
    Examples:
    • Color
    • Hardness
    • Density
    • Texture
    • Shape
    • Size
    • Phase (solid, liquid, gas)
  2. Chemical properties: Properties that relate to the ability of a substance to react with other substances, or to transform from one substance to another. The chemical properties of a substance relate to its chemical composition and the way the atoms in the molecules are chemically bonded together.
     
    Examples:
    • Iron will rust.
    • Methane in natural gas will react with oxygen to produce carbon dioxide, water, and heat energy.
    • Baking soda will react with vinegar to produce carbon dioxide and water.

Physical and Chemical Changes in Matter
 

  1. Physical change: A change in form of a substance without a change in its identity. A substance may change in phase (solid, liquid, or gas), or it may change in some other physical property, but its chemical composition does not change.
     
    Examples:
    • Boiling of water (liquid water, ice and steam are just the liquid, solid and gas forms of H2O)
    • Freezing of water to form ice
    • Chewing of food
    • Sharpening of a pencil
    • Crystallization of sugar from a sugar solution
    • Melting of gold
       
  2. Chemical change: A change in the identity or chemical makeup of a substance. A change that involves a rearrangement of the way atoms are bonded is a chemical change.

    Examples:
    • Digestion of food
    • Combustibility = The ability to react with oxygen (e.g., burning a candle or a match, burning of fuel).  Methane reacts with oxygen to form carbon dioxide and water
    • Electrolysis of water (breaking water up into hydrogen and oxygen)
    • Reactivity = The ability to be changed chemically because of a reaction with another chemical substance, such as the development of a gas (e.g., from the reaction of vinegar and baking soda)
    • Formation of a precipitate (e.g., silver nitrate and salt water, or cooling a solution of sodium nitrate and water) - see p. 492-494
    • Change in color (such as burning toast, or reaction of chlorox bleaching colored cloth, or cooking of an egg)
    • Oxidation (or rusting) of steel wool in water

    A chemical change can be expressed as a chemical equation.  The same elements (and same number of atoms of each element) will be present on each side of the equation.

Law of Conservation of Matter (or Law of Mass Conservation)

The Law of Conservation of Matter (or Law of Mass Conservation) states that matter is neither created nor destroyed during a chemical reaction.  The atoms present at the beginning of a reaction are rearranged to form new molecules.  No atoms are gained or lost in a reaction.  As a result, chemical equations are balanced.  The same elements (and same number of atoms of each element) will be present on each side of the equation.


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Content provided by Dr. Margaret Venable, Dr. Michael Denniston, and and Dr. David Wilner.

Some of this information is used with permission of Chemtutor.

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created November 16 - December 2, 2006
Page modified June 24, 2007