Chemical changes in matter produce new substances with
properties different from the original substances.
In a chemical change, compounds may change into other compounds.
Then these compounds may be broken down into elements.
Alternatively, elements may combine to form compounds.
1. In all of these types of changes, new substances with new properties form.
Various types of evidence are used to identify the formation of a new substance through a
chemical change.
2. Gas production is a form of evidence indicating a chemical change taking place.
a. For example, when baking soda and vinegar react one of the products formed is
carbon dioxide gas. When the two reactants combine, bubbles form indicating the
production of CO2 gas.
b. Another way to see this gas formation is to put the two reactants in a sealed flask
with a deflated balloon around the top of the flask. When the two reactants
combine, they form bubbles and the balloon inflates.
3. Temperature changes are sometimes used as evidence to indicate a chemical
reaction has occurred.
a. However, a temperature change does not always indicate a chemical change.
Thermal energy is transferable in many processes, including physical changes.
For example, when ice is placed in warm water, the temperature changes but no
chemical reaction takes place. However, when two combined substances with
similar beginning temperatures produce (exothermic) or lose (endothermic) heat,
then a temperature change indicates a chemical reaction.
4. The formation of a precipitate can be evidence of a chemical change.
a. A precipitate is an insoluble compound formed out of solution in precipitation
reactions.
b. A common example of precipitate formation is seen when mixing two clear
solutions, silver nitrate and sodium chloride.
c. The result is a chemical change in which two products, sodium nitrate and silver
chloride, a white solid, are produced. Silver chloride is the precipitate. It is a
white solid.
5. Color changes, as with temperature changes, are sometimes used as evidence of a
chemical change. Not all color changes, however, indicate chemical changes.
a. For example, adding food color to water might appear to change the color of the
water, but no chemical reaction takes place.
b. Many times a chemical indicator changes color when the pH of a solution
changes. When the two clear solutions, silver nitrate and potassium iodide are
mixed, a bright yellow compound called silver iodide precipitates. This color
change is an indicator of a chemical change, as well as a sediment formation.
A chemical formula is a combination of element symbols and
subscript numbers used to show the composition of a substance,
either as a compound or in pure element form .
Chemical formulas make it easier to describe substances by providing information about the
number and kinds of atoms creating a molecule of that substance.
Coefficient : number
of molecules
2CO2
Subscript: number of
atoms
These formulas identify each component element by its chemical symbol and indicate the atoms’
number of each element found in each molecule of that substance.
If a molecule contains more than one atom of a particular element, this quantity is indicated by
using a subscript after the chemical symbol.
For example, a molecule of carbon dioxide consists of one carbon atom and two oxygen
atoms. Its chemical formula is CO2. If there is only one atom of an element in a molecule of a
compound, then there is no subscript number written.
Another example, glucose, is represented by the chemical formula C6H12O6. For each
molecule of glucose, there are six carbons atoms, twelve hydrogen atoms, and six oxygen atoms.
Some elements occur naturally in pairs of atoms rather than as single atoms. These are called
diatomic molecules. Hydrogen gas (H2), nitrogen gas (N2), and oxygen gas (O2) are common
diatomic molecules.
Chemical formulas are used in chemical equations to describe chemical reactions.
Chemical equations represent chemical reactions symbolically with the reactants
written on the left hand side of an arrow (represented by an arrow symbol and
usually read as “yields”) and the products written on the right hand side. If there is
more than one reactant or product, then they are separated by a plus sign.
Reactants
2H2 + O2 → 2H2O
Products
The Law of Conservation of Mass states the quantity of each element does not change in a
chemical reaction. Thus, each side of a chemical equation must represent the same quantity
of each element in the reaction.
H2 + O2 → H2O This equation, however, is not balanced.
: 2H2 + O2 → 2H2O ( balanced equation, follows the Law of Conservation
Chemical Changes:
A chemical reaction occurs when the chemical identity of the
reactants is different than that of the products.
1. A new substance or substances forms with properties different from the original
reactants.
2. Often times, chemical reactions result in changes easily observed. Evidence of
chemical reactions includes temperature changes, formation of a precipitate, color
changes and gas production.
3. During chemical reactions involving the gain or loss of heat, temperature changes
indicate the formation of new substances.
a. In endothermic reactions, heat is absorbed when transforming from reactants
to products (gain in enthalpy). When it takes more energy to break the bonds
of the reactants than is released when new chemical bonds form, the reaction
is endothermic (feels colder).
b. A reaction is exothermic (loss in enthalpy) when more energy is released in
forming new bonds than it takes to break the original bonds of the reactants
(feels warmer).
4. In precipitation reactions, a precipitate is a solid formed out of solution.
a. A precipitate forms because the solid produced is insoluble in aqueous
solutions. This is a new substance formed with new properties and is evidence
of a chemical change.
5. When gas is produced during a reaction, it is also evidence of a new substance
formed. For example, when baking soda (solid) is mixed with vinegar (liquid), carbon
dioxide (gas) is one of the products formed. The new products formed from this
reaction, including CO2 gas, have properties different from the original reactants.
6. A color change represents more than just a physical change.
a. . The color of a substance may change when its chemical composition
changes. An example of a color change as evidence of a chemical reaction is
seen when crystal violet dye is added to a solution containing hydroxide ion.
The intensity of the color decreases over time until it disappears.
b. The product of the reaction is colorless in water solution.
Physical Properties :
7. . The density of a substance is a physical property defined as mass per unit volume.
a. Density is calculated by dividing the mass of an object by its volume.
i. . Often times in chemistry, the density of a substance is compared to
the density of water (1.0 g/ml).
ii. Objects that sink in water have density values greater than 1 (g/ml or
g/cm3) and objects that float on water have density values less than 1
(g/ml or g/cm3).
8. Ductility, Malleability, transparency , hardness are other examples of physical
properties .