Download Matter, Mass and Weight

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Chemistry 4
Chapter 2
Matter and Energy
Matter, Mass and Weight
Matter is defined as anything occupying space and having mass.
Mass of an object is determined by its resistance to change in motion (inertia).
Weight is the force of gravity acting on an object.
The weight with which the earth attracts an object is equal to:
Weight = mass x acceleration of gravity *
State of Matter: Gas, Liquid, Solid
A gas is a fluid form of matter ( one that flows) that fills any container it occupies. A gas consists of
a great number of particles ( molecules ) that do not interact with one another except in collision.
The particles are far apart compared with their dimensions and are in constant motion.
A solid is a rigid form of matter that maintains the same shape, whatever the shape of its container.
The particles of a solid are close together. The particles of a solid can only rotate and vibrate in fixed
position relative to each other.
Liquid is a fluid form of matter that takes the shape of the part of the container it occupies. In a
liquid, the force between particles are also sufficient to keep the volume of a sample constant but not
strong enough to prevent nearby particles from sliding past one another.
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Homogeneous and Heterogeneous Matter
A system is homogeneous if it has a uniform appearance and composition throughout. When water
and alcohol are mixed, they dissolve in each other to form a homogeneous system called solution.
A system is heterogeneous if it has visibly distinguishable parts. Phase is a region of matter
separated by a visible boundary. A mixture of cooking oil and water is a heterogeneous system. Two
different layers of oil and water are two different phases
Physical Change and Chemical Change
A physical property of a substance is a characteristic that we observe or measure without changing
the identity of the substance. A chemical property of a substance is a characteristic that we observe
or measure only by changing the identity of the substance.
Changes that alter the physical form of matter without changing its chemical identity are called
physical changes ( fusion, solidification, vaporization, condensation, sublimation, deposition...)
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A chemical change occurs when the chemical identity of a substance is destroyed and a new
substance is formed. A chemical change is described by a chemical equation. Formulas of initial
substances are written to the left of an arrow that points to the formulas of substances formed or
products. For example, the reaction of magnesium and oxygen to form magnesium oxide is
2 Mg + O2 ---------> 2 MgO
Water can be decomposed by electricity:
2 H2O ---------> 2 H2 + O2
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Pure Substances and Mixtures
A pure substance is one with constant composition. It has its own set of physical and chemical
properties which may be used to identify the substance. A pure substance cannot be separated into
parts by physical means.
A mixture consists of two or more different chemicals. A physical change can be used to separate a
mixture into pure substances. For example salt and water can be separated by boiling off the water.
Properties of a mixture depend upon its composition.
Elements and Compounds
Elements are substances that cannot be decomposed into simpler substances by physical or chemical
means. Like all pure substances, it has its own set of physical and chemical properties. At room
temperature, 75 elements are solids, 11 are gases, 2 (mercury and bromine) are liquids. Each
element is represented by a symbol: H for hydrogen, O for oxygen, C for carbon, Cl for chlorine.
Some elements are not stable as a single atoms but form stable units called molecules. Symbols for
hydrogen and oxygen are H and O but their molecular formulas are H2 and O2
A compound is a pure substance that can be decomposed by chemical change into two or more other
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pure substances. Water is a compound because it can be decomposed by electrolysis into hydrogen
and oxygen.
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Electrical Character of Matter
Beside gravitational force there are also magnetic and electrostatic forces. Electrostatic forces exist
between objects that carry an electrical charge. Two objects having the same charge (both positive
or both negative) repel each other. Two objects having opposite charges (one positive and one
negative) attract each other.
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Conservation Laws and Chemical Changes
Law of conservation of mass
In a chemical change, mass is conserved; it is neither created nor destroyed.
total mass of reactants = total mass of products
Law of conservation of charge
The law of conservation of charge states that the net electric charge in an isolated system remains
constant. In a chemical change, the algebraic sum of charges of the reactants is equal to the sum of
charges of the products . For example the net electric charge is unchanged in the reaction
NaCl ---------> Na+ + Cl-
Law of conservation of energy
Energy of an isolated system always remains constant. Energy may be converted from one form to
another, but cannot be created or destroyed.
A system can exchange its energy with its surrounding in two forms: heat and work. When a gas is
in contact with an object at higher temperature, energy flows in as heat. This flow of energy into the
system increases the thermal motion of its particles and the temperature of the gas increases.
A chemical change that absorbs heat from its surrounding is called endothermic reaction. A
chemical change that releases heat to its surrounding is called exothermic reaction. The form (as
heat or as work) in which energy is released depends on how the reaction takes place. For example in
normal condition the reaction between hydrogen and oxygen is exothermic
2 H2 + O2 ---------> 2 H2O + energy as heat
But this same reaction in a fuel cell can generate electricity and produce work
2 H2 + O2 ---------> 2 H2O + energy as work
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