air pressure: the that air puts on an area

The air molecules are more spread out as one goes higher. So, air pressure with height.

Storms usually occur when the air pressure over an area is very .

Air pressure at the peak of a mountain is much than what it is at sea level.

conduction: the transfer of heat energy within an object, or between objects that are directly touching each other, due to between the particles in the objects

In a solid, heat is conducted as the gradually spread to the right.

Heat energy can be transferred in several ways. Radiation is the transfer of energy via . Conduction is the transfer of molecular heat energy via . Convection is the transfer of heat via the .

gas: a state of matter without any defined

Plants use carbon dioxide gas and water to make food for themselves in the presence of sunlight. This process is known as .

Nitrogen, oxygen, and carbon dioxide are the main gases that make Earth's .

Unlike a solid or liquid , the molecules of a gas are not packed close together. They move about in all directions.

is a gas that is less dense than air under normal conditions. That is why balloons filled with it float up in the air.

kinetic energy: the energy an object has because of its

When water is , it has kinetic energy. This energy is used to generate electricity in hydroelectric power plants.

When a roller coaster is at highest position on the slope, it has stored potential energy. As it begins to move down the slope, this stored energy is converted to energy.

liquid: a state of matter with a defined but no defined and whose molecules roll past each other

These three illustrations represent views of the differences between a solid, a liquid, and a gas.

Liquids, as well as , are often characterized as fluids. Fluids are materials that under the action of an applied force.

matter: material that has and takes up some amount of

of matter.

mixture: a combination of substances that can be from one another

is a mixture of crystals of quartz, feldspar, and mica.

The ocean is a mixture of salt and many other in water.

pure substance: a material with a particular

is the hardest substance on Earth.

Matter can be categorized as mixtures and pure substances. Pure substances could be or . They can be obtained when the components of a mixture are separated by methods.

solid: matter with a volume and shape

Alphabet blocks are solids because they have a definite shape and a fixed volume.

A crayon is a solid. The that make the crayon are very close to each other.

temperature: a measure of the average of the atoms in a system, used to express in degrees

Lord Kelvin (William Thomson), the physicist who first identified absolute zero, devised a temperature scale that begins at . The Kelvin temperature scale is widely used in the physical sciences. This diagram compares the Celsius temperature

thermal energy: energy in the form of

Heat is the of energy from a hot object to a cooler one. When you feel that heat, in a cup of hot chocolate or the warmth of sunlight, you are actually feeling .