Saturday, March 26, 2016

Structure and Measurement of Matter

Solids – Liquids – Gases:
The molecules in a solid are packed closely together in regular structure. They do not have enough energy to break free of the forces of attraction which bind them to their neighboring molecules. They can only vibrate. This is why solids have a fixed shape and affixed volume and do not flow like liquids.

The molecules in a liquid have just enough energy to break free of the forces which bind them to their neighbors. This is why liquids are able to flow and do not have a fixed shape. However, the forces are strong enough to hold the molecules close together, giving liquids a fixed volume.

The molecules in a gas have so much energy that the force of attraction between them is negligible. They can move freely and at great speed. The molecules in a gas are much further a part than those in a liquid or a solid. This is why gases can be compressed easily.

Brownian Motion:
The molecules in liquids and gases are continually moving in a completely random fashion. This is known as Brownian motion, after the botanist Sir Robert Brown who first studied the nature of their movement. He demonstrated that pollen grains placed in water move erratically. This motion is due to the pollen grains unseen impact with water molecules. The tiny water molecules are able to move the much larger pollen grains because there is large number of water molecules and they are moving very fast.

Diffusion is the gradual mixing of two or more different gases or liquids. Diffusion happens when the molecule of the substances collides and intermingles. For example, the scent of flowers spreads through a room because its molecules diffuse through the air. The process of diffusion supports the idea of moving molecules, since the particle must be moving in order to mix.
Atoms and Helium Atoms:
Molecules are made up of smaller particles called atoms. Atoms are formed of even smaller particles called electrons, protons and neutrons. The structure of an atom is shown here using the example of a Helium atom. The central nucleus of an atom is formed of protons and neutrons. Protons have a positive electrical charge and neutrons have no charge. Protons and neutrons are approximately 2000 times more massive than the electrons which orbit the nucleus. Electrons have a negative charge, equal in magnitude to the positive charge of the protons. The number of electrons in an atom is the same as the number of protons in the nucleus. 

The number of protons in a nucleus is called the proton number (Z). The total number of protons and neutrons in a nucleus is called the nucleon number (A).

The nucleon and the proton number of an atom are written next to the symbol for the element to which the atom belongs. For example, Helium is written as: 24He

Measuring Mass:
The mass of an object is the measure of how much matter it contains. Mass is measured in kilograms (Kg). To find the mass of an object, simple balancing scales like the ones shown are used to compare the unknown mass with a known mass.

These quantities are used in the equation:
Mass (M) = Density (D) X Volume (V)
Volume (V) = Mass (M) / Density (D)
Density (D) = Mass (M)/Volume (V)

Measuring Volume:
The volume of an object is the measurement of the amount of space it occupies. It is measured in cubic meters (m3) or cubic centimeters (cm3). The volume of regular shaped solids is found using a ruler and mathematical formula.

For example, The volume of a rectangular block is found using the equation: length X breadth X height.

The volume of liquid can be found by pouring it into a measuring cylinder. The volume of an irregular shaped solid is measured by displacement as in the diagram.

Measuring Density:
Objects which are the same size and shape can vary greatly in mass. For example, one cubic centimeter of cork is much lighter than a cubic centimeter of lead. This is because the materials have a different density. Molecules of lead are heavier and more closely packed together than those of cork. This makes lead a more dense material than cork.
To find the density of a solid or a liquid its mass and volume must be measured using the methods described above.

These quantities are used in the equation:
Density (D) = Mass (M) / Volume (V)

Density is measured in kilograms per cubic meter (kg/m3) or grams per cubic centimeter (g/cm3)