Science

Science

1. Materials and their Properties

   • Reversible reactions like melting, dissolving, diffusing, freezing are called PHYSICAL REACTIONS
     • Physical changes involve states of matter and energy. No new substance is created during a physical change, although the matter takes a different form. The size, shape, and color of matter may change. Also, physical changes occur when substances are mixed, but don't chemically react. One way to identify a physical change is that such a changes may be reversible, especially phase changes.
   • Irreversible reactions like burning are called CHEMICAL REACTIONS
     • Chemical changes involve chemical reactions and the creation of new products. Typically, a chemical change is irreversible.
   • All matter is made of atoms. There are over a hundred different types of atom, called elements, and each one has a symbol. The atoms of a particular element are identical to each other.
     • When atoms of different elements join together they make a compound. Compounds are represented by formulae that show how many atoms of each element are in the compound. Formulae can be worked out from the charge of the ions that make up the compound.
   • The forces holding together atoms are called bonds. There are different types of bond; there are metallic (for example, iron), ionic (for example, sodium chloride) and covalent (for example, water).
     • The atom is comprised of three major particles—protons, neutrons and electrons. There are four forces (Electromagnetic, Strong, Weak, and Gravity) that are responsible for the behavior of the particles and thus keep the atom together.
   • Atoms are building blocks of elements.
     • Different from atoms of other elements
       • Two or more different atoms bond in simple ratios to form compounds
   • An Element is a substance that cannot be broken down into anything simpler by chemical means. It is made up of atoms of one kind only. Some elements can consist of single atoms, whereas as others can consist of molecules, such as O2, S8 and P4.
   • A Compound is formed when two or more elements become chemically combined together. The elements react chemically and chemical bonds exist between the atoms involved.
   • A Mixture is formed when two or more different substances are mixed together. There is no chemical combination.
   • All matter is made from atoms. Atoms are very, very small. A molecule is formed when atoms join together by chemical bonds.
     • There are over a hundred different types of atom, called elements. The atoms of a particular element are identical to each other. They cannot be changed chemically into any different element. For example a piece of pure sulfur consists only of sulfur atoms. No chemical process can break it down into a different substance.
     • Compounds are substances that contain atoms of at least two elements chemically combined. For example: magnesium oxide consists of magnesium atoms and oxygen atoms chemically bonded together.
   • Molecules form when two or more atoms form chemical bonds with each other. It doesn't matter if the atoms are the same or are different from each other.
     • Molecules made up of two or more elements are called compounds. Water, calcium oxide, and glucose are molecules that are compounds. All compounds are molecules; not all molecules are compounds
       • Single atoms of elements are not molecules. A single oxygen, O, is not a molecule. When oxygen bonds to itself (e.g., O2, O3) or to another element (e.g., carbon dioxide or CO2), molecules are formed.
   • Why does warm water evaporate faster than cold water? When a liquid evaporates, the higher energy particles escape from the liquid’s surface. This can produce a cooling effect, for example, when alcohol evaporates from the back of the hand and the hand feels cold. Melting involves the breakdown of the solid structure into free moving particles.
   • Does the mass of the water change once ice cubes are melted? In any chemical or physical process the mass of the materials at the start is the same as the mass of the materials at the end. That is, unless some of the products are allowed to escape. For example, if a piece of wood is burned in oxygen, the mass of the wood and the oxygen is equal to the mass of the ash and the other gases produced.
   • How many molecules does one drop of water contain?" An average size drop of water has about 0.05 ml, and a drop of that size would have 1.7x10^21 molecules."
     • Eighteen centimetres cubed of water contains 600 000 000 000 000 000 000 000 or 6 x 1023 water molecules. Even one drop of water will contain millions of molecules (approximately 1021). (Remember, 1 million = 106.) It is hard to comprehend the large numbers involved here