MELC 23

• physical change - occurs when the appearance of an object is altered without forming a new type of substance.
• chemical change - involves a transformation of one substance into a new type by altering its chemical composition.
• the law of conservation of matter - states that matter (mass) can neither be created nor destroyed.  It can, however, be rearranged.
• total mass of reactants = total mass of products
• reactants - are the starting material/s in a chemical reaction.
• products - are the substances formed as a result of a chemical reaction
• chemical equations - identifies the starting and ending chemicals as reactants and products:
• chemical equations - is an expression in which symbols and formulas are used to represent chemical reactions.
• chemical formula - is a shorthand method of representing a compound using chemical symbols and numerical subscript.
• chemical reaction - a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. This rearranges the constituent atoms of the reactants to create different substances as products.
• combination/synthesis reaction - a reaction when 2 or more reactants combine to form a single product. A + B = AB
• decomposition reaction - a single reactant breaks down into simpler ones (2 or more products). AB = A + B C
• single displacement or replacement reaction - when one element replaces another element from a compound. the more active element takes the place of the less active element in a compound. A + BC = AC + B
• double displacement - when the positive ions (cations) and negative ions (anions) of different compounds switch places, forming two entirely different compounds. AB + CD = AD + CB
• combustion - when oxygen combines with a hydrocarbon to form a water and carbon dioxide.
• acid-base reaction - is a special kind of double displacement reaction that takes place when an acid and base react with each other
• evidences of chemical reactions -

  • production of light
  • evolution of gas
  • temperature change
  • change in intrinsic properties (color, odor)
  • formation of precipitate
• chemical notation or chemical equation is a standard way of writing chemical formulas, used by Chemists to express the proportionate number of atoms in each element present in the compound.
• coefficient is a numerical number attached to the front of the chemical formula, denoting the number of molecules or moles in a compound.
• subscript is a number written in the lower right side of the chemical formula and shows the number of atoms of each type in the molecules.
• You look at how many atoms you have on each side of the equation and add coefficients to the molecules to balance out the number of atoms, that’s why it is known as the Inspection Method.