Acids, Bases, Chemical Changes
Vocabulary
1. Physical change: a change in matter in which the appearance or state (solid, liquid, or gas) of the matter is altered, but the matter is not converted into a new substance. Examples: chopping wood, shaping clay, melting ice
2. Chemical change (chemical reaction): a change in matter resulting in the formation of one or more new substances that are chemically different from the original substances. Examples: iron rusting, wood burning, vinegar and baking soda reacting and giving off a gas
3. Compound: a substance made of two or more elements chemically combined. Example: Sodium chloride (NaCl) is a compound formed between the elements sodium (Na) and chlorine (Cl).
4. Chemical Formula: a combination of symbols that represent a compound. Example: NaCl is the chemical formula for sodium chloride
5. Chemical Equation: the use of chemical formulas and other symbols instead of words to summarize a chemical reaction. Example: H2 + O2 → 2H2O
6. Reactants: substances you have at the beginning of a chemical reaction
7. Products: substances you have when a chemical reaction is complete
8. Synthesis Reaction: a chemical reaction in which two or more elements or compounds combine to make one more complex substance (A + B → AB)
9. Replacement Reaction: a chemical reaction in which one element replaces another in a compound, or two elements trade places (AB + CD → AC + BD) or (A + BC → B + AC)
10. Decomposition Reaction: a chemical reaction in which a compound is broken down into simpler products (AB → A + B)
11. Endothermic Reaction: any chemical reaction in which heat is absorbed during the reaction (this may make the container feel cool to the touch).
12. Exothermic Reaction: any chemical reaction in which heat is given off during the reaction (this may make the container feel warm or hot to the touch).
13. Precipitate: a solid that forms from a solution during a chemical reaction. Precipitates are usually evidence that a chemical reaction has taken place.
14. Acid: an acid is any substance that produces hydrogen ions (H+) in water.
15. Base: a base is any substance that produces hydroxide ions (OH−) in water.
16. Indicator: any substance that changes color in the presence of an acid or a base. Examples: litmus paper, cabbage juice, methyl orange, phenolphthalein
17. pH scale: a range of values from 0 to 14 that expresses the concentration (amount) of hydrogen ions in a solution. The lower the pH, the higher the concentration of hydrogen ions.
Notes
The only definite proof that a chemical reaction has taken place is the formation of new substances. In other word, the old stuff is GONE, and new stuff has been made!
However, there are 5 indications that a chemical reaction has possibly taken place:
§ There is a temperature change
§ Light is given off
§ There is a color change
§ A gas is given off
§ A precipitate is formed
Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.
In a synthesis reaction, two or more substances combine to make one more complex substance:
2Na + Cl2 → 2NaCl
In a decomposition reaction, one complex substance is broken down into two or more simpler substances:
CaCO3 → CaO + CO2
In a replacement reaction, one element replaces another in a compound, or two elements in different compounds trade places:
2Cu2O + C → 4Cu + CO2
An acid is a substance that has the following properties:
§ Sour taste
§ Reacts with metals and carbonates
§ Turns litmus paper red
§ Produces hydrogen ions (H+) in water
§ Has a pH less than 7
A base is a substances that has the following properties:
§ Bitter taste
§ Does not react with metals or carbonates
§ Turns litmus paper blue
§ Produces hydroxide ions (OH−) in water
§ Has a pH greater than 7
Substances with a pH of exactly or very close to 7 are neutral (not an acid or a base). Water (H2O) is a neutral substance and has a pH of exactly 7.