1、 Definition: chemistry is a natural science that studies the composition, structure, properties and change law of substances.
2、 Physical and chemical changes
Physical change: a change that does not produce other substances.
Chemical changes: changes in other substances. Chemical change is also called chemical reaction.
3、 Physical and chemical properties
Physical properties: properties that do not require chemical changes. Such as color, state, smell, density, hardness, melting point, boiling point, etc.
Chemical property: a property expressed by chemical changes.
4、 Research on candle and its combustion
1. Before lighting: observe the properties of the candle: cylindrical, milky white, solid, insoluble in water and less dense than water.
2. When burning: ① changes during combustion: after the candle is lit, the candle at the wick melts and burns continuously and quietly without making a sound, and the candle becomes shorter slowly during combustion.
② Observe the flame: the flame of the candle flickers slightly, and black smoke is generated above the flame. The flame of the candle can be divided into three layers: the flame in the innermost layer is dim, and the bottom is light blue, which is the flame core; The second layer of flame is bright and conical, which is an internal flame; The third layer of flame surrounding the outermost layer is yellow, bright but not dazzling. It is an external flame.
③ Temperature comparison of three parts of flame; The outer flame temperature is the highest, the inner flame temperature is the second, and the flame core temperature is the lowest. How to determine by experiment?
④ What is the substance after combustion? How to test? Cover a dry beaker over the flame. After a while, water mist will be generated on the beaker wall. After a while, take down the beaker and quickly pour a small amount of clarified lime water into it. After oscillation, the lime water becomes turbid, indicating that water and carbon dioxide are generated during candle combustion.
3. After extinguishment: light the white smoke just extinguished by the candle: the flame will relight the candle along the white smoke.
3. Know several "must" and "not necessarily" of oxides!
(1) Alkaline oxide must be metal oxide! But metal oxides are not necessarily alkaline oxides（ For example, mn2o7 is acidic oxide, Al2O3 is amphoteric oxide and Na2O2 is peroxide).
(2) Acidic oxides are not necessarily non-metallic oxides（ Such as mn2o7); Non metallic oxides are not necessarily acidic oxides（ Such as CO, no).
(3) Acidic oxides and alkaline oxides may not all react with water to form corresponding acids and bases（ (e.g. SiO2, Fe2O3)
(4) The oxide that reacts with water to form acid is not necessarily acidic oxide, such as NO2; The oxide that reacts with water to form alkali is not necessarily alkaline oxide, such as Na2O2.
(5) Acid oxides are acid anhydrides corresponding to acids, but anhydrides are not necessarily acid oxides! Such as acetic anhydride [(ch3co) 2O].
For example, CO2 is the anhydride of H2CO3.
When one or several molecules of water are removed from an oxygenated acid, the remaining part is called the anhydride of the acid.
(6) The aqueous solution of acid (or base) must be acid (or base), but the solution of acid (or base) is not necessarily acid (or base)!
(7) A monobasic acid molecule can ionize an H +, and it is not necessarily monobasic acid (such as NaHSO4) that ionizes an H +.
(8) There must be acid radical ions in salt, and those with acid radical ions are not necessarily salts (such as H2SO4).
