Science Quarterly 2-Chemistry
In the chemical reaction CH4 + 2O2 → CO2 + 2H2O, there are reactants on the left of the equation and products on the right. If you count the atoms in the reactants, there are
1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms
If the reactants on the left side of a chemical equation are C3H8 + 5O2, the products in a balanced equation could be
3CO2 + 4H2O
A chemical change is different than a physical change because in a chemical change
A new substance is formed and in a physical change no new substance is formed
The chemical formula for sucrose (sugar) is C6H12O6. In some parts of the sucrose molecule, oxygen is covalently bonded to hydrogen. This makes the sucrose molecule
A polar molecule
One characteristic property of a substance is the way it reacts chemically with another substance. Another way of saying this is that substances react chemically in characteristic ways. This means that
A substance will always react the same way when tested with the same chemical
If two substances react and the temperature of the mixture decreases, the reaction is
Endothermic
If two substances react and the temperature of the mixture increases, the reaction is
Exothermic
In a chemical reaction, if the reactants are heated, the reaction usually happens
Faster
The amount of gas that can dissolve in water
Increases as the temperature of the water decreases
The solubility of a substance
Is the mass of a substance that will dissolve in a certain volume of water at a certain temperature
If liquid A is more polar than liquid B, you might expect
Liquid A to evaporate more slowly than liquid B
In a chemical reaction, Substance A reacts with Substance B and forms a new substance AB. The chemical equation for this reaction is A + B = AB where A and B are the reactants and AB is the product. If you add more and more of only Substance A, there will be
More and more of product AB but limited by the amount of B
When certain substances dissolve, the solution gets warmer. This type of dissolving process is exothermic. In exothermic dissolving
More energy is released when water molecules bond to the solute than is used to pull the solute apart.
When certain substances dissolve, the solution gets colder. This type of dissolving process is endothermic. In endothermic dissolving
More energy is used to pull the solute apart than is released when water molecules bond to the solute.
If more reactants are used in a chemical reaction, more products will be produced. This is because
More reactants have more atoms to react to form more products
Water is not a good dissolver of oil mainly because
Oil molecules are non-polar
Sodium chloride is made up of sodium ions and chloride ions which bond together in a salt crystal because
One is positive and one is negative so they attract
The water molecule is a polar molecule. A water molecule is polar because
Oxygen has a stronger attraction for electrons than hydrogen
Sucrose dissolves well in water because
Polar water molecules attract the opposite polar areas of the sucrose molecules
The atoms in a water molecule are held together by covalent bonds. This means that the bonded atoms
Share electrons
The water molecule is
Slightly negative near the oxygen atom and slightly positive near the hydrogen atoms
Water cannot dissolve all substances that are made from ionic bonds. This is probably because
Some ionic bonds are too strong for the attractions of water molecules to pull them apart
Sugar is more soluble at higher temperatures. This is mainly because at higher temperatures
Sugar and water molecules move faster and their interactions make the sugar molecules separate from one another
In a chemical reaction
The atoms of the reactants unbond, rearrange, and then rebond to form the products
A paper clip can stay on the surface of a sample of water because of water's strong surface tension. Water's surface tension is mostly a result of
The attraction of water molecules
Water is able to dissolve sodium chloride because
The polar areas of water molecules attract the oppositely charged ions of sodium chloride
Water molecules are attracted to each other because
The positive area of one water molecule is attracted to the negative area of another
Dissolving often involves a solid dissolved in a liquid. In this type of dissolving
The solid is the solute and the liquid is the solvent
In a chemical reaction, mass is conserved. This means that
The type and number of atoms in the reactants equals the type and number of atoms in the products
Carbon dioxide gas dissolves pretty well in water because
The unequal sharing of electrons in the covalent bond between the carbon and oxygen in carbon dioxide makes the molecule polar
When sucrose dissolves in water
The water molecules cause the sucrose molecules to separate from one another
Alcohol molecules are not as polar as water molecules. If you mixed sucrose in alcohol you might expect the sucrose
To dissolve better in water than in alcohol
Some chemical reactions require a substance called a catalyst. The main purpose of a catalyst is
To speed up the reaction
A water molecule has the chemical formula H2O. This means that the water molecule is made up of
Two hydrogen atoms and one oxygen atom
One clue of a chemical change is the formation of a precipitate. A precipitate is formed when
Two liquids react and a solid is formed
Examples of chemical change
digesting food,rusting,burning wood,baking a cake
Examples of physical change
melting of an ice cube, mixture of sand and water,sugar dissolving in water,paper being shredded
