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Chemical and Physical Changes
The term chemical change, or chemical reaction, refers to the transformation of one or more substances into others. We say that a chemical change has occurred if there is a change in composition. If a change does not involve a change in composition, then it is called a physical change. At the molecular level, a chemical change involves a reorganization of atoms into different formula units. For physical changes, the molecules remain intact; they simply move apart (as in vaporization of a liquid into a gas; or breaking of a rock), or move closer together (as in condensation of a gas into a liquid).
Exercise 1 and Answer watch below video:
Exercise 2 and Answer watch below video:
Exercise 3 and Answer watch below video:
Reactants and Products
We refer to the substances involved in a chemical change as reactants and products. Reactants are transformed into products. Therefore, as a chemical change occurs, the amount of reactants will be decreasing and the amount of products will be increasing.
Example: Suppose a drop of water added to a white solid results in the formation of a blue solid. Identify the reactants and products.
Answer: the white solid and water are reactants; the blue solid is a product. This is something that you would actually observe if you were to add a drop of water to copper sulfate (CuSO4, a white solid). The reaction produces a compound called copper sulfate pentahydrate (CuSO4.5H2O), which is a blue solid. Here's a video that demonstrates this reaction:
Example: During a chemical change, the amount of substance X increases from 5.0 g to 6.0 g. Is X a reactant or a product?
Answer: Since the amount of X increased, we call it a product of the reaction.
Exercise: A mixture contains 25.0 g of substance X and 10.0 g of substance Y. A few minutes later, due to a chemical change, it is found that the mixture now contains 30.0 g of substance X and 5.0 g of substance Y. For the reaction described here...
A. X is the reactant and Y is the product
B. Y is the reactant and X is the product
C. Both X and Y are reactants
D. Both X and Y are products
Exercise and Answer watch below video:
How can we tell if a chemical change has occurred?
Each substance has a unique set of properties that makes it different from other substances. If there is a change in the properties as a result of putting two or more substances in contact, or subjecting a sample of matter to heat, light or electricity, then it is very likely that a chemical change has occurred. Some sure signs that a chemical change has occurred include: an unusual change in color, odor, or taste and production of heat and light. Some signs that a chemical change may have occurred include: production of heat, absorption of heat, production of gas bubbles in water, and formation of a solid when two liquids are mixed (precipitation).
An apparent lack of visible change does not imply that a chemical change has not occurred. Example: sodium hydroxide dissolved in water, NaOH(aq), is a colorless liquid. The (aq) label means "in aqueous solution," which means that the substance is dissolved in water. Acetic acid in water, CH3COOH(aq), is also a colorless liquid. If you mix the two liquids, you still get a colorless liquid; you will not be able to observe a visible change. But there is, in fact, a chemical change. NaOH(aq) is bitter, CH3COOH(aq) is sour, but the resulting mixture is salty.WARNING: Do not taste anything in the lab. CH3COOH(aq) smells like vinegar while NaOH(aq) is odorless. The resulting mixture could be odorless or could still smell a little bit like vinegar (if there are acetic acid molecules left unreacted). If you were to measure the temperature of the liquids before and after mixing, you will also find that the temperature of the resulting mixture is higher; this reaction generates heat; we say it is an exothermic reaction.
Production of heat generally accompanies most physical and chemical changes. So, generation of heat alone does not imply that a chemical change has occurred. Example: steam condensing into a liquid generates a lot of heat, but it is just a physical change. Production of heat and light together, as in the burning of paper, is a definite sign that a chemical change has occurred; these are called combustion reactions.
Production of gas bubbles in water could occur by simply stirring it, shaking it, heating it to its boiling point, or pulling a vacuum above it. In these cases, the change involved is just a physical change. However, if bubble formation results simply by mixing two liquids (no heating or shaking involved), then the gas bubbles are very likely to be the product of a chemical change. If a chemical reaction produces a gas that is not very soluble in water, the gas will be observed leaving the water as bubbles ("fizz"). A familiar example is carbon dioxide, CO2, which fizzes out when baking soda is mixed with vinegar. The fizz observed when opening a can of carbonated drink does not involve a chemical change; the can is pressurized; opening it lowers the pressure and lowers the solubility of carbon dioxide.
Precipitation usually indicates a chemical change. However, it can happen if you mix different liquids such that a substance dissolved in one of the liquids suddenly becomes insoluble; in this case, the precipitation is not considered a chemical change.
A change in color, odor or taste does not necessarily imply that a chemical change has occurred. For example, our eyes perceive a mixing of yellow and blue as green. So, mixing a yellow liquid with a blue liquid to produce a green liquid does not necessarily mean that a chemical change has occurred. But if mixing a yellow liquid with a blue liquid produces a red liquid, then we can confident that a chemical change has occurred. If you mix something sweet and something sour and your taste buds senses a "sweet and sour" taste, it does not necessarily mean that a chemical change has occurred. But if you sense a bitter taste instead, then a chemical change has occurred.
Example: Which of the following involves a chemical change?
A. A liquid is sprayed in a stinky room and the odor disappears.
B. The windows of a stinky room are opened and a fan turned on; the odor disappears.
Answer: A. Odor is due to molecules triggering a response from your brain when they encounter receptors inside your nose. The spreading out the molecules in case B gets rid of the odor because fewer molecules are able to reach the receptors inside your nose. That does not involve a chemical change. In case A, whatever was sprayed must have reacted with the molecules responsible for the odor, producing something that is odorless.
Exercise: Which of the following observations is a definite sign that a chemical change has occurred?
A. Mixing a blue-colored liquid with a yellow-colored liquid, yields a green-colored mixture.
B. Mixing a blue-colored liquid with a colorless liquid yields a yellow-colored liquid
Exercise: Which of the following observations implies that a chemical change has definitely not occurred?
A. Mixing two colorless liquids yields a colorless liquid.
B. A piece of paper, held next to a flame, catches fire and leaves a black residue.
A chemical equation is a symbolic representation for what happens in a chemical change.
For example, the chemical equation for the reaction of calcium carbonate, CaCO3, with hydrochloric acid, HCl(aq), is
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
In a chemical equation the formulas of the reactants are written on the left hand side, separated by + signs. In the example above, the reactants are CaCO3 and HCl. The formulas of the products are written on the right hand side, separated by + signs. In the example above, the products are CaCl2, H2O, and CO2. The reactants and products are separated by an arrow. It is preferable, but not required, to indicate the physical states of the reactants and products by using the following labels: (s) for solid, (l) for liquid, (g) for gaseous, and (aq) for aqueous solution.
Numbers written in front of the formulas are called coefficients. In the example above, the coefficient of HCl is 2. Note that we don't say that "2 HCl" is a reactant in this reaction. We say that "HCl" is a reactant and its coefficient in the chemical equation is 2. Unwritten coefficients are implied to be 1. In the example above, the coefficients of CaCO3, CaCl2, H2O, and CO2 are all 1.
Exercise: Consider the chemical equation for the hydrogenation of acetylene:
C2H2 + 2H2 → C2H6
Which of these is the formula of a reactant in this reaction?
A. C2H2, B. 2H2, C. C2H6
Exercise: Consider the following observations of a chemical change: A small amount of orange-red solid is heated in a test tube. A little explosion occurred and a green powder was obtained. The mass of the green solid was found to be less than that of the original orange-red solid. Which of the following chemical equations is consistent with the observations above?
A. Mg(s) + O2(g) → 2 MgO(s)
B.(NH4)2Cr2O7(s) → 2 N2(g) + Cr2O3(s) + 4 H2O(g)
C. 2 HgO(s)→ 2 Hg(l) + O2(g)
D. Pb(NO3)2(aq) + 2 NaCl(aq) → PbCl2(s) + 2 NaNO3(aq)
The purpose of the coefficients is to "balance" the chemical equation. Since no atoms are created or destroyed during a chemical change, the number of atoms of each element indicated on both sides of the arrow must be equal. To count the number of atoms, we simply multiply the coefficient by the subscript. Consider the following chemical equation:
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
The number of Cl atoms on the left-hand side is 2. Cl is found in HCl, which has a coefficient of 2; its subscript in HCl is implied to be 1; therefore, 2x1 = 2. On the right-hand side, the number of Cl atoms is also 2. On the right, Cl is found in CaCl2, which has a coefficient of 1. Its subscript in CaCl2 is 2; 1x2 = 2.
For interactive tutorials:
- Chemistry: Interactive Tutorial on Balancing Equations
- Classic Chembalancer
- A tutorial on balancing chemical equations
The coefficients in a chemical equation are relative counts. This means that there isn't a unique set of coefficients to balance chemical equation. In fact, there is an infinite number of sets of coefficients that we can use to balance a chemical equation. In the example above, the coefficients are 1, 2, 1, 1, and 1. We can multiply all of these numbers by the same factor and come up with another set of coefficients that will also give us a balanced equation. For example, we can multiply all of these coefficients by 2, and we get another balanced equation:
2 CaCO3(s) + 4 HCl(aq) → 2 CaCl2(aq) + 2 H2O(l) + 2 CO2(g)
We can even multiply it by a fraction, like 1/2:
1/2 CaCO3(s) + HCl(aq) → 1/2 CaCl2(aq) + 1/2 H2O(l) + 1/2 CO2(g)
It is customary, but not required, to use the smallest set of whole number coefficients. How can you tell that you have the smallest set of whole number coefficients? Find the greatest common factor; if it's one, then you have it. How do you know that your greatest common factor is one? Divide all the numbers by the smallest; if one of the resulting numbers is not a whole number, then your greatest common factor is one and you have the smallest set of whole number coefficients.
You might find it disconcerting to see fractional coefficients. You could argue that we cannot have half of a molecule. Coefficients do not necessarily mean individual counts; in fact, they could be a group counts. Because atoms and molecules are so small, it is convenient to count them in groups called moles. A mole of molecules is about 602 billion trillion molecules. In the example above, 1/2 CO2would be interpreted not as half of a CO2 molecule, but half a mole of CO2molecules (or 301 billion trillion CO2 molecules).
Exercise: Which of the following is not a balanced chemical equation?
A. 2 H2 + O2 → 2 H2O
B. 4 H2 + 2 O2 → 4 H2O
C. Both A and B
D. None of the above
Strategies for Balancing Equations
Once you have established the formulas of reactants and products, the only thing you should be doing is adjusting the coefficients.
- If balancing by inspection, first try to balance elements that appear only once on either side. For example, to balance CH4 + O2 → CO2 + H2O, it is best to try to balance C and H before O. O appears twice on the right side. In this example, we can see that the coefficients of CH4 and CO2 must be equal since these are the only instances where C appears and the subscript of C in both cases is the same 1. Similarly, the coefficents of CH4 and H2O must be be in a 1:2 ratio. There are 4 H atoms in CH4, but only 2 in H2O. There should be twice as many H2O molecules compared to CH4 in order to have the same number of H atoms on both sides.
- Balance by adjusting coefficients (in the deficient side) upward. In the example above, balance H by adding another H2O to the right side (increasing the coefficient of H2O up to 2), since there are fewer H atoms on the right.
If you're comfortable with simple algebra, assign variables names to the coefficients and set up algebraic equations to solve for the values of the variable. Start by assigning any one of the variables to any number. Example: find the values of w, x, y, and z, needed to balance w CH4 + x O2 → y CO2 + z H2O
To balance C: w=y. Eq. 1.
Here, w is the number of C on the left, y is the number of C on the right
To balance H: 4w =2z. Eq. 2.
Here, 4w is the number of H on the left, 2z is the number of H on the right.
To balance O: 2x = 2y + z. Eq. 3.
Here, 2x is the nmber of O on the left, 2y+z is the number of O on the right
We can assign any number to any one of these variables and we will be easily able to solve for the others.
Using Eq. 1.: w=y, or 1=y.
Using Eq. 2. 4w=2z, or 4(1) = 2z.
Therefore: z= 4(1)/2 = 2
Using Eq. 3. 2x = 2y+z.
Therefore: 2x = 2(1)+2, or x = (2(1)+2)/2 = 2
Therefore,: w=1, x=2, y=1, and z=2
The balanced equation is: CH4 + 2 O2 → CO2 + 2 H2O
Had we chosen to assign a value of 2 for w, we would have gotten w=2, x=4, y=2, and z=4.
Exercise: Which of the following is the balanced chemical equation for the following reaction:
Mg + O2 → MgO
A. Mg + O → MgO
B. Mg + O2 → MgO + O
C. Mg2 + O2 → 2 MgO
D. 2 Mg + O2 → 2 MgO
Exercise: Consider the following chemical equation where coefficients are represented by the letters w,x,y, and z: w Al2O3 + x H2SO4 → y Al2(SO4)3 + z H2O
If w=1, then which of these is true? A. y=2, B. z=3, C. x=6
Exercise: Consider the combustion of propane: C3H8 + O2 → CO2 + H2O(unbalanced)
Using the smallest set of whole numbers, the sum of the coefficients in the balanced chemical equation is:
A. 7, B. 9, C. 12, D. 13
Exercise and Answer watch below video:
Exercise and Answer watch below video:
Classifying Simple Chemical Reactions
Combination (or composition, or synthesis)
A combination reaction involves two or more reactants and only one product. The simplest examples involve the combination of two elements to form a compound.
- 2 Al(s) + 3 Br2(l)→ 2 AlBr3(s)
- 2 Na(s) + Cl2(g) → 2 NaCl(s)
- S8(s) + 8 O2(g) → 8 SO2(g)
- 2 Fe + 3 O2(g) → Fe2O3
Compounds can also combine to form just one compound. Examples:
Ammonia reacts with a molecular acid to form an ammonium salt.
Metal oxides react with water to form metal hydroxides.
- MgO(s) + H2O(l) → Mg(OH)2(s)
Nonmetal oxides react with water to form acids.
- SO2(g) + H2O(l) → H2SO3(l)
Metal hydroxides react with carbon dioxide to form metal bicarbonates.
- NaOH(s) + CO2(g) → NaHCO3(s)
A decomposition is the opposite of a combination reaction. It involves only one reactant, and two or more products.
2 HgO(s)→ 2 Hg(l) + O2(g)
CaCO3(s) → CaO(s) + CO2(g)
If a reaction requires heat to occur, it is customary to put a triangle above the arrow.
Here are videos that demonstrate the decomposition of HgO:
The decomposition of CaCO3 occurs when limestone is heated to make cement, Watch:
Exercise: Which of the following is a decomposition reaction?
A. 2 CO(g) + O2(g) → 2 CO2(g)
B. Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
C. Ba(OH)2(aq) + H2SO4(aq) → 2 H2O(l) + BaSO4(s)
D. 2 H2O2(aq) → 2 H2O(l) + O2(g)
Displacement (or single replacement)
A displacement reaction involves the replacement of an element in a compound by another element.
In the following examples, copper is displaced by magnesium and aluminum.
Mg(s) + CuCl2(aq) → MgCl2(aq) + Cu(s)
2 Al(s) + 3 CuCl2(aq) → 2 AlCl3(aq) + 3 Cu(s), Watch Video:
In the following example, Mg displaces H from an acid:
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
Note that hydrogen is a molecular element; it is made up of diatomic molecules.
The term activity series refers to a list that we can use to predict which metallic element can displace another (under "standard conditions"). A subset of this series which includes the most common metallic elements is:
[ K Ca Na ] [ Mg Al Mn Zn Cr ] [Fe Ni Sn Pb] H [ Cu Bi Sb ] [ Hg Ag Pt Au ]
The metal listed first can displace one that is listed later. A mnemonic for this is
Peter Carly Simon Made A Mangy Zebra Carry Iron Nails To Liverpool.
He Caught Billy, A Mexican Silver Plated Goat.
Peter is for potassium, Carly is for calcium, etc. The first group (K, Ca, and Na) are so reactive that they can readily displace H2 from cold water. The second group readily displaces H2 from steam; they react slowly with room temperature water. The first, second, and third groups are able to displace H2 from acids. For more information about the activity series of metals:
A displacement reaction can also involve displacement of a nonmetal by another nonmetal, as in the following example:
Cl2(aq) + 2 NaBr(aq)→ 2 NaCl(aq) + Br2(aq)
In this example, chlorine displaces bromine from sodium bromide,
Metathesis (or double replacement or double decomposition)
A metathesis reaction generally involves two electrolytes (compounds that yield ions in solution). You can think of this reaction as involving an exchanging of partners.
CaCl2(aq) + 2 AgNO3(aq) → 2 AgCl(s) + Ca(NO3)2(aq)
If the product is insoluble in water, it is labeled with (s), as in AgCl in the example above, and the reaction is called a precipitation reaction. A precipitate can also be labeled with a downward-pointing arrow.
CaCl2 + 2 AgNO3 → 2 AgCl↓ + Ca(NO3)2
A metathesis reaction can also involve an acid and a base (an oxide or hydroxide). The reaction in this case is called acid-base neutralization, and the product is water and a salt. The term salt does not necessarily refer to NaCl. In Chemistry, salt refers to any ionic compound, except if the anion is oxide or hydroxide.
Example: write the chemical equation for the reaction of hydrochloric acid reacts with calcium hydroxide
Answer: Ca(OH)2(s) + 2 HCl(aq) → CaCl2(aq) + 2 H2O(l)
The salt formed is calcium chloride.
A metathesis reaction can also involve an acid (or a base) and a salt. One that is commonly encountered in introductory laboratory courses involves the reaction of carbonates or bicarbonates with an acid.
Example: baking soda + vinegar:
NaHCO3(s) + HC2H3O2(aq) → NaC2H3O2(aq) + H2CO3(aq)
H2CO3 (carbonic acid) readily decomposes to water (H2O) and carbon dioxide (CO2).
H2CO3(aq) → H2O(l) + CO2(g)
CO2 is not very soluble in water so most of the CO2 formed comes out of solution as gas bubbles ("fizz"). The overall change can be represented by the following chemical equation:
NaHCO3(s) + HC2H3O2(aq) → NaC2H3O2(aq) + H2O(l) + CO2(g)
Watch above videos:
A similar reaction occurs between a sulfite or bisulfite and an acid. The sulfite or bisulfite ion combins with the hydrogen ions from the acid to form sulfurous acid (H2SO3), which decomposes to SO2(g) and H2O.
Example: write the chemical equation for the reaction of hydrochloric acid with solid sodium sulfite
Answer: Na2SO3(s) + 2 HCl(aq) → 2 NaCl(aq) + H2SO3(aq)
The sulfurous acid further decomposes: H2SO3(aq) → H2O(l) + SO2(g)
So, the overall reaction can be written as:
Na2SO3(s) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l) + SO2(g)
Exercise: What is the salt produced by the neutralization reaction of magnesium hydroxide, Mg(OH)2, and hydrochloric acid, HCl? A. MgCl, B. MgCl2, C. HOH, D. none of these
Exercise: What is chemical equation for the neutralization of stomach acid, HCl,
by milk of magnesia, Mg(OH)2?
A. HCl + Mg(OH)2 → HMg + Cl(OH)2
B. 2 HCl + Mg(OH)2 → 2 HOH + MgCl2
C. HCl + Mg(OH)2 → H2O + MgCl
D. HCl + Mg(OH)2 → H2O + MgHCl
Exercise: What is the chemical equation for the neutralization of nitric acid, HNO3, by calcium oxide (CaO)? A. HNO3 + CaO → HO + CaNO3
B. 2 HNO3 + CaO → H2O + Ca(NO3)2
C. HNO3 + CaO → NO4 + CaH
D. 2 HNO3 + CaO → H2O + CaNO3
The term redox is short for reduction-oxidation. Redox reactions involve electron transfer. For example, in the reaction,
Na(s) + Cl2(g) → 2 NaCl(s),
we can identify Na atoms having lost electrons. Na atoms in elemental sodium are neutral. In a compound, as in NaCl, sodium is always found as a cation with a charge of +1.
We say that, in this case, sodium is oxidized; oxidation involves a loss of electron. We can also identify the chlorine atoms as having gained electrons.
In NaCl, we don't have neutral chlorine atoms; we have chloride ions, which have a charge of -1. It therefore appears that each chlorine atom in Cl2 ended up with one extra electron. You should be easily able to recognize some simple redox reactions. Any time an elemental substance is involved as either a reactant or product, you can conclude that the reaction is a redox reaction. Combination, decomposition, or displacement reactions involving at least one elemental substance are all redox reactions. More sophisticated methods of determining whether a reaction is a redox reaction (using oxidation numbers) will be discussed elsewhere.
Exercise: Which of the following chemical reactions is not a redox reaction?
A. C(s) + O2(g) → CO2(g)
B. Cl2(g) + 2 KBr(aq) → 2 KCl(aq) + Br2(l)
C. 2 KClO3(s) → 2 KCl(s) + 3 O2(g)
D. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Exercise: Which of the following chemical reactions is a redox reaction?
A. NH3(g) + HCl (g) → NH4Cl(s)
B. CaCO3(s) → CaO(s) + CO2(g)
C. 2 CaO(s) → 2 Ca(s) + O2(g)
D. Ag+(aq) + Cl-(aq) → AgCl(s)
Combustion involves a reaction of a material with oxygen (O2), accompanied by the production of heat and light. The product of combustion are the oxides of the elements present in the material.
Example: the burning of propane is:
C3H8(g) + 5 O2(g) \rightarrow 3 CO2(g) + 4 H2O(g)
Note that the products are the oxide of carbon (carbon dioxide, CO2) and the oxide of hydrogen (water, H2O). If there is insufficient O2 available, carbon monoxide (CO) may also be formed.
If the compound contains nitrogen, N2 is formed unless a very large excess of oxygen is used (in which case, oxides of nitrogen are formed such as NO and NO2).
If the compound contains sulfur, SO2 and SO3 are also produced.
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