CHEMICAL EQUILIBRIUM

Subject :Chemistry

Term : Second Term

Week :Week 3

Topic : CHEMICAL EQUILIBRIUM

Behavioural Objectives :By the end of the lesson,

1. Students will be able to define equilibrium and explain the factors affecting the position of equilibrium.
2. Students will be able to identify and differentiate between the three states of matter.
3. Students will be able to describe the properties of the three states of matter.

Materials Needed:

• Whiteboard and markers
• PowerPoint presentation
• Worksheets for students
• Examples of reversible reactions
• Examples of the three states of matter

Content

CHEMICAL EQUILIBRIUM

Chemical equilibrium is a state where the rates of the forward and reverse reactions in a chemical reaction are equal. This means that the concentrations of the reactants and products remain constant over time. Chemical equilibrium is a dynamic process, meaning that the reactants and products are still reacting, but the rates of the forward and reverse reactions are equal, so there is no net change in concentration.

Here are some examples of chemical equilibrium:

1. The reaction between nitrogen gas and hydrogen gas to form ammonia gas is an example of chemical equilibrium:

N2(g) + 3H2(g) ⇌ 2NH3(g)

In this reaction, the forward reaction is the formation of ammonia, and the reverse reaction is the decomposition of ammonia back into nitrogen and hydrogen. At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction, so the concentrations of nitrogen, hydrogen, and ammonia remain constant.

2. The dissolution of calcium sulfate in water is another example of chemical equilibrium:

CaSO4(s) ⇌ Ca2+(aq) + SO42-(aq)

In this reaction, the forward reaction is the dissolution of calcium sulfate, and the reverse reaction is the precipitation of calcium sulfate. At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction, so the concentrations of calcium ions and sulfate ions remain constant.

3. The reaction between acetic acid and ethanol to form ethyl acetate and water is yet another example of chemical equilibrium:

CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l)

In this reaction, the forward reaction is the formation of ethyl acetate and water, and the reverse reaction is the decomposition of ethyl acetate and water back into acetic acid and ethanol. At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction, so the concentrations of acetic acid, ethanol, ethyl acetate, and water remain constant.

These are just a few examples of chemical equilibrium, but there are many more reactions that exhibit this behavior. Understanding chemical equilibrium is important in fields such as chemistry, biology, and environmental science.

 

 

Examples

Example 1: The Haber Process

The Haber process is used to produce ammonia from nitrogen and hydrogen gases. The chemical equation for the reaction is:

N2(g) + 3H2(g) ⇌ 2NH3(g)

At a certain temperature and pressure, the equilibrium constant for this reaction is 4.6 x 10^-4. If the initial concentrations of N2, H2, and NH3 are all 0.1 M, what will be the equilibrium concentrations of these species?

Solution:

Let x be the change in concentration of NH3 at equilibrium, which means the concentrations of N2 and H2 will also change by a factor of x.

Using the equilibrium constant expression, we have:

Kc = [NH3]^2 / [N2][H2]^3

Substituting the given values, we get:

4.6 x 10^-4 = (0.1 + x)^2 / (0.1 – x)(0.1 – 3x)^3

Solving for x gives:

x = 0.023 M

Therefore, the equilibrium concentrations of N2, H2, and NH3 are:

[N2] = [H2] = 0.077 M

[NH3] = 0.146 M

 

Example 2: Dissociation of Acetic Acid

Acetic acid (CH3COOH) is a weak acid that dissociates in water to form acetate ions (CH3COO^-) and hydronium ions (H3O+). The chemical equation for the dissociation is:

CH3COOH ⇌ CH3COO^- + H3O+

At a certain temperature and pressure, the equilibrium constant for this reaction is 1.8 x 10^-5. If the initial concentration of acetic acid is 0.1 M, what is the equilibrium concentration of hydronium ions?

Solution:

Let x be the change in concentration of hydronium ions at equilibrium.

Using the equilibrium constant expression, we have:

Kc = [CH3COO^-][H3O+] / [CH3COOH]

Substituting the given values, we get:

1.8 x 10^-5 = x^2 / (0.1 – x)

Solving for x gives:

x = 2.67 x 10^-3 M

Therefore, the equilibrium concentration of hydronium ions is:

[H3O+] = 2.67 x 10^-3 M

 

 

Example 3: Reaction of Nitric Oxide and Oxygen

Nitric oxide (NO) and oxygen (O2) react to form nitrogen dioxide (NO2). The chemical equation for the reaction is:

2NO(g) + O2(g) ⇌ 2NO2(g)

At a certain temperature and pressure, the equilibrium constant for this reaction is 4.6 x 10^-31. If the initial concentrations of NO and O2 are both 0.2 M, what is the equilibrium concentration of NO2?

Solution:

Let x be the change in concentration of NO2 at equilibrium, which means the concentrations of NO and O2 will also change by a factor of x/2.

Using the equilibrium constant expression, we have:

Kc = [NO2]^2 / [NO]^2[O2]

Substituting the given values, we get:

4.6 x 10^-31 = (x/2)^2 / (0.2 – x/2)^2(0.2 – x/2)

Solving for x gives:

x = 3.

 

EQUILIBRIUM

Equilibrium can be defined as the state of a system where there is no observable or detectable change in the properties of the system with respect to time. Examples of system in equilibrium

1. a balanced see – saw
2. a saturated solution of NaCl

 

STATIC AND DYNAMIC EQUILIBRIUM

Static equilibrium is attained when a system in equilibrium is in a state of rest or stationary state.

Example of this is balanced see – saw

 

Dynamic equilibrium can occur during a physical change or a chemical change that is reversible.  Dynamic equilibrium involving a physical change is referred to as physical equilibrium while that involving a chemical change is a chemical equilibrium.

 

A system is said to be in a state of dynamic equilibrium when both the forward and backward reactions take place at the same rate. Example

    N2O4(g)        2NO2(g)

 

EQUILIBRIUM IN REVERSIBLE REACTION

A reversible reaction is one which proceeds in both forward and backward directions under suitable conditions.

 

For example: NH4Cl(s)        NH3(g)  +  HCl(g)

A reversible reaction is said to be in dynamic equilibrium when both the forward and backward reaction are occurring at the same rate, thereby producing no net change in concentrations of the reactants or products.

 

Properties of system in equilibrium

1. A chemical reaction at equilibrium is in dynamic state with the rates of the forward and backward reactions are equal.
2. The equilibrium position can be reached from reactants alone or form product alone
3. For equilibrium to be established, the system must be closed

 

EVALUATION 

1. When is a reversible reaction said to be in dynamic equilibrium
2. List three properties of a system in a state of equilibrium

QUESTIONS

1. What is chemical equilibrium? a. A state where the rates of the forward and reverse reactions are equal b. A state where the reactants and products are completely consumed c. A state where the reaction has stopped

Answer: a

2. Which of the following is true about chemical equilibrium? a. The concentrations of the reactants and products remain constant b. The concentrations of the reactants and products change over time c. The concentrations of the reactants and products eventually become zero

Answer: a

3. What is the equilibrium constant? a. A constant that depends on the initial concentrations of the reactants and products b. A constant that depends on the temperature and pressure of the system c. A constant that depends on both the initial concentrations and the temperature and pressure of the system

Answer: b

4. If the equilibrium constant (Kc) for a reaction is greater than 1, which of the following is true? a. The forward reaction is favored b. The reverse reaction is favored c. The reaction does not reach equilibrium

Answer: a

5. Which of the following is true about Le Chatelier’s principle? a. If a system is disturbed, it will shift to minimize the disturbance b. If a system is disturbed, it will shift in the opposite direction to the disturbance c. If a system is disturbed, it will not shift at all

Answer: a

 

6. If the concentration of one of the reactants in a reaction at equilibrium is increased, which of the following is true? a. The equilibrium will shift to the left b. The equilibrium will shift to the right c. The equilibrium will not shift

Answer: a

7. Which of the following factors does not affect the equilibrium constant of a reaction? a. Temperature b. Pressure c. Initial concentrations of the reactants and products

Answer: c

8. Which of the following is not a type of chemical equilibrium? a. Homogeneous equilibrium b. Heterogeneous equilibrium c. Electrochemical equilibrium

Answer: c

9. Which of the following is an example of a heterogeneous equilibrium? a. The dissolution of NaCl in water b. The Haber process c. The reaction between HCl and NaOH

Answer: a

10. Which of the following is not a way to increase the yield of a product in a chemical reaction at equilibrium? a. Increasing the concentration of the reactants b. Removing the product as it is formed c. Increasing the temperature of the system

Answer: c

 

Summary:

Equilibrium is a state where there is no observable or detectable change in the properties of the system with respect to time. Equilibrium can be static or dynamic. Static equilibrium is attained when a system in equilibrium is in a state of rest or stationary state. Dynamic equilibrium can occur during a physical change or a chemical change that is reversible. A system is said to be in a state of dynamic equilibrium when both the forward and backward reactions take place at the same rate.

Review Questions:

1. What is equilibrium?
2. Give an example of a system in equilibrium.
3. What is static equilibrium?
4. Give an example of a system in static equilibrium.
5. What is dynamic equilibrium?
6. When does dynamic equilibrium occur?
7. What is physical equilibrium?
8. What is chemical equilibrium?
9. How can a system be in a state of dynamic equilibrium?
10. What happens in a system that is in a state of dynamic equilibrium

 

FACTORS AFFECTING THE POSITION OF EQUILIBRIUM

The position of equilibrium depends on the following set of condition

1. Temperature 
2. Concentration
3. Pressure (for gases)

Variation in any of these factors will upset the balance of the system and results in a shift in the position of the equilibrium.  These factors and their effect on chemical systems in equilibrium were studied by Le Chatelier’s who formed the Le Chatelier’s principle.

 

Le Chatelier’s principle states that if a chemical system is in equilibrium and one of the factors involved in the equilibrium is altered, the equilibrium position will shift so as to neutralize the effect of the change.

Importance of Le Chatelier’s principle in chemical industry:  It helps to

1. Define optimum conditions for chemical process
2. Reduce undesirable reversibility 
3. Predict the effect of an altered factors on the equilibrium

 

EFFECT OF TEMPERATURE 

For an endothermic reaction, increase in temperature shifts the position of equilibrium to the right. That is, the forward reaction is favoured while a decrease in temperature favours the backward reaction.  The reverse is the case for exothermic reactions.

 

EFFECT OF CONCENTRATION

In a system of equilibrium, if the concentration of reactants is increased or product decreased, the forward reaction will be favoured, that is; equilibrium position shifts to the right.  Also, if the concentration of the products is increased or reactant decreased, the backward reaction will be favoured, equilibrium position shifts to the left.

 

EFFECT OF PRESSURE

Pressure of gas is proportional to the number of moles.  Increase in number of moles implies increase in pressure and vice visa.  If the forward reaction proceeds by decreases in number of moles (pressure), an imposed increase pressure will favour forward reaction.  Reverse is the case if it proceeds by increase in number of moles (pressure). In other words, when high pressure is applied, equilibrium will shift to the side with less number of moles while when low pressure is applied, equilibrium will shift to the side with greater number of moles.

N2(g)        +    3H2(g)            2NH3(g)

1 mole            3 moles            2 moles

Increase in pressure favours forward reaction while decrease in pressure favours backward reaction

 

EFFECT OF CATALYST

A catalyst does not change (affect) the position of equilibrium.  Addition of catalyst, however increase the rate of both forward and backward reaction by lowering the activation of energy of the reaction.

Equilibrium position is attained at faster rate when a catalyst is present.

 

EVALUATION

1. State Le Chatelier’s principle
2. Explain the effect of
3. Increase in temperature
4. decrease in pressure

on the following reaction

2SO2(g)    +    O2(g)        2SO3(g)    H = -3957KJmol-1   

 

HABER PROCESS

Haber process is an industrial process for the manufacture of ammonia.

Ammonia, NH3, is an important chemical for the manufacture of fertilizers. During the process, the following equilibrium is established

N2(g)    +    3H2(g)        2NH3(g)        H =    -96KJmol-1

1 mole          3 moles         2 moles

 

CHOICE OF TEMPERATURE

Since forward reaction is exothermic, Le Chatelier’s principle shows that the greatest yield of ammonia, NH3, would be obtained at low temperature.  However, if the temperature is too low, time taken to reach equilibrium would be much.  A temperature of 5000C gives reasonable yield of ammonia.

 

CHOICE OF PRESSURE

Since forward reaction leads to decrease in number of moles (pressure), Le Chatelier’s principle shows that high yield of ammonia would be obtained at high pressure.  The pressure of 25atm gives a satisfactory yield.

 

CHOICE OF CATALYST

Iron is used as catalyst in Haber process.  Iron is found to be effective in speeding up the rate of which equilibrium is reached.  It is used in form of small pellets which increased the surface area available for contact.

 

EQUILIBRIUM CONSTANT (K) AND LAW OF MASS ACTION

The law of mass action states that at constant temperature, the rate of reaction is proportional to the active masses of each of the reactants;

The active mass of the reactants is the concentration of the substance raised to the power of coefficient (number of mole) of that substance in the equation of reaction under study.

NOTE: Active mass = molar concentration x activity coefficient

In most reactions, the active masses of the reactant may be taken as their molar concentrations.

Example:

aA     +    bB        products

Thus,    r α [A]a    and    r α [B]b

where [   ]  represents concentration in moldm-3

    r    α     [A]a x [B]b

    r    =    k [A]a   [B]b

where k is called equilibrium constants 

Applying the law to the reversible reaction represented by the equation below:

aA(aq) + bB(aq)            cC(aq)    + dD(aq)

    Kc=    [C]c [D]d

    [A]a  [B]b

Where Kc is called equilibrium constant

For reactions involving gases, the equilibrium constant is expressed in terms of the partial pressures of the gases involved in the reaction.

Therefore, for the general gaseous reaction:

aA(g)  +  bB(g)            cC(g)  +  dD(g)

    Kp = PCc x PDd

          PAa x PBb

Where Kp is equilibrium constant for the gaseous reaction, PA, PB, PC and PD are partial pressures of gases A, B, C and D respectively.

 

NOTE: For a given reaction, concentrations of solids are ignored. A Kc or Kp value greater than one tells you that product formation is favoured at equilibrium; while a Kc or Kp value less than one tells you that reactants formation is favoured at equilibrium. A system in equilibrium has Kc or Kp value equal to one.

 

GENERAL EVALUATION/REVISION

1. State the conditions for the optimum yield of ammonia in Haber process.
2. State Le Chatelier’s principle
3. Define the following terms and state how they vary across a period and down a group: Ionization energy, Electronegativity, Electron affinity
4. Define oxidation, reduction, oxidizing agent and reducing agent in terms of electron transfer
5. An organic compound contains the following by mass: 0.188g of C, 0.062g of H and 0.25g of O. If the vapour density of the compound is 16, determine its molecular formula.

 

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by O.Y. Ababio (6th edition) Pages 265-275

 

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY.

1. When the concentration of both reactants and product are equal
2. K = 1       b. K > 1     c.  K ≥1 d. K = 0
3. In the reaction

2SO2(g)    +    O2(g)            2SO3(g)        H = -xkJ/mole

    increase in temperature favours the

1. backward reaction      b. forward reaction     c. yield of SO3
2. How is the equilibrium constant for the forward reaction (Kf) related to that of the reverse reaction (Kr)? a. Kr is the additive inverse of Kf b. Kr is the multiplicative inverse of Kf c. Kr is same as Kf d. The product of Kr and Kf is zero
3. The equilibrium constant expression involves a. solid, liquid and gaseous species b. gaseous species only c. species in aqueous solution only d. species in aqueous solution and gases only
4. Increase in pressure favours the forward reaction for an equilibrium reaction that proceeds with a. increase in pressure b. decrease in number of moles c. no change in pressure d. increase in number of moles 

 

SECTION B

1. State the Le Chatelier’s principle
2. Calculate (a) the equilibrium constant (Kp) for the reaction below at 45oC

    (b) the equilibrium constant for the backward reaction below:

        H2(g)  +  I2(g)  →  2HI(g)

Given that the partial pressure for H2, I2 and HI are 0.065atm, 0.45atm and 0.245atm respectively. From your answer, deduce which of the reaction is favoured.

EQUILIBRIUM IN REVERSIBLE REACTION

Equilibrium in a reversible reaction occurs when the forward and backward reactions occur at the same rate, resulting in a constant concentration of reactants and products over time. Reversible reactions are those that can proceed in both the forward and reverse directions. The rate of a reversible reaction can be influenced by various factors, including temperature, pressure, and the concentration of reactants and products.

 

The equilibrium constant (Kc) is a quantitative measure of the position of the equilibrium for a reversible reaction. The equilibrium constant is defined as the ratio of the concentrations of products to the concentrations of reactants at equilibrium, with each concentration term raised to the power of its stoichiometric coefficient. If the value of Kc is greater than 1, the equilibrium lies towards the products, while if the value of Kc is less than 1, the equilibrium lies towards the reactants. If the value of Kc is equal to 1, then the reactants and products are present in equal concentrations.

 

Le Chatelier’s principle is a useful tool for predicting how a system at equilibrium will respond to changes in various factors such as temperature, pressure, and concentration. Le Chatelier’s principle states that if a system at equilibrium is subjected to a change, the system will shift in a direction that minimizes the change. For example, if the concentration of a reactant is increased, the equilibrium will shift to the right to form more products in order to restore equilibrium.

 

Reversible reactions and equilibrium are important concepts in many fields, including chemistry, biology, and environmental science. They play a vital role in understanding chemical reactions and processes, and their applications range from the production of fertilizers to the buffering of blood pH in the human body

 

Evaluation

1. What is a reversible reaction? a. A reaction that proceeds only in the forward direction b. A reaction that proceeds only in the reverse direction c. A reaction that can proceed in both the forward and reverse directions

Answer: c

2. What is equilibrium in a reversible reaction? a. A state where the concentrations of reactants and products are equal b. A state where the rate of the forward reaction is equal to the rate of the reverse reaction c. A state where the reaction has stopped

Answer: b

3. What is the equilibrium constant (Kc) for a reversible reaction? a. A measure of the rate of the reaction b. A measure of the position of the equilibrium c. A measure of the temperature of the system

Answer: b

4. If the value of Kc for a reversible reaction is greater than 1, which of the following is true? a. The equilibrium lies towards the products b. The equilibrium lies towards the reactants c. The equilibrium is in the middle

Answer: a

5. What is Le Chatelier’s principle? a. A principle that states that a system at equilibrium will shift in a direction that minimizes a change b. A principle that states that a system at equilibrium will shift in a direction that maximizes a change c. A principle that states that a system at equilibrium will not shift at all

Answer: a

6. If the concentration of a reactant in a reversible reaction is increased, which of the following is true? a. The equilibrium will shift towards the products b. The equilibrium will shift towards the reactants c. The equilibrium will not shift

Answer: a

7. What is the effect of increasing the temperature on a reversible reaction? a. The equilibrium will shift towards the products b. The equilibrium will shift towards the reactants c. The equilibrium will not shift

Answer: a

8. What is the effect of increasing the pressure on a reversible reaction involving gases? a. The equilibrium will shift towards the side with more moles of gas b. The equilibrium will shift towards the side with fewer moles of gas c. The equilibrium will not shift

Answer: a

9. What is the effect of adding a catalyst to a reversible reaction? a. The equilibrium will shift towards the products b. The equilibrium will shift towards the reactants c. The equilibrium will not shift

Answer: c

10. Which of the following is an application of equilibrium in reversible reactions? a. Production of fertilizers b. Buffering of blood pH in the human body c. Both a and b

Answer: c

 

1. What are the three conditions that affect the position of equilibrium? a. Temperature, pressure, and concentration b. Temperature, light, and pressure c. Temperature, volume, and concentration
2. What is Le Chatelier’s principle? a. A principle that states that the position of equilibrium will shift to neutralize the effect of a change in one of the conditions affecting equilibrium b. A principle that states that the concentration of products is always higher than the concentration of reactants at equilibrium c. A principle that states that the position of equilibrium is fixed and cannot be changed
3. What happens to the position of equilibrium if the concentration of reactants is increased in a reversible reaction? a. The equilibrium position shifts to the right b. The equilibrium position shifts to the left c. The equilibrium position does not change
4. What happens to the position of equilibrium if the pressure is increased in a reversible reaction involving gases? a. The equilibrium position shifts to the side with more moles of gas b. The equilibrium position shifts to the side with less moles of gas c. The equilibrium position does not change
5. What is the effect of adding a catalyst to a reversible reaction? a. The equilibrium position shifts to the right b. The equilibrium position shifts to the left c. The equilibrium position does not change
6. How does an increase in temperature affect the position of equilibrium in an endothermic reaction? a. The equilibrium position shifts to the left b. The equilibrium position shifts to the right c. The equilibrium position does not change
7. How does an increase in temperature affect the position of equilibrium in an exothermic reaction? a. The equilibrium position shifts to the left b. The equilibrium position shifts to the right c. The equilibrium position does not change
8. What is the equilibrium constant (Kc) for a reversible reaction? a. A measure of the rate of the reaction b. A measure of the position of the equilibrium c. A measure of the temperature of the system
9. What is the law of mass action? a. A law that states that at constant temperature, the rate of a reaction is proportional to the active masses of the reactants b. A law that states that the rate of a reaction is proportional to the concentration of the products c. A law that states that the rate of a reaction is proportional to the concentration of the reactants
10. How is the equilibrium constant (Kp) expressed for a reversible reaction involving gases? a. In terms of the concentrations of the reactants and products b. In terms of the partial pressures of the reactants and products c. In terms of the molar masses of the reactants and products

Simplified Questions

1. What are the three states of matter?
2. What is the state of matter of a pencil?
3. What is the state of matter of water when it is frozen?
4. What is the state of matter of air?
5. What happens to a liquid when it is heated?
6. What happens to a solid when it is heated?
7. What happens to a liquid when it is cooled?
8. What happens to a gas when it is cooled?
9. What is evaporation?
10. What is condensation?

 

Lesson Plan Presentation: Equilibrium and States of Matter

Grade Level: SS 2

Time Allotted: 60 minutes

Introduction (10 minutes):

1. Greet the students and introduce the topic of equilibrium and states of matter.
2. Ask the students if they have ever heard of the term “equilibrium” and the three states of matter.
3. Use a PowerPoint presentation to introduce the topic, define equilibrium, and explain the factors affecting the position of equilibrium.

Activity 1: Equilibrium (20 minutes):

1. Divide the class into groups of three.
2. Distribute worksheets to the students that contain questions about Le Chatelier’s principle, the effect of temperature, concentration, and pressure on equilibrium, and examples of reversible reactions.
3. Instruct the students to work on the worksheet in their groups and answer the questions.
4. Circulate among the groups to provide assistance and answer any questions.

Activity 2: States of Matter (20 minutes):

1. Use examples of the three states of matter (solid, liquid, and gas) to explain their properties and differences.
2. Ask the students to identify examples of solids, liquids, and gases in their environment.
3. Provide worksheets to the students that contain questions about the three states of matter.
4. Instruct the students to work on the worksheet independently and answer the questions.
5. Collect the completed worksheets.

Conclusion (10 minutes):

1. Ask the students to summarize what they have learned about equilibrium and the three states of matter.
2. Provide a brief recap of the topics covered in the lesson.
3. Answer any questions that the students may have.
4. Thank the students for their participation and attention.

Assessment:

1. The completed worksheets will be used to assess the students’ understanding of equilibrium and the three states of matter.
2. Students’ participation in group activities and independent work will also be used as assessment criteria

Weekly Assessment /Test 

Equilibrium:

1. What is the definition of equilibrium?
2. What is Le Chatelier’s principle and how is it used in chemistry?
3. What is the effect of an increase in temperature on an endothermic reaction?
4. What is the effect of an increase in pressure on a reaction that produces fewer moles of gas?
5. What is the effect of an increase in concentration of reactants on a reaction in equilibrium?
6. What is the difference between a physical equilibrium and a chemical equilibrium?
7. What is the equilibrium constant and how is it calculated?
8. How does a catalyst affect a chemical reaction at equilibrium?
9. Give an example of a reversible reaction and explain how it reaches equilibrium.
10. How does Le Chatelier’s principle help in predicting the effect of an altered factor on a system in equilibrium?

States of Matter:

1. What are the three states of matter?
2. What is the definition of a solid?
3. What is the definition of a liquid?
4. What is the definition of a gas?
5. What is the difference between a solid and a liquid?
6. What is the difference between a liquid and a gas?
7. What is the difference between a solid and a gas?
8. What is melting?
9. What is boiling?
10. What is evaporation?