Welcome to our Free HESI A2 study guide series, HESI A2 Chemistry Study Guide is the number seven module in our HESI A2 Study guide series.

There are 25 questions in the chemistry section on the HESI A2 exam. 

Depending on your requirements when applying, this section might not appear in your exam. 

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Scientific reasoning and the metric system

In this part of  HESI A2 chemistry study guide we will discuss the scientific reasoning and the metric system

Metric system

The metric system uses several basic units:

  • Meter – for measuring length
  • Liter – for measuring volume
  • Gram – for measuring mass

These will use a base unit which can then be increased or decreased in units of 10. 

To indicate an amount, you simply combine the prefix and the base unit. 

SI units of measurement

Seconds (s) are used by SI to measure time. 

To measure fractions of seconds, nanoseconds and milliseconds are used.

A millisecond is 1/1,000 of a second, while a nanosecond is 1/1,000,000,000 of a second.

When we measure larger increments of time, minutes and then hours come into play. 

A second is:

  • 1/60 of a minute
  • 1/3,600 of an hour
  • 1/86,400 of a day

There are other SI base units as well:

  • Ampere (A) – which measures electrical current
  • Kelvin (K) – which measures thermodynamic temperature
  • Candela (cd) – which measures luminous intensity
  • Mole (mol) – Which measures the amount of substance at a molecular level
  • Meter – Which is used to measure length
  • Kilogram – Which is used to measure mass

Multiple and subdivision metric prefixes

Multiple prefixes are as follows:

  • Deka (da) – 101
  • Hecto (h) – 102
  • Kilo (k) – 103
  • Mega (M) – 106
  • Giga (G) – 109
  • Tera (T) – 1012

Prefixes for subdivisions are as follows:

  • Deci (d) – 10-1
  • Centi (c) – 10-2
  • Milli (m) – 10-3
  • Micro (µ) – 10-6
  • Nano (n) – 10-9
  • Pico (p) –  10-12

Generally, any prefixes that are bigger than 103 will be capitalized if using abbreviations.

Pluralization is assumed, so there is no need to add an “s” to any abbreviations. 

Using logic and evidence to review a scientific explanation

Collecting data

Any experiment should be measurable, especially to ensure its overall validity. 

For every trial, data should be collected and this includes forming data tables. 

To begin, researchers should first work out not only what the critical data need is, but the reasons why it is needed.

Also, understanding in advance how the data will be used once the experiment ends is also of the utmost importance.

Research data needs to fulfill these three critical areas:

  • It needs to be accurate
  • It needs to be repeatable
  • It needs to be reproducible

Also, a reliable and constant method procedure for data collection should be established. 

By carrying out practice tests, a researcher is able to validate the measurement system used while calibration of any equipment used should be carried out and retested from time to time to ensure the continual validation of collected data. 

Scientific process skills

In science, the skill of observation is a critical one. 

Without any bias altering the results, scientists should be able to take accurate dates from observing nature or their experiments. 

Hypothesizing is another critical skill to have and develop. 

So that they can logically work out what should happen in their own tests, scientists should be able to combine their overall knowledge of theory as well the results of experiments they, or others, have carried out previously. 

Skills of ordering and categorizing are critical to the data-analysis process. 

Any data gathered should be easily readable, showing key results and this can be achieved by arranging it in the correct manner. 

Comparing, as a skill, is key to this too. 

This means scientists take their own results and then compare them to others that have been published already. 

From their results, they should be able to draw logical conclusions too. 

This can then be applied, along with theoretical knowledge as a way to determine cases of special behavior and to formulate logical experiment designs. 

Communication is another critical skill, as scientists should be able to get their results and conclusions across to others. 

This leads to others being able to review and test their work which is one of the most critical factors when it comes to making scientific progress.

Scientific statements

Scientific statements include hypotheses, assumptions, scientific models, and scientific laws. 

A hypothesis is an educated guess of what will probably occur. 

When an experiment is being designed, the starting point thereof will include a hypothesis. 

These are not just guesswork, however, and could include knowledge of previous experiments, but also theories or other knowledge. 

Assumptions take the form of statements.

These don’t have any proof but for the purpose of performing a given experiment, are talent as fact. 

In some cases, these can be true, while in others, a certain set of conditions under which the experiment will be carried out will need to be met for the assumption to be true

Assumptions are a necessity, as they are the perfect way to help simplify experiments.

Scientific models describe a physical behavior using mathematical statements. 

A scientist’s knowledge of the actual system determines the quality of these models, however. 

When new discoveries prove a model to be inaccurate, they are discarded. 

Models cannot represent an actual system perfectly, but they can help for a better understanding of the behavior within it by simplifying everything. 

Scientific laws deal with natural behaviors and these statements have not changed over a long period because they produce repeatable results and accuracy in all testing. 

A behavioral statement that takes into account all current observations and consolidates them is a theory. 

When compared to scientific laws, theories have been developed more recently and because of that, can still be proved to be incorrect. 

If they stand to testing and scrutiny over a long period of time, they too can become scientific laws. 

Objects and events


Causes are events or acts that lead to something happening while an effect occurs because of the cause taking place. 

English terms that signal causes include:

  • Due to
  • Because
  • Since

English terms that signal effect include:

  • This leads to
  • Therefore
  • Consequently

A single cause can have multiple effects while a single effect can have several causes. 

There is also the possibility for an effect to become the cause of another effect and when this occurs, a cause and effect chain has taken place. 


Because objects come in various shapes and sizes, they will need to be measured in various ways. 

For this reason, knowing which unit of measurement to use is imperative when it comes to length, width, and even weight. 

Take working with a patient, for example.

You are going to measure their height in meters but the diameter of one of their veins in millimeters. 

Weighing them would be in kilograms, but the weight of their internal organs would use grams.

Scientific inquiry

Scientific method

This is a general method used to test ideas via the use of experiments as a way to either confirm or refute them. 

Starting off the process, one would first need to formulate the problem in question. 

To allow for a focused analysis, the limits of what is to be observed must be clearly defined. 

A hypothesis is then the next step carried out once the problem is defined, while a possible solution to the problem can come from an educated guess. 

Experimentation is the next step to test the hypothesis and for the most part, includes the design of a complete experiment related to the problem. 

When it comes to experimentation, observation is key.

It can be broken down into two types: quantitative and qualitative.

Quantitative uses numeric measurements while qualitative is when observation is based on preference or feeling. 

Patterns or trends that are present are then discovered by looking at the measurement data. 

Generalizations or conclusions regarding the results will then be made from the trends that are observed.

The experiment is over when the original hypothesis is supported by the conclusions drawn. 

This can then be tested through repetition of the experiment many more times.

If not, a new hypothesis is developed and a redesigned or new experiment is drawn up to verify it.

Experimental design

It’s not easy to design meaningful experiments that produce results. 

The design thereof is not a slap-dash affair at all and all stages should be planned very carefully. 

This ensures that the right data can be taken in an accurate manner. 

If at all possible, experiments should always take place in a controlled manner, as this means that conditions, except the manipulated ones, remain constant.

This ensures that shifting conditions don’t affect the results either. 

Consider a placebo group in drug trials as an excellent example of this. 

While the conditions are the same between the two groups, the placebo group won’t receive the medication that the other group receives. 

Data collection should always be at the forefront of any experiment as well. 

Any equipment collecting data should regularly be checked for errors as well. 

All data collected will need to be analyzed as well and finally, presented in a clear manner. 


Any experiment that’s expected to produce valid results must be carefully controlled. 

So all conditions have to be maintained, other than that which is being tested, and should a control group be needed, a set of data is necessary. 

The normal state or condition of the manipulated variable is represented by the control group.

The variables expected to have an effect on the experiment’s outcome are the positive controls.

Negative controls are commonly placebos and will confirm that when it comes to the outcome of the experiment, the variable has no effect. 

More valid conclusions will result when an experiment is better controlled.  


Variables are found in each experiment carried out but only one of them, that’s the one being tested, will be changed on purpose. 

The name given to this variable is the independent or manipulated variable. 

When the manipulated variable is changed, observation of the responding or dependent variable is carried out and any changes are recorded. 

Atomic structure

A conceptual view of atomic structure

Atom pieces

We know that atoms make up all matter, but what do they consist of?

There are two primary parts to an atom: the nucleus and electrons.

The nucleus is further broken into protons and neutrons. 

They have electric charge and mass and their properties are measurable. 

Due to the presence of protons, the nucleus has a positive charge, while the electrons, which orbit the nucleus, are negatively charged. 

In terms of mass, when compared to the electrons that surround it, the nucleus is far heavier. 

Molecules are formed when atoms bond together and those that are electrically neutral have an equal number of electrons and protons.

An atom is an ion when it has either a positive or negative charge because the number of protons and electrons in it are not equal. 

Atom models

We know that atoms are very small.

For example, the head of a pin could be filled with five trillion hydrogen atoms. 

The average distance between the nucleus and the furthest electron from that point is known as the atomic radius.

There are various atom models with the most common showing the proton, nucleus, and electrons.

The electrons are usually close to the nucleus and move around it, much in the same way as the earth rotates around the sun.

Most of these models, however, are not to scale. 

Atomic number

The number of protons that an atom has in its nucleus shows the atomic number of an element which is unique to each.

The atomic number (referred to as “Z”) will be equal to the number of electrons if an atom has a neutral charge. 

Atomic mass 

Also known as mass number (and referred to as “A”) this looks specifically at the nucleus of an atom and the total number of neutrons and protons found therein. 

Simply put, atomic mass (A) = protons present (Z) + neutrons present (N) or A=Z+N.

You might hear the term relative atomic mass, but this refers to atomic weight and shouldn’t be confused with atomic mass.

Instead, the ratio of an average mass per sample to 1/12 of the mass of a carbon-12 atom is the atomic weight. 


While they will vary in their number of neutrons, isotopes are atoms of the same element. 

This means that they not only have the same atomic number but also the same number of protons. 

The symbol of the element is used to denote them with the mass number and atomic number preceding the symbol in superscript and subscript. 

When isotopes don’t decay, they are non-radioactive (stable) with around 80 elements including at least one, and in some cases more. 

Radioactive isotopes can carry out spontaneous nuclear reactions because they have unstable nuclei. 

When this occurs, particles or radiation will be emitted by the element. 

In total, there are 256 isotopes currently known. 


A careeremployer teacher showing how the  the atomic bondings work.


Orbiting the nucleus, electrons are subatomic particles and only make up a fraction of the mass of an atom. 

They can be referred to as shells, layers, or clouds with only a minuscule amount of the atom’s mass accounted for by the orbiting electrons.

Electrons, when compared to the nucleus are a lot smaller in size.

They also have wave-like properties and are negatively charged.

They orbit at varying distances from the nucleus and are also part of the lepton family, occupying the lowest energy level that they are able to.

When all the electrons are in the lowest positions they can be in an atom, they are said to be in a stable electron arrangement.

A valence shell is a name given to the atom’s outermost electron shell, with the electrons within it named valence electrons.

Bonding behavior of these electrons is determined by their number.

Atoms tend to behave in a way that allows them to fill or empty their valence shells.

Chemical bonds

A negative-positive reaction between electron(s) and the nucleus results in a chemical bond between the atom or nuclei of multiple atoms.

The atom remains cohesive because of this attraction but other bonds can also form between molecules as well as other atoms. 

There is a maximum number of electrons that can be contained within the four shell (energy) levels of an atom. 

Before electrons will be added to the valence level, each of these four levels must first be filled. 

Electrons found at the further points of the nucleus have more energy than those that are found closer to it. 

Here’s a breakdown of what these four shells can hold:

  • 1st shell (K shell): Maximum of two electrons
  • 2nd shell (L shell): Maximum of eight electrons
  • 3rd shell (M shell): Maximum of 18 electrons
  • 4th shell (N shell): Maxim of 32 electrons

 Within these shells, subshells can also exist.

It’s in the out valence shell that chemical bonds form and break and this occurs when atoms gain, lose or share an electron. 

When there is the separation of a charge, with one end negative and the other positive a polar bond occurs and this is a covalent type of bond. 

An example of a polar bond is seen in water between hydrogen and oxygen.


In the nucleus, because the positive charge of the protons is balanced by the surrounding electrons and their negative charge, most atoms are neutral. 

As soon as they come into contact with each other, electrons are transferred between atoms. 

This results in either a positive or negative charge because the molecule or atom created doesn’t have an equal number of protons. 

If electrons are gained by an atom, a negative ion is created and if electrons are lost, a positive ion is created.

When ions with opposite charges come together, an ionic bond is the result and the compound that results from that will be neutral. 

When neutral particles are ionized into charged particles, ionization has taken place. 

When atoms share electrons, the bond is said to be covalent with those shared equally forming a polar bond and those unequally, a nonpolar bond.

Hydrogen bonding occurs when there is interaction in the same area with a hydrogen bond and a molecule. 

The same molecule can see hydrogen bonding between two different parts.


Matter in its most basic form is an element.

Containing unique properties, elements cannot be broken down any further. 

Of all the elements, the atom is the smallest and we’ve covered much about them earlier in the course. 

Should two or more elements form in a chemical combination, they become a compound. 

When compared to the constituent elements that they come from, compounds have very different properties. 

In an element or compound, elements are the smallest independent found within them. 

While it’s in a single-atom form that you will find most elements in nature, there are some – known as diatomic elements – that exist in pairs, including oxygen, nitrogen, and hydrogen 

Chemical symbols are used as a way to represent all the compounds and elements that we know of and this is done via one or two letters from the alphabet.

The first letter used is usually the name of the element or compound but this isn’t always the case. 

If more than one element of the same compound is found in the element or compound, a subscript number is used to represent it. 

The Periodic Table

The Periodic Table of Chemical elements

Organized according to periodic law, the periodic table is an arrangement of elements using a tabular format. 

The properties of these elements are based on two things:

  • Their atomic number
  • Their atomic structure

The table helps to identify those elements with similar properties while it also shows periodic trends of chemical and physical properties. 

Horizontal rows, called periods and vertical rows, known as families or groups are used to arrange elements by atomic number within the table. 

They are then further broken down into three more categories for nonmetals (to the right of the table), metals (to the left of the table) and metalloids (in the middle of the table). 

Overall, most of the elements are metal with only eight metalloids and seventeen nonmetals present.

Other information contained for each element includes the symbol that represents it, its atomic number,  and how many protons it has in its nucleus. 

Chemical reactivity

The tendency of substances to partake in chemical reactions is known as reactivity. 

Substances with a high reactivity will take part in a chemical reaction much more readily than those with a lower reactivity. 

There must be uncommitted electrons available for a reaction to take place, however, because of the fact that a transfer of electrons takes place. 

Based on its position in the table, periodicity allows for the prediction of the reactivity of an element. 

Those elements found in high-numbered groups on the right-hand side of the table are less likely to react because, in their outer levels, they have a fuller electron complement. 

Substance properties

A book with glasses to read substance properties of chemicals

Extensive and intensive properties

There are two types of physical properties: extensive and intensive.

The quantity of the sample, or the amount of matter, is not a factor when it comes to intensive properties.

So there is no change in these properties should the sample size increase or decrease. 

Examples of intensive properties include:

  • Magnetization
  • Concentration
  • Temperature
  • Specific heat
  • Malleability
  • Ductility
  • Density
  • Boiling point
  • Melting point
  • Hardness
  • Color

The quantity of the sample or the amount of matter found in it does have an effect on extensive properties. 

So if the sample size increases or decreases, these properties will change.

Examples of extensive properties include:

  • Electric charge
  • Number of moles
  • Entropy
  • Energy
  • Mass 
  • Weight
  • Volume

Matter’s physical properties

If it can be observed or measured, then any property of matter is a physical one. 

Examples of this would include, temperature and mass, for example.

Mass looks at an object and measures the amount of substance within.

Weight is the force of gravity acting on the object.

The amount of space the object occupies is measured by volume while density is the amount of mass per volume. 

Specific gravity compares a substance’s density against that of water.

Matter’s chemical properties

In order to measure and observe a property, a chemical change must be carried out. 

A perfect example of this is the formation of water when hydrogen gas is burned in oxygen. 

Because water, a different chemical substance is formed after burning, this is seen as a chemical property of hydrogen. 

Any physical changes to the water, for example, freezing it, or boiling it, won’t result in the hydrogen returning. 

Properties of water

Water has several important properties which include:

  • High polarity
  • Hydrogen bonding
  • Adhesiveness
  • Cohesiveness
  • High specific and latent heat
  • Vaporization at high heat

All living things are made up of water which at room temperature is found in liquid form. 

Hydrogen bonds in water are not that easily broken, and that’s due to its high specific heat.

This means that it has both a high boiling and vaporization point and remains resistant to both motion and heat. 

Water is different from most substances which in their solid forms are denser.

In other words, in its solid state, it floats in its liquid state (think of ice floating in a glass of water). 

Because it is drawn towards itself, water is said to be cohesive but because it draws other molecules in, it is also adhesive.

Water is an excellent solvent due to its adhesive and cohesive qualities and substances will dissolve easily in it if they have molecules and polar ions. 

A substance that attaches to water is referred to as hydrophilic. 

Hydrogen bonds

Hydrogen bonds are a type of chemical bond that forms between a hydrogen atom and a highly electronegative atom, such as fluorine, nitrogen, or oxygen.

It is within a single molecule or between molecules that these bonds are formed. 

In water, which is considered polar, these bonds form between the hydrogen atoms of one water molecule (which are positively charged) and the oxygen atoms of another water molecule (which are negatively charged).

When the number of electrons are reduced during the bonding process with oxygen, hydrogen becomes oxidized.

These bonds, which there are many of, give water many of its unique properties, such as its high surface tension, high boiling, and melting points, and its ability to act as a solvent for many substances. 

The bonds, however, don’t last for very long. 

Furthermore, water forms droplets instead of spreading out into a thin film, for example, and this is due to its hydrogen bonding.

Hydrogen bonds don’t only occur in water, but also in our DNA.

Mechanisms of passive transport: Osmosis and diffusion

The diffusion process occurs when molecules move from a high concentration area to a low concentration area until their concentrations become equal.

This process occurs without the use of energy and it is a spontaneous process.

In facilitated diffusion, a specific carrier protein is responsible for the transportation of specific molecules. 

These carrier proteins differ with regards to their shape, overall size, as well as the charge they carry.

An example of a carrier protein would be glucose. 

Water molecules diffuse from a high water concentration to a low water concentration through a selectively permeable membrane in a process known as osmosis. 

In contrast to active transport mechanisms, passive transport mechanisms require no energy from the cell but instead rely on kinetic energy. 

The states matter can take on

A Careeremployer chemist showing the states matter

There are three main states that matter can take: solid, liquid, and gas.

Solid: Having a defined volume and shape, particles in solids are closely packed together without much space to move and they vibrate around a position that is fixed. This results in the formation of strong bonds. 

Liquid: While they have a defined volume, liquids do not take on a shape (unless put in a container). Weak bonds form in liquids because their particles are spread out more when compared to solids. While these bonds are weaker, they still do not break easily. 

Gas: There is no defined shape or volume when it comes to a gas and much like liquids, will take on the shape of any container they are placed in. Particles within gas are spaced very far apart and can move easily but because of the distance between them, no bonds are formed. 

Matter can also exist in a fourth state, known as plasma, which is characterized by the presence of free electrons and ions, but this won’t be asked in the exam. 

It’s possible to move through the various states of matter by either adding or removing heat. 

For example, a solid can be heated to its specific melting point, and then, it becomes a liquid. 

In theory, that’s true, but once it reaches its melting point, extra heat is necessary.

This is because to become a liquid, it must move past the latent heat of fusion. 

In turn, liquids can be heated and once they reach their boiling point, they will become a gas but only if the latent heat of vaporization is overcome when more heat is added to the boiling point. 

Chemical reactions

A Careeremployer chemist performing experiments to note  chemical reactions.

Chemical reactions don’t all happen at the same rate.

Some can occur quickly, while others take place over time, in some cases, even billions of years. 

Depending on how often there is an interaction between atoms and molecules will determine the rate of chemical reactions.

Other factors also play a role including the properties of the reacting materials, their shape for example, as well as temperature. 

They can also be influenced by inhibitors (decrease reaction rates) or catalysts (increase reaction rates). 

In some reactions, heat and light are released as energy and in others between atoms, molecules, as well as ions, there is a transfer of either hydrogen ions or electrons. 

In some cases, there can be breakage in chemical bonds due to light or heat.

This will result in reactive radicals which can go on to make new bonds. 

Chemical reactions: Reading and balancing

When looking at a chemical equation, you will see how they indicate what chemical reaction is taking place. 

Before the arrow, you will find all the reactants on the left-hand side of the equation. 

On the right-hand side, you will find the products of that reaction. 

The arrow indicates the reaction or change itself, while the number before the coefficient is the element that shows, in moles, the ratio of reactants to products.  

Let’s look at this a little more in depth by considering hydrogen and oxygen and the equation for making water. 

  • That equation is: 2H₂(g) + O₂(g)  → 2H₂O(l)     

The number 2 found before both the hydrogen symbol and the water symbol is the coefficient and it indicates that two moles are found in hydrogen and water.

What about oxygen? 

Well, although it doesn’t show a number 1 before it, oxygen does have one mole. 

Notice the (l) after the water symbol?

Well, that indicates it is a liquid. 

For reactions that produce a gas, a (g) would be used, and for those that make a solid, an (s), while (aq) is for aqueous, or a substance that water can dissolve. 

An unbalanced equation, or one in which the coefficients on each side of the arrow are not equal, will not adhere to the law of conservation of mass. 

This law says that matter is not created, but only changed. 

In unbalanced equations, on each side of the arrow, the coefficients are not equal. 

Chemical equations can be balanced by writing out the formulas for compounds/elements found in the reaction. 

Next, find out if there are an equal number of atoms on each side of the equation by counting them. 

Note that it’s not possible for there to be fractional amounts.

To balance the equation, you need to make the smallest possible whole number coefficient and this can be achieved by multiplying the coefficient you have. 

Here’s an example of an unbalanced equation:

  • H₂ + O₂ → 2H₂O

The balanced equation would be:

  • 2H₂ + O₂ → 2H₂O

It’s balanced as it shows that to make two moles of water requires two moles of hydrogen and one mole of oxygen. 

Law of Conservation of Mass

Earlier, we mentioned the Law of Conservation of Mass.

Let’s expand on that a little.

The main idea to remember here is that the law states that matter is neither created nor destroyed when a chemical reaction takes place. 

So the total mass of material before a reaction takes place will be exactly the same after it has ended. 

Because of this, balanced equations can be used to predict how molecules will combine since either side of the equation has the same number of atoms.

Mechanisms within reactions

A chemist showing how the Mechanisms of reactions work.

When a chemical reaction happens, electrons are between one atom (or molecule) and another. 

The octet rule governs reactions and reactivity and indicates that atoms lose or gain electrons until their outer energy levels contain eight electrons.

These reaction changes could be in their configuration or composition and because of this, one (or more) products that were not present before the reaction occurred, could result. 

Reactions need a reactant, which is the substance that will be changed during it, a reagent, or a partner to the reaction (which won’t be transformed as much),  as well as the reaction’s final result which is the product. 

Other mechanisms related to reactions are reaction conditions (environmental factors) which include concentration, pressure, and temperature, for example. 

Five reaction types

Let’s look at the five different types of reactions that you will need to know. 

Combination reactions

A combination reaction, also known as a synthesis reaction, is when two or more reactants combine resulting in a single product. 

The reactants in this type of reaction are normally in their elemental form, and the products are typically compounds. 

The overall chemical equation for this type of reaction is normally shown as A + B → AB.

AB is the product formed by reactants A and B. 

Decomposition reaction

A decomposition reaction is one where a single reactant breaks down into two or more simpler products. 

In this reaction type, a compound is usually the reactant, while the products are usually elements or simpler compounds. 

Decomposition reaction equations are usually shown as AB → A + B, where AB is the reactant and A and B are the products.

Decomposition reactions are either endothermic or exothermic.

Depending on how much energy is required to break the bonds between atoms in the reactant compound, this amount of energy will vary.

For the most part, these reactions are endothermic, so for the chemical reaction to occur, heat will need to be added which then creates thermal decomposition. 

Electricity will cause electrolytic decomposition with the decomposition of water into both oxygen and hydrogen an example of this. 

When there is a reorganizing of substances and their chemical nature doesn’t change, the separations can then be mechanical or chemical. 

In terms of chemical or physical properties, the types of products that come from this can differ from the original mixture. 

In terms of separation processes, these can include filtration, crystallization, distillation, and chromatography. 

Single replacement reactions

Single replacement reactions are also referred to as single displacement or single substitution reactions.

In this type of reaction, displacement of one reactant by another occurs in a compound. 

The equation for this is A + BC → AC + B with A and B the elements and C the compound.

In most cases, the element replaced (B) is a metal, while the element that replaces it (A) is a nonmetal.

These types of reactions can either be anionic or cationic. 

Double replacement reactions

Double replacement reactions are also referred to as double displacement, substitution, metathesis, or ion exchange reactions.  

Here, two different compounds will exchange ions or bonds with a different compound formed as a result of this. 

The equation for this is  AC + BD → AD + BC and when this type of reaction takes place, there is an exchange of ions between the reactants. 

Combustion reactions

Combustion reactions involve the combination of a fuel with oxygen (or another oxidant).

As a result of this reaction, heat will most certainly be a product, but in some cases, light as well as certain other products may result.  

When heat is produced, the reaction is said to be exothermic because energy is released. 

It would also be exothermic, however, if light resulted from the reaction.

These reactions can take on many forms, including slow, rapid, incomplete, turbulent, and others.

The compounds formed through these reactions are determined by both the fuel and oxidants used. 

As an example, water vapor is a result of the combustion of hydrogen and oxygen in rocket fuel, while if wood and air burn, carbon compounds, unburned carbon, and nitrogen result. 


While they won’t change their form during a reaction, a catalyst can influence one by changing the rate at which it takes place. 

It does this by taking away from the number of steps needed to reach the final product. 

The catalyst won’t change its overall weight mass during the reaction either. 

The minimum amount of energy necessary to start the reaction is known as activation energy. 

This helps to produce enough energy when particles collide so the reaction can begin. 

More particles will react when a catalyst is added and as a result, there is a drop in the activation energy, so the reaction can start far easier. 

Baths, salts, and acids


pH or the potential of hydrogen measures the concentration of hydrogen ions found in a substance.

This measurement is the number of moles of H+ per liter of whatever solution is being measured. 

The pH scale runs from 0 to 14 and all substances will have a reading that falls somewhere on it. 

A higher H+ concentration shows that a substance has a lower pH while a lower H+ concentration signifies a higher pH. 

If we look at pure water, it’s considered to have a neutral pH and therefore will fall at 7 on the pH scale. 

If a substance has a rating below this, it will be seen as acidic while any rating above 7 is considered a base. 

Vinegar, for example, is an acid while soap is a base. 

Detecting pH levels is easily done by using a pH indicator that detects hydrogen or hydronium ions. 

These indicators are halochromic, which means they will show a certain color depending on the amount of hydrogen or hydronium ions detected, and from that, one can determine the pH level of a substance. 


We’ve touched on bases above, but let’s look at them in more detail. 

Normally, these are found in an aqueous solution but they are easily identifiable due to having traits including;

  • They are bitter when tasted
  • They are soapy/slippery when touched
  • They make litmus paper turn blue, even if it was previously red from having touched an acid
  • In reactions with acids, they will produce salts

The word most often used to describe bases is alkaline. 

When dissolved, bases produce hydroxide ions (OH-) which is unlike acids which produce hydrogen ions (H=) when dissolved. 

In some cases, certain nonmetal oxides, even though, when in molecular form, they don’t have any hydroxides, are described as bases. 

This is due to the fact that when they react with water, they will produce hydroxide ions. 


Because of their consistent properties which characterize them, acids are a unique class of compound. 

One of these properties, which gives acids their unique characteristics isn’t readily observable and this is their propensity to take on an electrical charge after disassociating from their parent molecules. 

Properties of acids that are easier to observe include the following:

  • They taste sour
  • Litmus paper will turn to red when placed in an acid
  • In reactions with certain metals, they produce gaseous H2
  • In reaction with bases, they will produce salt precipitates

There are less observable properties too. 

Included in this is the fact that inorganic acids have high boiling points and are readily dissolved in water. 

Weak or strong acids or bases

It is the tendency of atoms to donate or accept charged particles that give acids and bases their characteristic properties.

The degree to which its atoms ionize when placed in a solution gives an indication of the strength of a base or acid. 

So an acid is considered to be strong should all of its atoms ionize while it’s weak when only a few do so. 

Considering an acid’s reactivity is another method to think of the strength of a base or acid. 

Due to the fact that they will form and break bonds quickly, with almost all of their atoms ionizing, highly reactive acids or bases are considered to be strong. 


Properties of salt include:

  • Their formation from acid/base reactions
  • They are ionic compounds
  • They have both nonmetallic and metallic ions
  • They dissociate in water
  • They are made up of tightly bonded ions

In a hydrolysis reaction, water and salt will form a base and an acid should they react. 

Common salts include sodium chloride, sodium bisulfate, potassium dichromate, and calcium chloride. 

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