Experimental Chemistry | Experimental Design | Measurement | Related Topics

Lesson Objectives

• Experimental Design
  • Measuring mass
  • Measuring Volume
  • Measuring Time
  • Measuring Temperature
  • Properties of Gases
  • Measuring Rates
• Methods of Purification and Analysis
  • Terminology
  • Crystallization
  • Decanting and Centrifuging
  • Immiscible Liquids
  • Sublimation
  • Solar Evaporation
  • Distillation
  • Chromatography

When we begin studying chemistry, we initiate with the very basic concepts and measurements that form the base of all the experiments that we carry out in the latter part of the course. Hence, in the first section of the syllabus, we start by learning basic terminology and experimental methods to clear our concepts at the initial level. We will start off by discussing several different types of measurements, along with their Units and the instruments that are used to measure them.

1.1 Measuring Mass:

Mass:

• How much matter a substance has.
• It is important to note at this stage, that mass is NOT Mass is a physical property of a substance which remains unaffected under any condition such as gravitational pull whereas weight changes in accordance to the gravitational pull. (this will further be explored in later chapters).

Instruments:

• Beam Balance (this is the normal balance that is used traditionally to measure mass where we calibrate the mass of a substance in comparison to another mass)
• Top-pan/ digital/ electronic balance

Units:

Ton(t), Kilogram(kg), gram(g), milligram(mg) Where 1t = 1000kg 1kg = 1000g 1g = 1000mg We can note here that when we say we are measuring our weight in kg; we are not actually measuring the weight. It is ACTUALLY the mass that we measure!

1.2 Measuring Volume:

Volume:

• How much space the matter is occupying
• Volume can be changed for liquids and gases. In case of solid substances, volume is fixed and cannot be changed.

Instruments:

There are several different measuring instruments for volume and their use depends on the type of measurement we are required to take. (The experimental aspect of all the apparatus will be discussed in the alternate to practical paper in detail)

Units:

Decimeter cubic (dm³), centimeter cubic (cm³), meter cube (m³), liter (l) Where 1l = 1 dm³ = 1000cm³ 1m³ = 1000 dm³

1.3 Measuring Time

Time:

• We all know what time is but to put it in a formal definition, time is the interval between any two specific points, a measurable period, where some event can exist.

Instruments:

• Stopwatch/stop clock

Units:

Hours (hr), minutes (min), seconds (s), milliseconds (ms) Where 1 hour = 60 min 1min = 60 sec 1sec = 1000 ms

1.4 Measuring Temperature:

Temperature:

• Generally, temperature can be called the degree of hotness of something, meaning how hot something is. We also add coldness into the definition sometimes.
• Keep in mind, coldness is just the absence of heat. So, when we are measuring the coldness of a substance, it is the measure of the absence of heat.

Instruments:

• Mercury thermometer
• Alcohol thermometer

Both types of thermometers are used in the measurement of temperature. The choice of thermometer for an experiment is beyond the scope of Chemistry Olevel. However, the topic is discussed in Physics Olevel.

Units:

We have several different units for measuring temperature including kelvin, Fahrenheit and Celsius. However, our basic SI unit for temperature measurement is Kelvin even though the most commonly used unit is Celsius.

1.5 Properties of Gases:

Some of the common gases along with their properties are as follows. After becoming familiar with the properties of several gases, we can also discuss the collection method for gases.

Collection of Gases:

Based on the mass and solubility of a gas, it can be decided how the experimental setup should be in order to collect that particular gas. There are three basic types of experimental setups for collecting gases.

• Insoluble gas
• Light gas
• Heavy gas

Insoluble Gas Collection:

In case we are collecting a gas that is insoluble, we can set up the apparatus as shown where the gas will be collected over water. As the gas enters the inverted cylinder, it displaces the water and gets trapped inside.

Lighter Gas Collection:

• A lighter gas will rise above air and we can use this to our advantage.
• We invert a cylinder and as the gas rises up, it gets collected in the cylinder.
• In this case, we have to use a drying agent so that heavy vapors don’t pull the gas down. The choice of drying agent is also very important. In case, the gas is acidic, we use conc. Sulphuric acid but we cannot use it for alkaline gases otherwise a reaction will take place. In alkaline gas collection, like ammonia, we chose Calcium Oxide (CaO) as a drying agent.
• This is called upward delivery, or downward displacement (of air).

Heavier Gas Collection:

This is quite similar to lighter gas collection. However, we don’t need any drying agent in this case. Heavier gas sinks down into a container and hence it is called downward delivery or upward displacement (of air).

1.6 Measuring Rates:

We commonly use two ways to measure the rate of a reaction.

• We attach a syringe to a reaction that gives off gas and measure the rate of gas discharge

Formula = gas given of f/ time

• We place the reaction apparatus on a balance and measure the rate of change of mass

Formula = mass decrease / time

1.7 Terminology

Now we will move on to some of the basic experiments that are commonly performed in chemistry and you should be aware of them. These are techniques that are often part of bigger experimental setups and are also used in the practical paper. Starting off by some basic definitions:

Pure Substances and Mixtures:

Pure Substance:

A pure substance is something that contains only one type of substance. This means that it is free some impurities or any unwanted additions to it.

Mixture:

• A mixture is the opposite to a pure substance, meaning a substance that contain two or more substances.
• There is a difference between mixtures and compounds that will be discussed in detail later on. What we need to know at this stage is that a mixture is create by a physical change (i.e. It can be reversed by physical means, whereas a compound cannot be reversed by physical means only)
• Mixtures can be in the form of solutions or alloys as well (alloy is commonly when different types of metals are mixed together)

Impurity:

Any substance that is added into a pure substance, converting it to a mixture. Usually, these are unwanted substances are in the next topic we will discuss ways to remove these impurities from mixtures to turn them into pure substances.

Purification:

As the name suggests, it is the method of creating pure substances. In other words, it is the removal of impurities from mixtures to turn them into pure substances.

Solute, Solvent and Solution:

• A solute is a substance that we add into a solvent to create a solution. Solutes dissolve into the solvent.
• Substances that dissolve into solvents are called soluble substances and substances that do not dissolve are called insoluble substances.
• A solvent, as mentioned, is the substance that dissolves the solute in itself to create a solution.
• In short, a solution is a mixture of solute and solvent.

1.8 Crystallization:

The first technique that we will discuss is crystallization. In order to understand Crystallization, we need to understand a few terminologies:

• Crystal: A solid that is drawn out of a solution during the process of crystallization
• Crystallization: A process where we cool a hot solution down, allowing crystals to form of dissolved solid.
• There are two main uses of crystallization:
  • Separation of soluble and insoluble substance
  • Purification of solids

Separation of soluble and insoluble substance:

When we use crystallization to separate a soluble and an insoluble substance, we follow the following processes:

• Dissolving: mixing a solute in a solvent
• Saturated solution: a solution which has no more capacity of dissolving a solute
• Filtration: The process of separating insoluble substances from a solvent by using a filter paper
• Heating
• Evaporation: process of a liquid turning into a gas at any temperature (below boiling point)
• Crystallization

Steps:

• We firstly choose a solvent in which one of our two substances is soluble and the other is not. This is important otherwise we cannot separate the substance using crystallization. We dissolve the substances to create a saturated solution.
• Since one of the substances is insoluble, it can be filtered out using a filter paper. We filter out and separated the substance. This is done by passing the solution a filter paper. The insoluble substance is held back as residue (the substance in a mixture which is insoluble and stays back on the filter paper during filtration) and the filtrate is collected in a container such as a beaker.
• A filtrate is the substance that passes through the filter paper completely.
• After filtering the solution, we are left with one of the substances mixed in the solvent. Next, we heat the excess solution away and are left with very little amount of solution. Make sure to not heat excessively since that can damage the crystals by removing the water of crystallization.
• Water of Crystallization: The fixed amount of water that is part of a crystal.
• The remaining hot solution is left to evaporate. The crystals start to appear, and we have our solid substance in the form of crystals.

Purification of solids:

When we use crystallization to purify a solid, we follow the following processes:

• Melting: Turning a solid to a liquid by the use of heat
• Cooling and crystallization
• Filtration

Steps:

• The solid that we are purifying is firstly melted into a liquid form.
• The next step is to allow the molten solid to cool and crystallize. This removes the impurities in the substance, and we get pure crystals. The first crystals are the purest.
• The remaining impure molten solution is filtered out and the crystals are held back as residue on the filter paper.
• This process is mainly used to get pure silicon that is used in the creation on microchips for technology.

1.9 Decanting and Centrifuging:

Decanting:

Decanting is a process in which we separate heavy, insoluble substances that are settled at the bottom of a container by pouring off the liquid from the top of the container.

Centrifuging:

• Not all the impure substances settle down the bottom of a container, making decanting harder.
• In order to settle the suspensions: light-weight impurities that are undissolved and suspended inside the solvent, we carry out centrifuging.
• In this process, we spin the mixture at a high speed, causing the particles to fling to the bottom of the container.

2.0 Immiscible Liquids:

• Liquids that do not mix together are called immiscible liquids.
• For example: Oil and water are immiscible
• In order to separate immiscible liquids, we use an apparatus called a separating funnel which has a small tap at the bottom. We fill the funnel with both the substances. The lighter liquid stays on top, and we the liquid which is heavier down the tap into another container.
• However, it shall be noted that this separation is wholly NEVER completed.

2.1 Sublimation:

• Used when there is a sublime substance present in a mixture
• Sublime Substance: a substance that changes from a solid directly into a gas when heated.
• In sublimation, we heat a mixture with the sublime substance and place a cold glass funnel on top of the mixture. The sublime substance rises up in the form of a gas and as soon as it comes in contact with the cold funnel, it condenses back into solid as a sublimate (condensed solid that is the product of sublimation).
• This solid is then removed and the impurities are left inside the original container.

2.2 Solar Evaporation:

As the name suggests, solar evaporation is when we leave our solution to evaporate in the sun, i.e. Without providing heat ourselves.

2.3 Distillation:

• Distillation: The process used to separate a liquid from impurities by boiling and condensing back its vapors.
• Distillate: The pure liquid that is collected as the result of distillation process.

Types of Distillation:

• Simple Distillation:
  • In simple distillation, we separate a pure liquid from a solution with impurities.

• • We begin by placing the solution in a flask and heat the solution in the distillation apparatus. A check for temperature is kept using a thermometer placed on top (inverted) in the flask.
  • As the liquid begins to boil, its vapors rise up and they pass through the Liebig Condenser. This is the part of the distillation apparatus where the vapors are cooled back to distillate and a container is placed at the end of the condenser to collect the distillate.
  • In order to condense the vapors, cold water is gushed into the tube of the condenser. Keep in mind there are two tubes of this condenser, one is the inner tube that contains the vapors and the other is the outer tube that has the water flowing through it.
  • A Very IMPORTANT point to note here is that the water always enters at the bottom part of the tube and exits at the top which allows the entire tube to be cooled down. Also, since the vapors leave the tube at the end, this setting makes sure that the coolest water reaches the leaving vapors, in case there are any left in the condenser and hence, no vapor is wasted at the end of the tube.
  • Mostly, the distillate is collected in a conical flask.
• Fractional Distillation:
  • This type of distillation is carried out in order to separate more than one type of miscible liquids from a solution. (In case, liquids were immiscible, we can simple use the previous mentioned method using separating funnel).
  • The apparatus is set up quite similar to the simple distillation. However, we add a fractional column as well above the flask with the solution which contains glass beads.
  • We separate the liquids by the help of the difference in boiling points. A liquid with a lower boiling point boils first and turns to vapors. However, other liquids can still evaporate at a slow pace and their vapors can also join the other vapors. This is prevented by the glass beads which allow such vapors to condense back and fall into the flask.
  • The thermometer is very important since it is used to keep track of the different boiling points. When one of the liquids gets separated completely, temperature rises, and another conical flask is then placed for the next liquid and so on.
  • This is mostly used to separate crude oil, fermented liquor and liquefied air.

• Fraction: a distillate that lies in a certain boiling point range in the fractional distillation.

  Chromatography:

• Chromatography is a process in which we separate various different soluble substances by using their difference in solubilities.
• Locating Agent: In case of chromatograms that are not clearly visible, we add a locating agent to the chromatogram in order to make it visible. The locating agent reacts with the substances and produces a visibly colored product.
• Chromatogram: The pattern that we get as a result of the chromatography process. The chromatogram is formed on a substance that is absorbent of the solvent we are using.

Steps:

• We take a chromatography paper and draw a base line. The base line is the starting position and is marked as 0.
• We place the substance to be separated in small dots on the base line and place the chromatography paper inside a solvent that can dissolve all those substances. A point to note here is that the solvent level must be below the base line otherwise the solvent will dissolve all the solute before the process can begin and we will not get any diagram.
• Slowly, the solvent moves up the chromatography paper and takes the solutes with it. The difference in solubilities mean that different solutes travel different distances on the chromatography paper.

• The different marks that are made are used to determine the identity of the solutes. In case of colorless chromatograms, we add locating agents.
• In case our substance was pure, there will only be one dot on the chromatogram.
• Solvent front: Solvent front is the ending point till where the solvent has risen up on the chromatography paper (in a set time frame).
• Retardation Factor (RF VALUE): The RF value is calculated to identify the substances in the mixture. This is calculated by:

Formula: Distance moved by substance / Distance moved by the solvent (distance from base line to solvent front)

Lesson Tags

Experimental Chemistry | Detailed Notes For Preparation & Revision | O Level Chemistry 5070 and IGCSE Chemistry 0620

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