1.1.1 Understanding the composition of matter
Matter is all around us and constitutes our bodies, the food we consume, and the materials we interact with daily. Chemistry explores the properties of matter, while energy is often considered a property of matter itself. Energy enables matter to perform work or generate heat.
Chemistry focuses on transformations of matter where both mass and energy are conserved, such as in chemical reactions. The change in mass caused by chemical changes is usually negligible compared to the speed of light, allowing for approximation in calculations without compromising accuracy.
Exploring the Atomic Theory
The atomic theory originated from the observation that certain substances always combine in definite proportions. This led to the proposition that matter is composed of elements that cannot be chemically broken down. Experiments involving the reactions of hydrogen, oxygen, carbon, and other elements provided evidence supporting this theory.
According to the atomic theory, elements are the fundamental building blocks of matter and cannot be further broken down. Atoms of different elements combine in fixed, simple ratios, and possess different masses. All matter is composed of atoms, which cannot be created or destroyed but rearranged during chemical reactions. The properties of matter depend on the arrangement and bonding of these atoms.
Examining the Historical Evidence
Ancient philosophers, such as Uddālaka Āruni, Democritus, and Leucippus, proposed that matter consists of tiny, indivisible particles. Later, John Dalton classified atoms into different types called elements based on their masses. The development of atomic theory relied on verifiable evidence and observations made by early thinkers and scientists.
Chemical Symbols
In modern chemistry, atoms and elements are represented by symbols derived from their names. These symbols consist of one or two letters. For example, hydrogen is represented by H, while iron is represented by Fe. The periodic table provides a comprehensive list of chemical elements and their symbols. Atoms are the smallest units of matter with distinct chemical properties and can combine to form chemical substances.
Pure Substances and Mixtures
Matter can be categorised as either a pure substance or a mixture. Pure substances have a definite and uniform chemical composition. Elements are pure substances composed of one type of atom, while compounds are pure substances made up of two or more kinds of atoms in fixed ratios. Mixtures are combinations of two or more pure substances that retain their individual properties. Homogeneous mixtures have a uniform composition and properties throughout, while heterogeneous mixtures have a non-uniform composition and varying properties.
Melting Point Determination:
Determining the melting point of a substance is a useful method to assess its purity. When a substance is pure, it has a sharp melting point, meaning it melts at a specific temperature that closely matches the expected value. On the other hand, impurities in a substance lower its melting point, causing the melting process to occur over a range of temperatures.
To determine the melting point, a melting point apparatus and capillary tubes are used. Two known organic solids, such as aspirin and salol, can serve as samples for analysis. Safety precautions, including wearing eye protection and handling hazardous substances carefully, should be followed.
The procedure involves preparing separate capillary tubes with samples of each solid. Following the instructions provided by the teacher, small amounts of the two solids are mixed together. A third capillary tube contains a sample of the mixture. The melting point of the three samples (pure substances A and B, and the mixture) is then determined.
Some questions to consider in this experiment include recording qualitative and quantitative data in an appropriate format, commenting on the results by comparing the melting points of pure and impure substances, researching the structural formulas of the substances to explain differences in melting points, and evaluating the use of melting point data in analyzing the success of an organic synthesis.
Mixtures and Separation Techniques
Mixtures consist of more than one element or compound and can be separated using physical methods. There are two types of mixtures: homogeneous and heterogeneous. Homogeneous mixtures have particles evenly distributed throughout, such as air. Heterogeneous mixtures have non-uniform distribution, like natural milk with cream rising to the top.
Components of mixtures retain their physical and chemical properties. However, when elements chemically combine to form compounds, such as water, the resulting substance has distinct properties different from its individual components.
Various separation techniques can be employed to separate mixtures based on the unique properties of their components. These techniques include filtration, dissolution, crystallization, distillation, and paper chromatography.
Technique Description:
1. Filtration:
Filtration is a separation technique that involves pouring a mixture through a paper filter or other porous material. The filter allows the liquid components to pass through while trapping solid particles. This method is particularly useful when separating mixtures of solid particles from liquids or when separating solid mixtures with different particle sizes.
2. Dissolution (Solvation):
In dissolution, a mixture is added to water or an organic solvent. The soluble components in the mixture dissolve in the solvent, forming a solution. This technique is effective for separating soluble substances from insoluble ones.
3. Crystallisation:
Crystallisation is a process where a mixture is dissolved in hot water or an organic solvent. As the solution cools down, crystals of the dissolved substance form. These crystals are then isolated through filtration. Crystallization is commonly used to purify substances and separate different components based on their solubilities.
4. Evaporation or Distillation:
Evaporation and distillation are techniques used to separate mixtures with volatile components, such as liquids with different boiling points. In evaporation, the mixture is heated until one or more of its components vaporize. In distillation, the vapors are condensed back into a liquid. The liquid obtained is enriched in the more volatile component, while the less volatile components remain behind.
5. Paper Chromatography:
Paper chromatography involves placing a mixture on a piece of chromatography paper. One end of the paper is submerged in water or a solvent (the mobile phase). As the solvent travels up the paper, the different components in the mixture move along with it. However, each component interacts differently with the paper (the stationary phase) and the solvent, leading to different migration rates. This results in the separation of the components based on their affinities for the solvent and the paper.
Activity
Suggest and describe an appropriate separating technique for the following mixtures:
a. Salt and pepper
The suitable method is dissolution. Add water to the mixture; salt, being soluble in water, will dissolve while pepper remains as solid particles. Filtration is then used to separate the dissolved salt from the pepper.
b. Several Water-Soluble Dyes:
- Chromatography is the appropriate method. The dyes are placed on chromatography paper, and as the solvent moves up the paper, the dyes will separate based on their affinities for the paper and the solvent.
c. Sugar and water:
- The method is evaporation. Heat the mixture, and as water evaporates, sugar crystals will remain behind.
d. Iron and Copper Filings:
- The technique is magnetism. A magnet can be used to attract the iron filings, leaving the copper filings behind.
Linked Questions:
- When selecting a method to separate the components of a mixture, various factors are taken into consideration. (Tool 1)
- Purification of reaction products can be achieved through different techniques. (Tool 1)
- The type of mixture that forms between two substances is influenced by intermolecular forces, which play a role in determining their affinity for each other. (Structure 2.2)
- Alloys are typically regarded as mixtures despite the presence of metallic bonding because they consist of different elements or compounds that are blended together, resulting in a mixture with unique properties. (Structure 2.3 and Structure 2.4)
Revision notes
Matter is composed of atoms, which cannot be created or destroyed but rearranged during chemical reactions.
Elements are the fundamental building blocks of matter, while compounds consist of atoms of different elements chemically bonded together.
Mixtures contain more than one element or compound and can be separated by physical methods.
The kinetic molecular theory explains the physical properties of matter and changes of state.
Temperature is a measure of the average kinetic energy of particles.
Pure substances have a definite and uniform chemical composition, while mixtures retain their individual properties.
Chemical symbols represent elements and atoms.
The atomic theory states that all matter is composed of atoms, which cannot be chemically broken down.
Historical evidence and experiments supported the development of atomic theory.
Energy and matter are closely associated, but chemistry focuses on transformations where both mass and energy are conserved.
Melting point determination is a useful technique to assess the purity of a substance. Pure substances have sharp melting points, while impurities cause the melting point to be lower and occur over a range of temperatures.
To determine the melting point, a melting point apparatus and capillary tubes are used. Samples of known organic solids are prepared and mixed together, and their melting points are measured.
Mixtures can be classified as homogeneous (evenly distributed particles) or heterogeneous (non-uniform distribution). Components of mixtures retain their properties, while compounds formed by chemical bonding have distinct properties.
Various separation techniques, such as filtration, dissolution, crystallization, distillation, and paper chromatography, can be employed to separate mixtures based on the unique properties of their components.
When planning experiments involving mixtures, it is important to consider the physical and chemical properties of the substances and conduct a risk assessment to ensure safety.
Linked questions to explore include factors influencing the choice of separation methods, purification of reaction products, the role of intermolecular forces in mixture formation, and why alloys are considered mixtures despite containing metallic bonding.
Quiz:
What is the atomic theory?
How are elements represented in chemistry?
What is the difference between pure substances and mixtures?
Explain the concept of temperature.
Can atoms be chemically broken down? Why or why not?
What is the purpose of determining the melting point of a substance?
How can mixtures be classified based on the distribution of their particles?
Name three separation techniques used to separate mixtures.
How does a compound differ from a mixture in terms of their properties?
Why are alloys considered mixtures, even though they contain metallic bonding?