Organic Chemistry: Alkenes and Their Properties

This section focuses on organic chemistry, specifically alkenes and their properties, which is an important topic in GCSE science.

Crude Oil and Hydrocarbons

Crude oil is a fossil fuel formed primarily from dead sea creatures, such as plankton. It serves as a source for many fuels and chemicals, including plastics.

Definition: Crude oil is a mixture of hydrocarbons, which are compounds made up of only hydrogen and carbon atoms.

Most hydrocarbons in crude oil belong to a group called "alkanes" (not alkenes as mistakenly stated in the original text).

Alkanes and Their Properties

Alkanes are hydrocarbons with single bonds between carbon atoms. They have a general formula of CnH2n+2, where n is the number of carbon atoms.

Example: Some common alkanes include:

• Methane (CH4)
• Ethane (C2H6)
• Propane (C3H8)
• Butane (C4H10)

The properties of alkanes change with the number of carbon atoms in the molecule.

Combustion of Hydrocarbons

When hydrocarbons (alkanes) are burned, two types of combustion can occur:

1. Complete combustion: Occurs with a plentiful supply of oxygen

   • Carbon oxidizes to form carbon dioxide (CO2)
   • Hydrogen oxidizes to form water (H2O)

2. Incomplete combustion: Occurs when there isn't enough oxygen

   • Produces carbon monoxide or carbon soot

Highlight: Complete combustion of hydrocarbons produces carbon dioxide and water, while incomplete combustion can result in the formation of carbon monoxide or carbon soot.

Understanding the properties and reactions of hydrocarbons is essential for GCSE science students, as it forms the basis for more advanced topics in organic chemistry.

Osmosis in Plant and Animal Cells

Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration through a partially permeable membrane. This process is crucial for maintaining cellular balance in both plants and animals.

Definition: Osmosis is the movement of water from a higher water concentration to a lower concentration through a partially permeable membrane.

The effects of osmosis vary between plant and animal cells due to their structural differences:

1. Osmosis in animal cells:

   • Animal cells lack a cell wall, making them more susceptible to changes in size due to osmosis.
   • In dilute solutions, animal cells can burst due to excessive water intake.
   • In concentrated solutions, animal cells can shrink as water leaves the cell.

2. Osmosis in plant cells:

   • Plant cells have a rigid cell wall that provides support and prevents bursting.
   • In dilute solutions, plant cells become turgid (swollen) but do not burst.
   • In concentrated solutions, plant cells become sunken as water leaves the cell.

Vocabulary:

• Turgid: Swollen and rigid, used to describe plant cells in a dilute solution.
• Cytoplasm: The gel-like substance inside cells, which is a relatively concentrated solution with a low water concentration.

Examples of osmosis in animals and plants include:

• Water absorption in plant roots
• Rehydration of dried fruits
• Regulation of water balance in animal cells
• Absorption of water in the intestines

Highlight: The cytoplasm of cells is a relatively concentrated solution with a low water concentration. When a cell is placed in water, osmosis occurs, causing water to move from outside the cell into the cytoplasm.

Questions for review:

1. What does the term "turgid" mean, and for which type of cell is it used?
2. What type of membrane is required for osmosis?
3. In concentrated solutions, what happens to animal cells?
4. What structural feature do animal cells lack that makes them more susceptible to osmotic changes?