## Wednesday, 15 July 2015

### Particulate theory of matter

1. Solids, liquids and gases
Click here to watch a video from youtube, for an overview on the kinetic particle theory for solids, liquids and gases.

The following is a summary of the particles in solids, liquids and gases:

Solids
• orderly arrangement of particles
• particles are packed closely together, and vibrate about a fixed position. They are also held together by strong forces of attraction
• solid cannot be compressed, and has a fixed shape.
Liquids
• particle are less closely packed together, and are able to move throughout the liquid. Particles are also held together by moderately strong forces of attraction.
• liquids cannot be compressed, and they do not have a fixed shape (take the shape of the container).
Gases
• particles are far apart, and move about randomly and in all directions. Forces of attraction between particles are weak.
• Gases can be compressed (they take the volume of the container), and do not have a fixed shape.
2. Changes in state
Click here to watch a video from youtube, that shows what happens to the particles during state change.

Most substances goes from solid to liquid then to gas as temperature increases, and vice versa when temperature decreases. This means that not many solids sublime on heating. Towards the end of the above video, we see the sublimation of solid carbon dioxide. Carbon dioxide is one of the substances that sublimes (change from solid to gas). Other substances that sublime include iodine and ammonium salts.

During a change in state for pure substances, the temperature remains constant, until all of the substance is changed from one state to another. Take the melting of ice for example. During melting of pure ice, the temperature remains constant, as the heat gained is used to separate the particles in the solid further apart, to form the liquid. It is only when all the pure ice has melted does temperature then start to increase.

3. Diffusion
Diffusion is a process where particles move from a region of higher concentration to a region of lower concentration. Examples of diffusion in our daily life includes being able to smell the perfume when someone applies it some distance away; being able to smell the aroma of coffee some distance away from it.

The rate of diffusion is affected by the density of a substance. The higher the density of a particle, the slower the rate of diffusion. For gases, diffusion is inversely proportional to the molecular mass. The higher the molecular mass, the slower the rate of diffusion. [Hint: This is related to why ammonia diffuses faster than hydrogen chloride].

The rate of diffusion is also affected by temperature. The higher the temperature, the faster the particles move, and as such, the faster the rate of diffusion.

The following is an amazing video on diffusion I found on youtube, which provides what you need to know for secondary school diffusion! Click here to watch the video. The video ends off stating that ammonia diffuses faster than hydrogen chloride. Why is it so?

Ammonia has a formula of NH3. This means that it consists of 1 nitrogen atom and 3 hydrogen atoms per molecule. From the periodic table, the mass number of nitrogen is 14, and the mass number of hydrogen is 1. The molecular mass of ammonia = 14 + 3x 1 = 17.

Hydrogen chloride has a formula of HCl. This means that it consists of 1 chlorine atom and 1 hydrogen atom per molecule. From the periodic table, the mass number of nitrogen is 35.5 and the mass number of hydrogen is 1. The molecular mass of hydrogen chloride = 1 + 35.5 = 36.5.

The lower the molecular mass, the faster the rate of diffusion of the gas. Hence, ammonia diffuses faster than hydrogen chloride.

Questions
- Quizzes and Tests on Particulate Theory of Matter