Reflection of learning in Science and personal growth
Acids and bases
Strong and weak acids
A strong acid is a substance which ionizes completely in water to produce many hydrogen ions. There are no molecules remaining and the solution contains a high concentration of ions, thus making it a good electrical conductor.
A weak acid on the other hand is a substance which ionizes partially in water to produce few hydrogen ions. Most of the acid molecules remain as acid molecules and the solution contains a low concentration of ions thus making it a poor electrical conductor.
Importance of water for acidity
The properties of acids are due to the presence of hydrogen ions. Upon mixing with water, ionization of acids occurs and thus hydrogen ions are produced. Therefore, acids only behave as acids when they are dissolved in water.
Properties of acids
1. Sour taste
2. Change the colour of indicators
3. Contain hydrogen ions and conduct electricity
4. React with metals, carbonate and bases
Reactions of acids and bases
5. Acids react with carbonate to produce carbon dioxide, salt and water
6. Acids react with bases to produce salt and water
7. Acids react with metal to produce salt and hydrogen
8. Bases react with ammonia to produce salt, water and ammonia gas
Bases and Alkalis
What is a base?
-It is the oxides or hydroxides of metal that reacts with an acid to form a salt and water only.
What is an alkali?
-It is the metal hydroxide which is soluble in water and produces hydroxide ions in water.
Note: *All alkalis are bases but not all bases are alkalis.
Strength of an alkali
The strength of an alkali depends on its degree of ionization in water to form hydroxide ions. The properties of alkalis are due to hydroxide ions.
A strong alkali is one that ionizes completely in water to produce hydroxide ions and there will be no molecules left. A weak alkali is one that ionizes partially in water to produce a few hydroxide ions. Most of the alkali molecules remain as molecules.
Properties of bases
1. Alkalis have a soapy feel and a better taste
2. Alkalis can change the colour of indicators
3. Bases can react with acids to form salt and water only - Neutralisation
Colour change of indicators
| Indicators | Observation(s) |
| Litmus paper | Acid turns blue litmus paper red, while alkali turns red litmus paper blue |
| Methyl Orange | Colour change at pH4 Acid turns methyl orange red Alkali turns methyl orange yellow |
| Phenolphthalein | Colour change at pH9 Alkali turns phenolphthalein pink Phenolphthalein remains colourless in acid |
| Universal Indicator | In neutral solutions, universal indicators remain green Strong acids turn universal indicator red Strond alkalis turn universal indicator violet |
Reflection and Refraction
1. Light is a form of energy capable of stimulating the light-sensitive cells in the retina of the eye.
2. Light travels in a straight line.
3. The path along which light energy travels is called a ray. It is represented by a straight line with an arrow to show its direction of motion.
4. We are able to see an object only when the light rays coming from it enter our eyes.
Term used in reflection
Incident ray - is a ray of light striking a surface
Reflected ray - is a ray of light reflected (bouncing off) a surface
Normal - imaginary line perpendicular to the surface where reflection or refraction occurs
Angle of incidence (i) - is the angle between the incident ray and the normal
Angle of reflection (r) - is the angle between the reflected ray and the normal
Laws of reflection
• Angle of Incidence = Angle of Reflection
• Incident ray, reflected ray and normal lie on the same plane
Regular and diffused reflection
Regular reflection occurs at smooth surfaces. Diffused reflection occurs at rough surfaces.
Image on a mirror
The characteristics of the image formed in the plane are:
• The image is of the same size as the object.
• It undergoes lateral inversion.
• It is upright.
• It is virtual.
• The distance of the image from the mirror is equal to the distance of the object from the mirror.
Refraction
Normal is the imaginary line perpendicular to the surface where reflection or refraction occurs.
Angle of incidence is the angle between the incident ray and the normal.
Angle of refraction is the angle between the refracted ray and the normal.
What is a refraction?
At the air-glass interface, light is
-partially reflected off the surface and
-partially transmitted through the medium.
The light that is transmitted through the medium bends as it travels from one optical
medium (air) into another (glass).This bending effect of light is known as refraction.
Refraction is the apparent change in direction of light when the ray of light passes from
one transparent medium (optical medium) into another.
What causes refraction?
• Light travels at different speed in different media.
• At the boundary of the two media such as air and glass, there is a sudden change in speed of the light. This change in speed causes the path of the light to change, resulting in refraction.
Amount of refraction
The amount of refraction depends on the optical density of the medium.
Optical density is a measure of the extent to which a substance transmits visible light.
The higher the optical density of a medium, the lower the transmittance, thus the greater is the refraction of light in it.
Why doesn’t refraction occur sometimes?
When a light ray strikes perpendicularly to the surface of an optical medium, its angle of incidence is zero, thus its angle of refraction is also zero. However, even though the light is not bend, its speed still decreases when it enters the optically denser medium.
Rules of refraction
When a light ray travels from an optically less dense medium(eg. air) into an optically denser medium(eg. glass) at an angle, it is always refracted towards the normal as light travels slower in the optically denser medium.
Critical angle
At a point where the refracted ray travels exactly along the glass surface, its angle of refraction is 90o.
The angle of incidence in glass measured is what is known as the critical angle of glass.
Critical angle is defined as the angle of incidence in the optically denser medium when the angle of refraction in the optically less dense medium is 90o.
The critical angle is different for different materials as it is dependent on the optical density of a material. A material with a greater optical density will have a smaller critical angle.
All the light is reflected at the interface as if it had hit a mirror – a perfect mirror.
This phenomenon is known as total internal reflection because 100% of the light is reflected within glass itself.
There is no refraction at all.
The light that is reflected follows the law of reflection such that the angle of incidence is equal to the angle of reflection.
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