Water Treatment, Chemical Changes, and Alloys
Topic 5: Water Treatment Methods
Drinking water is not obtained by simple decanting. Water obtained from rivers and lakes is often unfit for human consumption. It’s usually murky, and if left to stand, waste deposits at the bottom.
Decanting (using a decanting funnel): Muddy water is carefully poured into another container to separate and extract the sediments. After decanting, the water may remain murky due to small suspended particles. These particles can be separated by filtration using filter paper. The liquid that passes through the filter is called the “filtrate,” and the solid retained on the filter is the residue.
Obtaining Salt from Seawater: When seawater evaporates, salt (common salt) is left behind. Evaporation can separate any solid dissolved in a liquid. The remaining solid often forms small crystals in a process called crystallization.
Obtaining Pure Water: Distillation is the process of vaporizing a liquid and then condensing the vapor. It’s used to separate liquid solutions.
Separation of a Mixture of Water and Alcohol: Fractional distillation is used for this separation.
Chromatography: This method separates solids from a liquid solution. A simple example is chromatography on paper.
Topic 6: State Changes and Chemical Reactions
State Changes: Substances can undergo state changes (e.g., solid to liquid) and return to their original state after the change. The original substance remains the same.
Chemical Changes: In chemical changes, the initial substance transforms into different substances.
Indicators of Chemical Changes:
- Color change
- Formation of gas bubbles
- Formation of a precipitate
- Temperature change:
- Exothermic reaction: Temperature increases
- Endothermic reaction: Temperature decreases
Elements and Compounds:
- Element: A substance that cannot be broken down into simpler substances.
- Compound: A substance formed by the chemical reaction of two or more simpler substances.
Changes like boiling water or melting snow are easily reversible (physical changes). Chemical changes, which form new substances, are *not* easily reversible.
Recognizing Chemical Changes: We recognize a chemical change by observing one or more of the following:
- Gas bubble formation
- Solid (precipitate) formation
- Color change
- Temperature change
Elements are simple substances that cannot be decomposed. Compounds are substances that *can* be transformed into simpler substances. Elements are classified as metals, non-metals, and metalloids. Generally, metals are solid at room temperature, dense, and electrically conductive. Non-metals can be gases, liquids, or solids, have lower melting and boiling points, and do not conduct electricity.
- An element is composed of identical atoms, either as free atoms, molecules of identical atoms, or a giant structure of identical atoms.
- A compound is a substance composed of different atoms, either as molecules of different atoms or a giant structure of different atoms.
- Atoms are the smallest constituent particles of an element. Molecules are groups of a few atoms (same or different) strongly linked together. Giant atomic structures are sets of consecutive atoms (same or different) strongly linked together.
Topic 7: Air, Combustion, Respiration, and Corrosion
What is in the Air?
- Nitrogen: 78%
- Oxygen: 21%
- Argon: 0.9%
- Other: 0.1%
Combustion: Combustion is a rapid oxidation process accompanied by the emission of light and heat. Substances that burn on contact with *oxygen* (not water) and produce light and heat are called fuels.
Example: Carbon (s) + Oxygen (g) = Carbon dioxide (g)
Respiration: Respiration is a more controlled oxidation process. Oxygen reacts with substances in foods, such as glucose, to provide energy for living organisms.
Example: Glucose + Oxygen = Carbon dioxide + Water
Corrosion: Corrosion is the oxidation of a metal, such as the rusting of iron.
Example: Iron (s) + Oxygen (g) = Iron oxide (s)
Topic 8: Alloys
An alloy is a solid solution of two or more metals. It is obtained by cooling a mixture of molten metals.
- Brass: Copper (60%), Zinc (40%). Attractive gold appearance, easily moldable, harder than copper and zinc, corrosion-resistant.
- Bronze: Copper (90%), Tin (10%). Golden appearance, corrosion-resistant, hard.
- Cupronickel: Copper (70%), Nickel (30%). Silver appearance, properties similar to bronze.
- Tempered Steel: Iron (99.8%), Carbon (0.2%). Inexpensive, tough, easily moldable.
- Stainless Steel: Iron (74%), Chromium (18%), Nickel (8%). Harder than tempered steel, corrosion-resistant, brilliant.
- Duralumin: Aluminum (95%), Copper (5%). Low density, hard, more corrosion-resistant than aluminum.