9th Class Science: Is Matter Around Us Pure

Question: Explain why Hydrogen and Oxygen are considered element where as water is not considered an element.

Answer: The smallest particles of hydrogen and oxygen are hydrogen atom and oxygen atom respectively where as the smallest particle of water is a water molecule which is made up of hydrogen and oxygen further.

Question: Give reason:

1. Copper metal is used for making electric wires.

Answer: Copper being a good conductor of electricity is used for making electric wires.

2. Graphite is used to make electrode in a dry cell.

Answer: Bling as good conductor of electricity graphite is used to make electrode in dry cells.

Question: Is air a compound or a mixture? Give three reason for your answer.

Answer: Air is a mixture because:

1. Air can be separated into its constituents lixe oxygen, nitrogen etc.
2. Air shows the properties of its constituents.
3. Heat and light are not produced when air is created by mixing gases.

Question: Give reasons for supposing that water is a compound and not a mixture.

Answer: Water is a compound because:

1. Water cannot be separated into its constituents, hydrogen and oxygen.
2. The composition of water is fixed.
3. Water has a fixed boiling point of 100°C under standard atmospheric pressure.

Question: Define a compound show that Nace is a compound.

Answer: A substance made up of two or more elements chemically combined in a fixed proportion by mass is called a compounded. Nace is a compound formed by a fixed amount of Sodium and Chlorine. So, Nace is a compound.

Question: Define a mixture. Show sugar solution is a mixture.

Answer: A substance which contains particles of one or more substances in no fixed ratio. Sugar solution can be created by adding any amount of water and so it is a mixture. The sugar does not reacts chemically with water.

Question: Why brass a mixture not a compound. Show two reasons.

1. It shows the properties of zinc and copper.
2. It has a variable composition.

Question: Explain why a solution of salt in water is considered a mixture and not a compound.

Answer: Its so because:

1. It can have a variable composition.
2. Its constituents can be separated.
3. The constituents do not react chemically.

Question: 1. State the main points of difference between homogeneous and heterogeneous mixtures.

Answer: Homogeneous:

1. Those mixtures in which the substances are completely mixed together and are in distinguished. From one another are called homogeneous mixtures.
2. It has no variable boundaries of separation between constitutions.
3. The constituents mix up uniformly.
4. Soda water, air, vinegar, Alcohol and water mixtures sugar and water mixtures , copper sulfate, Solution.

Heterogeneous:

1. Those mixtures in which the substances remain separate and one substance is spread throughout the other substances as small particles, droplets.
2. It has a variable boundary of separation between constituents.
3. The constituents do not mix up uniformly.
4. Wood, Soil, Petrol and Water mixture, Chalk and water mixture.