Chemistry - The nature of substances and chemical reactions - elements, compounds, formulae and separating mixtures

Question 1

What is an atom?

Answer 1

This is the smallest particle of an element.

Question 2

What is a molecule?

Answer 2

A cluster of non-metal atoms that are chemically bonded together.

Question 3

What is an ion?

Answer 3

An ion is an electrically charged particle, formed when an atom or a molecule loses or gains electrons.
-loss of electron(s) = positively charged.
-gain of electron(s) = negatively charged.

Question 4

What is an element?

Answer 4

A pure substance that is listed in the periodic table and only has one type of atom in it.

Question 5

What is a mixture?

Answer 5

An impure substance made from different elements or compounds mixed together that are not chemically joined.

Question 6

What is a compound?

Answer 6

A pure substance made from more than one type of element chemically bonded together.

Question 7

What is an ionic compound?

Answer 7

When a negative ion joins with a positive ion.

Question 8

What are the differences between mixtures and compounds?

Answer 8

Mixture - The different substances are not chemically joined together
Compound - The different elements are chemically joined together

Mixture - Each substance in the mixture keeps its own properties
Compound - The compound has properties which are different from the elements it contains

Mixture - Each substance is easily separated from the mixture
Compound - It can only be separated into its elements using chemical reactions

Mixture – you can vary the amount of each substance in a mixture Definite Compound – you cannot vary the amount of each element in a compound

Question 9

How do you separate mixtures using filtration?

Answer 9

1. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper
2. The solid and liquid mixture is poured into the filter funnel
3. The liquid drips through the filter paper but the solid particles are caught in the filter paper

Question 10

How do you separate mixtures using evaporation?

Answer 10

One way to separate a soluble solid from its solution is to make crystals. This involves evaporating the solution to a much smaller volume and then leaving it to cool. As the solution cools, crystals form, and these can be obtained by filtration.

Question 11

How do you separate mixtures using distillation?

Answer 11

1. Salty water is heated and the water is evaporated
2. The water vapour cools in the condenser and drips into a beaker
3. The water has condensed and is now in the beaker, the salt stays behind

Question 12

What is chromatography?

Answer 12

Chromatography can be used to separate mixtures of coloured compounds. including ink dyes and food colouring.

Question 13

What is an experiment for chromatography?

Answer 13

Simple chromatography is carried out on paper. A spot of the mixture is placed on a pencil line near the bottom of a piece of chromatography paper – the line must be in pencil because pencil is insoluble in water and so will not move as the chromatography progresses. The paper is then placed upright in a suitable solvent, such as water.

As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.

Question 14

How do you calculate Rf value?

Answer 14

Rf = distance moved by the compound ÷ distance moved by the solvent

Question 15

What is a Rf value?

Answer 15

The Rf value is a measure of how soluble a particular substance is in a given solvent. The further the substance moves, the larger the value of Rf for that substance, and so the more soluble it is in the solvent that you are using.

Question 16

What is chemical change?

Answer 16

When a chemical reaction occurs, atoms are neither created nor destroyed. Instead, the atoms rearrange themselves to form new chemicals. This is known as a chemical change.

Question 17

What is a physical change?

Answer 17

The other kind of change in chemistry is a physical change, which is when a substance changes without becoming a new chemical. These physical changes are often changes in state such as melting, freezing, boiling, condensing and dissolving.

Question 18

What are the differences between chemical and physical change?

Answer 18

Physical - No new substances are formed
Chemical - New substances (known as products) are formed when the original chemicals (known as reactants) react

Physical - reversible
Chemical - irreversible

Physical - no significant energy change
Chemical - Energy is always given out (exothermic) or taken in (endothermic) resulting in a temperature change

Physical - Usually there is no colour change
Chemical - This can result in a colour change due to the formation of new products

Physical - The substance usually changes state
Chemical - New products can have a different state to the reactants but not always